# When does DC voltage become dangerous?



## BatteryCharger

Ok, theoretically in the right place you could probably be killed with 1ma @ 1v. More realistically speaking, when does DC voltage become dangerous to touch with your bare hands? 36v? 48? 75? How high does it need to be before you should be afraid of it like 120v sockets?

Just curious...


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## timberwolf

It varies, you usually start feeling it at about 50Volts, which also is about the limit in safety regulations. And as with any powersource, not the Voltage is dangerous but the current.
I may check this in more detail when I'm back at home.


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## endreein

IF you drop down 1 metre you could die
if you drop down 25 you could survive !!

same goes for voltage ,,don't be afraid for sockets They hurt but, normaly you wont die unless you put some wet steel rods in them and grab them with your hands for a long time !! ,,,, has somthing to do with how easy the current goes through You.


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## endreein

also you can touch 10000v electrical fenses , but they only hurt like **** and won't kill you same reason: not enough current !!


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## Wiggle

Probably not going to find an exact number people can agree on, but I'd say 60+ VDC.


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## jhellwig

36-48 is where you might start feeling it. It depends on varying conditions. 50v is where you start to have to aboe3y different wiring standards per the National Electric Code.

You must remeber that dc is much more hazardous than ac of the same voltage. Dc will sustain an arc at much lower levels than ac. If you get shocked by dc you will likely not let go of what you touch. Ac will trow you away.


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## Mr Happy

It depends how sensitive you are, whether you have sweaty hands, and so on. I can certainly feel a tingle from 24 V, and in the wrong circumstances 12 V could be enough to kill you.

If you always assume electricity is dangerous you won't go far wrong.


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## Apollo Cree

jhellwig said:


> You must remeber that dc is much more hazardous than ac of the same voltage. Dc will sustain an arc at much lower levels than ac. If you get shocked by dc you will likely not let go of what you touch. Ac will trow you away.



Conventional wisdom in electrical engineering is exactly the opposite. The most common form of electrocution death is from stopping the heart. 60 Hz AC is much more likely to cause heart stoppage than DC current of the same voltage. 

AC voltage is quite good at causing muscle contractions and causing you to "clamp on" to an exposed wire as well.


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## Magic Matt

The position of the shock, and distance between entry and exit points is critical. A voltage of 60V per cm across your body will cause tissue damage. If the entry and exit points are somewhere like your leg, you can survive even the frighteningly high voltages from lightning. If the voltage goes across your heart, even 100VDC could be fatal and stop it.


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## Apollo Cree

Here's some info from the CDC. 

http://www.cdc.gov/niosh/docs/98-131/overview.html

It lists around 16 mA as a current level where you may not be able to let go. 

Around 20 mA can paralyze your respiratory muscles and stop breathing. 

100 mA can cause ventricular fibrillation and stop the heart. This is particularly dangerous because even if you get disconnected from the current, your heart won't necessarily restart.


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## Magic Matt

Good info... just a word of caution though... the figures you've quoted are at ~600VAC 60Hz though, not DC.


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## JohnR66

I heard somewhere that low DC voltages that you may not notice can do electrolysis damage to cells. Supposedly, a person was killed while working on a vehicle with a 24 volt battery and some how manged to get pinned with part of his body in contact with one of the terminals and another to the body (ground) of the vehicle. He was trapped that way for a while (hours?) and died later due to the damage to his cells from the current.

I can't attest to the validity of this story, but it sounds plausible as electric currents passing through chemicals can break down the molecules or form new ones.


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## Apollo Cree

The problem with such a question is that the answer is so complicated. It's mostly the amount of current flowing into a vital organ that causes death. 

If you run a current between two fingers on the same hand, only a small percent of the current will flow through, for instance, your heart. If you run the current from your left hand to your right hand, a larger percentage of the current will flow through the heart. A medical patient with, for instance electrodes for a heart monitor could have a much lower current threshold if there's some sort of electrical problem because . 

Even though we say current is the problem, what we see most of the time is voltage. We are usually dealing with what we consider to be "constant voltage" sources. This would be something like a battery. It produces 1.5 Volts most of the time. If you have 0.01 mA flowing, it's 1.5 Volts. If you pull 500 mA out of it, it's still close to 1.5V. 

Now, assume you have an exposed voltage of 50 V somewhere. If you walk up and touch it and have on shoes with rubber or plastic soles, the electrical resistance of your shoes is so high that you might only get a few micro amps. Change to leather soles, you still probably don't get much current. Now, assume you're touching a metal piece of furniture with one hand and touch the 50V circuit with the other. You get considerably more current. Now, consider if you have sweaty hands and are making really good contact with a grounded metal table. Now, assume you're standing in a decorative fountain with wet hands working on the water pump and you don't realize that the 50V DC power supply isn't turned off.

The threshold of "safe" voltage varies widely in these different situations because the electrical resistance varies so widely. 

In the electrical engineer safety discussions, a "nightmare" scenario was something like "A technician is working on a piece of low voltage electronic equipment. The equipment has energized components with sharp edges, for instance voltage test pins. The technician slips and manages to spear a finger on each hand with a pin and pierce the skin. What's a safe voltage level here?" The answer was that there probably was no safe level. 

With all that said, you mostly worry about voltages above 50V. You understand that lower voltages can still be dangerous in certain conditions. You become more concerned in wet conditions, or any kind of medical situation. Me, I start getting nervous above 12V, and ratchet up the nervousness as voltage gets higher. 

You also understand that a lower voltage high current situation can cause thermal burn problems. For instance, shorting out a car battery with your class ring can cause a really nasty burn. 

By the way, telephone wiring is around 48 Volts when the phone is on the hook, and we normally don't worry about that too much. However, if you're messing with some phone line connection and thoughtlessly decide to hold the connector in your mouth to free both hands, it can be an unpleasant or even fatal surprise. Especially if the phone rings, when the voltage jumps to around 90V.


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## Apollo Cree

Magic Matt said:


> Good info... just a word of caution though... the figures you've quoted are at ~600VAC 60Hz though, not DC.



In terms of the hazards listed, the voltage is irrelevant. 100 mA will tend to cause fibrillation no mater whether it's 50 VAC with wet hands or 500VAC with dry hands. 

AC vs. DC is mostly relevant to the ventricular fibrillation. 

Normally, you "play it safe" with AC vs. DC and just assume they're equally dangerous. e.g. DC is less likely to cause fibrillation, but you treat it as if it were equally dangerous. 



By the way, I think we tend to think of AC as more dangerous than DC because the common AC power sources we are familiar with are higher voltage than DC power sources. 

House wiring is 120 VAC or higher. Electrical distribution lines on power poles are 7000VAC or higher. Car batteries are usually 12V. Batteries are commonly 1.5V or 9V. Electronic circuitry in consumer electronics is commonly under 5V.


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## Apollo Cree

JohnR66 said:


> I heard somewhere that low DC voltages that you may not notice can do electrolysis damage to cells. Supposedly, a person was killed while working on a vehicle with a 24 volt battery and some how manged to get pinned with part of his body in contact with one of the terminals and another to the body (ground) of the vehicle. He was trapped that way for a while (hours?) and died later due to the damage to his cells from the current.
> 
> I can't attest to the validity of this story, but it sounds plausible as electric currents passing through chemicals can break down the molecules or form new ones.



Entirely plausible. I'll bet the 24V current was still painful and he would have done something to stop it if he wasn't pinned down.


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## uk_caver

Apollo Cree said:


> In the electrical engineer safety discussions, a "nightmare" scenario was something like "A technician is working on a piece of low voltage electronic equipment. The equipment has energized components with sharp edges, for instance voltage test pins. The technician slips and manages to spear a finger on each hand with a pin and pierce the skin. What's a safe voltage level here?" The answer was that there probably was no safe level.


At some voltage, there'd presumably be an equal risk of damage from other factors (the technician jumping back after stabbing their fingers (from pain rather than electric shock), and tripping over something on the floor, etc), and at some lower voltage, even if the electrical risk was nonzero, it would be so relatively small compared to other factors as to be beyond worrying about.


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## InHisName

Apollo Cree said:


> The technician slips and manages to spear a finger on each hand with a pin and pierce the skin. What's a safe voltage level here?" The answer was that there probably was no safe level.


That is what I heard back in my Engineering days. They referred to an Everready cell killing one. Needing only 10+ ma to do it. Imagine one Eneloop and above stiuation and 210 lb technician being brought down. Gives me the willies.


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## Apollo Cree

uk_caver said:


> At some voltage, there'd presumably be an equal risk of damage from other factors (the technician jumping back after stabbing their fingers (from pain rather than electric shock), and tripping over something on the floor, etc), and at some lower voltage, even if the electrical risk was nonzero, it would be so relatively small compared to other factors as to be beyond worrying about.



The point wasn't necessarily that the exact scenario was likely. It was more of a reminder to not make any assumptions that a particular voltage is safe in all conditions. There's one urban legend about some teacher demonstrating to his students that a car battery wouldn't electrocute you. Bare foot in a bucket of salty water, and a hand in another bucket of salty water. Oops, bring in a new teacher and counseling for the kids.


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## march.brown

A friend of mine (many years ago) was changing a battery on a truck ... He made two mistakes ... Firstly, he left his wedding ring on ... secondly, he used a non-insulated spanner.

As he tightened the live battery terminal, the spanner was in contact with his wedding ring ... The ring unfortunately came into contact with the bodywork of the truck.

The ring suddenly became extremely hot and luckily did not weld itself to the spanner and the bodywork ... He was left with a ring shape burn on his finger and the ring had to be cut off due to the swelling of the finger.

Where you have an almost infinite ammount of current (as in this case), it can be very dangerous.

I worked for many years on 48 volt and 110 volt DC systems and you actually get used to these voltages ... I found that the back-EMF, off highly inductive devices (such as electro-magnetic relays), really makes you jump though ... You never really get used to that.
.


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## jtr1962

JohnR66 said:


> I heard somewhere that low DC voltages that you may not notice can do electrolysis damage to cells. Supposedly, a person was killed while working on a vehicle with a 24 volt battery and some how manged to get pinned with part of his body in contact with one of the terminals and another to the body (ground) of the vehicle. He was trapped that way for a while (hours?) and died later due to the damage to his cells from the current.


Well, they use the principal of electrolysis to kill hair cells, so there is some validity here. I think what happens is the electrolytic reaction produces lye, which in turn kills the hair cell. Something similar probably happens to other types of cells subjected to low but continuous current.

Also, everyone here is talking about voltage and current, but it's total energy in joules from the charge which actually kills. I think you need around 100 joules to stop the heart, give or take, although I've heard figures as high as 200. Accidentally discharging a 10,000 µF capacitor connected to a full-wave bridge across a 120 VAC would be enough ( stored energy = ½CV² = ½ ( 0.01 ) ( 169 )² = 142.8 joules ). However, accidentally discharging a 1000 µF cap wouldn't be, although it might cause some really nasty burns, plus a heck of a jolt. Notice however the squared term with voltage. A seemingly small value capacitor at a high enough voltage could be deadly. If you're working on CRT TVs, for example, all it takes is a cap of 0.3 µF charged to 25 kV to kill you. I think the capacitance of the picture tube is more than this. On the flip side, you would need HUGE capacitances at 12 VDC to kill you, roughly on the order of 1.5 _Farads_.

Obviously almost any battery can store well in excess of 100 joules ( a AA Eneloop stores roughly 9,000 joules, for example ), so given a low impedance path through the heart, even a 12 VDC battery can kill.


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## tino_ale

IIRC your kidney can also fail from current in your body. There has been occurence where someone sustained an electric shock, was well minutes after but would be found dead the next morning because the kidney were damaged during the process.

After any electrical incident where you suspect current has gone through your body you should seek medical assistance. Now, I agree it's not easy to define in what case the incident was "significant" enough to decide whether to have a check-up or not :scowl:


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## Apollo Cree

jtr1962 said:


> Also, everyone here is talking about voltage and current, but it's total energy in joules from the charge which actually kills.



I think joules is only going to matter for burns or tissue damage. A short shock without many joules can easily stop the heart and it may not restart on its own. A long shock below the dangerous current levels will tend to not do any damage except in extreme situations, even though it accumulates a lot of joules. 

The amount of current and length of time actually flowing through the various organs is usually the most important factor.


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## 65535

While it's true the lethal amount of current across the heart is very low, it takes a whole lot of voltage to allow that amount of current to flow through your heart.

Also joules is a measure of energy per time, the longer the time the more joules that flow, shorter time, less joules.

DC reacts to resistance more than AC, it takes a fair bit more DC volts to generate lethal current across the heart than it does AC. 24VAC will cause muscle contractions and some pretty bad feelings. It takes around 50VDC to feel the current flowing through your body.

You're more likely to die from higher voltages than low because they will travel farther through your body.

The actual chances of being electrocuted from normal circumstances of the average person is very low. It's hard to generate a short across your heart with current electrical codes. Sure you could burn yourself very badly, or cause muscle damage and soft tissue damage, but death is unlikely.


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## Mr Happy

It's worth bearing in mind that the electronic muscle stimulators with sticky pads that are sold to increase muscle tone use a pretty low voltage of about 12 V. If that voltage can cause muscle contractions around your body, it is not difficult to imagine it doing the same kind of thing to your heart muscle.

There is always a statistical variation where people are concerned. Just because 99 healthy people are unharmed by an electric shock, it does not prove that 1 person might not have an undiagnosed heart condition or genetic predisposition that causes a fatality.

To be safe, never get glib with electricity.


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## Apollo Cree

65535 said:


> While it's true the lethal amount of current across the heart is very low, it takes a whole lot of voltage to allow that amount of current to flow through your heart.



Not necessarily. Depends on where the voltage is applied, sweaty hands holding a metal wrench, in the bathtub using a telephone, patients connected to medical equipment, standing in the rain working on a "low voltage" piece of electrical equipment you though was turned off, accidentally cutting your finger on an electrical connector, etc. 



65535 said:


> Also joules is a measure of energy per time, the longer the time the more joules that flow, shorter time, less joules.



Joules is a measure of energy. Power is energy per unit time. 



65535 said:


> DC reacts to resistance more than AC



No. Period. End of story. See Ohm's Law. 



65535 said:


> it takes a fair bit more DC volts to generate lethal current across the heart than it does AC. 24VAC will cause muscle contractions and some pretty bad feelings. It takes around 50VDC to feel the current flowing through your body.



Current flows through things. Voltage is across things. AC current is more efficient than DC at causing ventricular fibrillation. The other potentially lethal effects of electrical current aren't necessarily more potent with AC than with DC. Long term low levels of DC current may have some effects that AC doesn't due to the effects of electrolysis, according to some posts in this thread. 



65535 said:


> You're more likely to die from higher voltages than low because they will travel farther through your body.



No. Current always flows all the way from one contact point through your body to the other contact point on your body. For instance from your hand touching a live circuit to your feet touching the ground. Higher voltage simply increases the amount of the current. (Except where the voltage is high enough to arc through the air and electrocute you from a conductor you're not touching.)



65535 said:


> The actual chances of being electrocuted from normal circumstances of the average person is very low. It's hard to generate a short across your heart with current electrical codes. Sure you could burn yourself very badly, or cause muscle damage and soft tissue damage, but death is unlikely.



Yes, chances of electrocution are low. 500 or so per year in the use. Thank goodness. 

No, it's not hard to get electrocuted. Work on a live house wiring circuit, and it's easy to electrocute yourself. The circuit breaker will not usually save you unless it's a GFCI breaker. Work on a consumer electronic device like a DVD player and it's easy to electrocute yourself. Once you remove the cover, there may well be exposed 115VAC terminals. 

It may be hard to electrocute yourself if you use all your devices the way they were designed to be used. Many CPF members are electrical tinkerers. Many of them aren't engineers or electricians or any other kind of trained electrical professional. Many of us do extreme electrical things with normally low-voltage equipment. Many of us will open things up and probe voltages with the power on. Want to bet you couldn't electrocute yourself if you open up a 35W HID spotlight even though the battery is only 12V? 

We should not get too callous about the dangers of electricity.

I'm an electrical engineer. I worked for years on electrical safety for consumer electronic equipment, factory equipment, international, US, and internal company safety standards, including the US National Electric Code. Not only do I understand the rules, I understand the underlying risks involved. 

When I give advice, I do go overboard on pointing out even the unlikely scenarios because I know someone will go out and try it.


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## Apollo Cree

Mr Happy said:


> Just because 99 healthy people are unharmed by an electric shock, it does not prove that 1 person might not have an undiagnosed heart condition or genetic predisposition that causes a fatality.



I suspect the the variation caused by how good a contact you make with the live circuit and ground makes more difference. Pressure, dampness, contact area, etc. makes an enormous difference. I've gotten just a glancing touch with household AC a number of time and gotten anything from just a tingle to a good kick.


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## Mr Happy

Apollo Cree said:


> I suspect the the variation caused by how good a contact you make with the live circuit and ground makes more difference. Pressure, dampness, contact area, etc. makes an enormous difference. I've gotten just a glancing touch with household AC a number of time and gotten anything from just a tingle to a good kick.


Right. What I'm trying to get across, though, is the problem with saying, "I once crossed a busy road safely without getting run over, therefore there is no danger involved in crossing a busy road."


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## Radiophile

I just saw this somewhere on TV - the reason 110v AC is so dangerous isn't the voltage or current, it's the frequency. The heart beats at the frequency the brain tells it to with electrical impulses. When house current goes through your heart it makes it try to beat at 60 hertz which is impossible, so it stops. Same thing with DC. Your heart matches the frequency and stops.

Screwing around with house current is stupid - period. Turn off the breakers or make sure you offer no path to ground for the current no matter how little.

I'm an amateur radio operator and tinkering with radios while they are operating is absolutely necessary for alignment or repair depending on the problem. The first thing I learned was to only have one hand in the radio at a time especially if it is a tube radio and high voltages are present. That applies even with solid state devices and insulated instruments.

A broadcast engineer friend of mine has an internal scar in his right index finger. He got it while aligning a CB for a friend at his home. His wife called his name for something and it distracted him. His finger wasn't visibly burnt on the outside, but it was fried inside. That happened 20 years ago, and he still feels the weather change from the internal scar in his finger.


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## liveforphysics

With my electric bikes, even soaking wet in the rain, I can touch the 48v battery sections indivdually and feel nothing more than a tickle. Doesn't feel much different than welding in the rain.

I try to avoid touching the dual 48v batteries when in series. 96vdc has never caused me any noticed physical damage, but it does trigger muscle action (at least in my body, doesn't seem to bother a buddy of mine). 

These batteries are 20Ah Lithium Polymer packs with 25C discharge ability, meaning they have the potental to continously dump 500amps. Fortunately, our skin (even wet) is fantastic protection from shocks at the lower voltages like <100v.

Go under the skin, and cause the current path to include the heart, and almost any amount of voltage or current can kill you.


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## n3eg

Apollo Cree said:


> Electronic circuitry in consumer electronics is commonly under 5V.


 
That's a bad thing to assume...

Most large flat panel monitors aren't lit by EL panels. I was surprised to draw an arc across two terminals in one, then found it was lit by two long fluorescent tubes with several hundred volts at 50kHz. Even EL panels run at 100 volts or more. The only true low voltage displays are LED displays, or ones backlit with LEDs like my modified TV at home.

I once worked on laser bar code scanners years ago - they ran on 10kV at 1ma. Yikes! Now they use 3 volt diodes...


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## SirJMD

liveforphysics said:


> With my electric bikes, even soaking wet in the rain, I can touch the 48v battery sections indivdually and feel nothing more than a tickle. Doesn't feel much different than welding in the rain.
> 
> I try to avoid touching the dual 48v batteries when in series. 96vdc has never caused me any noticed physical damage, but it does trigger muscle action (at least in my body, doesn't seem to bother a buddy of mine).
> 
> These batteries are 20Ah Lithium Polymer packs with 25C discharge ability, meaning they have the potental to continously dump 500amps. Fortunately, our skin (even wet) is fantastic protection from shocks at the lower voltages like <100v.
> 
> Go under the skin, and cause the current path to include the heart, and almost any amount of voltage or current can kill you.



Reason why it doesnt kill you, its due to the resistance of the skin.
Worth reading: http://www.darwinawards.com/darwin/darwin1999-50.html




65535 said:


> DC reacts to resistance more than AC



No, no, no and no. U = R * I. Electronics is no joke - please get your facts strait.




n3eg said:


> That's a bad thing to assume...
> 
> Most large flat panel monitors aren't lit by EL panels. I was surprised to draw an arc across two terminals in one, then found it was lit by two long fluorescent tubes with several hundred volts at 50kHz. Even EL panels run at 100 volts or more. The only true low voltage displays are LED displays, or ones backlit with LEDs like my modified TV at home.
> 
> I once worked on laser bar code scanners years ago - they ran on 10kV at 1ma. Yikes! Now they use 3 volt diodes...



Agree, Eventho alot of electronics uses logic 5V, its a bad idea to assume that all electronics uses max 5V. A switchmode powersupply can very well have around 500V at some areas.

I have a LED driver from a traffic light (230V supply), and it is doubling that input voltage to 460V. If i were to assume that it was 5V, i could easily have died.


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## recDNA

BatteryCharger said:


> Ok, theoretically in the right place you could probably be killed with 1ma @ 1v. More realistically speaking, when does DC voltage become dangerous to touch with your bare hands? 36v? 48? 75? How high does it need to be before you should be afraid of it like 120v sockets?
> 
> Just curious...


 
Ever get a little static shock? It's about 10,000 volts DC. Volts aren't the issue.


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## Magic Matt




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## 65535

n3eg said:


> That's a bad thing to assume...
> 
> Most large flat panel monitors aren't lit by EL panels. I was surprised to draw an arc across two terminals in one, then found it was lit by two long fluorescent tubes with several hundred volts at 50kHz. Even EL panels run at 100 volts or more. The only true low voltage displays are LED displays, or ones backlit with LEDs like my modified TV at home.
> 
> I once worked on laser bar code scanners years ago - they ran on 10kV at 1ma. Yikes! Now they use 3 volt diodes...




It's true, computers drop 120VAC to less than 24VDC.

TV's and other similar devices like microwaves use step up transformers which do generate high voltages, but most devices like video game systems immediately drop voltage down to under 30VAC to run.


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## PMM

Great thread 

Would seem that 60v is deemed safe working voltage per power tools 1/2 supply voltage per wire so if you get a short of bare wire touch you don't get the full 120v.

An interesting subject - I know friends in the past have been daft in making up a 90v dc bar out of 3v button cell computer batteries which people press there fingers against the end and get a shock from.

After reading all the above and looking back that could have been deadly.

Always been aware Current plays a massive part, I have had the pleasure of having my hair stand up thanks to the silly voltages of a Vandergraph Generator 

So a very enlightening thread


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## jtr1962

PMM said:


> An interesting subject - I know friends in the past have been daft in making up a 90v dc bar out of 3v button cell computer batteries which people press there fingers against the end and get a shock from.
> 
> After reading all the above and looking back that could have been deadly.


The key is to avoid giving the current a path through the heart. If you hold a stack of button cells between the fingers of one hand, you'll get a shock, but it's extremely unlikely to kill you as there is no path through the heart. However, those with rare heart conditions might still be in danger of being killed, perhaps from their heart stopping due to the stress.

Bottom line is never assume electricity is safe. I've gotten shocks from 5V electronics when test pins penetrated my skin. Thankfully I make sure that it can't happen on both hands at the same time. I've also gotten more than my share of high voltage shocks from vacuum fluorescent display power supplies. I needed to work on them live for troubleshooting purposes. Again, so long as I only touch the supply with one hand at a time it's highly unlikely I'll die, but I'll be the first to admit working with live high-voltage electronics is never a good idea. No matter how careful you are to avoid touching live components while probing, sooner or later it happens, even if you're highly experienced. I'm an EE and do this stuff for a living, yet I can attest to my share of "accidents". Thankfully I don't work with high-voltage step-up supplies on any kind of regular basis any more. I do regularly work with 120 VAC in my projects, but there's no reason for the device to be plugged in while I'm working on it. I also always make it a habit to discharge any high-voltage filter caps _several times_ before touching anything. Yes, several times. Sometimes electrolytic caps have a residual voltage memory. They might creep back up to 10 or 20 or more volts after you short them to bleed off the voltage. I also generally incorporate high value resistors to bleed off the voltage within several minutes tops just for added safety.


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## Mr Happy

PMM said:


> Would seem that 60v is deemed safe working voltage per power tools 1/2 supply voltage per wire so if you get a short of bare wire touch you don't get the full 120v.


This is not at all right and betrays a deep misunderstanding of electricity. The domestic electrical supply in the USA is 120 V at normal household outlets, and it remains 120 V when fed to power tools. If you get an electric shock from your drill or hedge trimmer it will potentially be 120 V. Countless people have been electrocuted while using power tools.


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## Apollo Cree

Originally Posted by *Apollo Cree* 

 
_ Electronic circuitry in consumer electronics is commonly under 5V._



n3eg said:


> That's a bad thing to assume...



Commonly, not always. The key word is "commonly".


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## PMM

Mr Happy said:


> This is not at all right and betrays a deep misunderstanding of electricity. The domestic electrical supply in the USA is 120 V at normal household outlets, and it remains 120 V when fed to power tools. If you get an electric shock from your drill or hedge trimmer it will potentially be 120 V. Countless people have been electrocuted while using power tools.




Sorry prob should explain myself better on reading I can see clarification on what I was trying to say.

... 1st I am in the UK

I am talking worksite / building industry i.e industrial 240v is stepped down in a 120v transformer and output center tapped voltage is 60v-0v-60v out to the tool not domestic home tools.

But I think our supply got dropped to 230 or even 220 as I see 55-0-55 commonly touted these days.


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## Mr Happy

PMM said:


> Sorry prob should explain myself better on reading I can see clarification on what I was trying to say.
> 
> ... 1st I am in the UK
> 
> I am talking worksite / building industry i.e industrial 240v is stepped down in a 120v transformer and output center tapped voltage is 60v-0v-60v out to the tool not domestic home tools.
> 
> But I think our supply got dropped to 230 or even 220 as I see 55-0-55 commonly touted these days.


Ah. Helpful hint: you can fill in your location in your user profile so people know where you are.

I don't think the supply did get dropped to 230 V as the whole country is connected by the National Grid and it would be too difficult to change all the installed equipment. What they did is declare the European voltage standard to be 230 V "plus/minus 10%". Since 230 V + 10% = 253 V the UK's 240 V supply is within range and it "magically" became 230 V without actually changing anything.

(By the way, last time I was back there I saw how the banning of light bulbs has taken effect. That is so crazy...)


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## shadowjk

Mr Happy said:


> ack there I saw how the banning of light bulbs has taken effect. That is so crazy...)



I thought only the high power bulbs were banned, that shouldn't make a differe..... oh wait, sorry, this is CPF.


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## LuxLuthor

Theoretically then, there should be no problem touching a 9V transistor battery to your tongue. First up to the plate?


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## wyager

n3eg said:


> That's a bad thing to assume...
> 
> Most large flat panel monitors aren't lit by EL panels. I was surprised to draw an arc across two terminals in one, then found it was lit by two long fluorescent tubes with several hundred volts at 50kHz. Even EL panels run at 100 volts or more. The only true low voltage displays are LED displays, or ones backlit with LEDs like my modified TV at home.
> 
> I once worked on laser bar code scanners years ago - they ran on 10kV at 1ma. Yikes! Now they use 3 volt diodes...



Correct. They used a mini Helium-neon laser that runs in the tens of KVs range. Modern diodes almost never run above 6v, and those red diodes never above 3.3v. I've heard that 60V will cause electrical breakdown of dry skin (it becomes conductive). Wet skin, I don't know. I know the medical industry will use saltwater covered electrodes to significantly reduce the required voltage for muscle tests. Also, voltage will never kill you. You could have a billion volts pass through you and be fine, as long as the current is lower. and @PMM, I believe that the AC wave of a 120V household plug extends to 120V either direction, so it's really 240V. They call it 120 because if you used a full-wave rectifier it would come out to be 120. It goes from wire [email protected], wire [email protected] to wire [email protected], wire [email protected] Only 2 wires are needed.



LuxLuthor said:


> Theoretically then, there should be no problem touching a 9V transistor battery to your tongue. First up to the plate?


No. Your tongue is saturated with water, and has no insulating layer. Skin is a decent insulator.
will


----------



## Quension

wyager said:


> and @PMM, I believe that the AC wave of a 120V household plug extends to 120V either direction, so it's really 240V. They call it 120 because if you used a full-wave rectifier it would come out to be 120. It goes from wire [email protected], wire [email protected] to wire [email protected], wire [email protected] Only 2 wires are needed.



If you're speaking of US 120V household electricity, that isn't correct. It's hard to tell since you're directing your comment to someone in the UK who never mentioned 120V domestic, and I don't know where you are located...


----------



## Mr Happy

wyager said:


> Also, voltage will never kill you. You could have a billion volts pass through you and be fine, as long as the current is lower.


Volts don't pass through things, current passes through things. Volts sit across things. As it happens, if you had a billion volts sitting across you then several million amps would be flowing through you and you would be vaporized to atoms. 

Likewise, the AC mains is called 120 V because if you connect it to a resistor (or person) it behaves like 120 V (not 240 V). Even though the voltage is alternating, you can never get +120 and -120 at the same time (though you can get a peak voltage of 170 V).


----------



## TorchBoy

wyager said:


> I believe that the AC wave of a 120V household plug extends to 120V either direction, so it's really 240V. They call it 120 because if you used a full-wave rectifier it would come out to be 120. It goes from wire [email protected], wire [email protected] to wire [email protected], wire [email protected] Only 2 wires are needed.


It's called 120 V because that's (approximately) what its RMS value is. The presence of 240 V AC in the United States I think comes from two phases which have a potential difference of 240 V between them. In other words, they're 180° out of phase. Using only one phase, you have a potential difference between 120 V AC and 0 V AC (ground). Using the two phases your potential difference is between 120 V AC and -120 V AC (not sure about the notation for negative AC).

A 9V battery on the tongue feels fuzzy, and might even be painful if applied to the tip of the tongue. Several of them in series can cause instant burns (as seen on YouTube).


----------



## Apollo Cree

wyager said:


> You could have a billion volts pass through you and be fine, as long as the current is lower.



NO. 

You could have a device that generates 100,000 volts open circuit that is current limited to, for instance, 1 mA. When you touch it, the voltage of the device will drop to some much lower voltage, perhaps 100V. 

You still need to be careful. Even if the device will only generate 1 mA long term, there will always be some capacitance in the device and things it's connected to. You will get a higher current when you first touch the device until the capacitance in the circuit is discharged. 

This capacitance is, for instance, the reason you get a small zap when you walk across a carpet and touch a doorknob. It's lethal voltage and current, but it only lasts a tiny fraction of a second. If you have a high voltage, low current device and it gets connected to something with a lot of capacitance, it CAN kill you. 



wyager said:


> I believe that the AC wave of a 120V household plug extends to 120V either direction, so it's really 240V. They call it 120 because if you used a full-wave rectifier it would come out to be 120. It goes from wire [email protected], wire [email protected] to wire [email protected], wire [email protected] Only 2 wires are needed.



NO!!! absolutely wrong. 

Common US household 120VAC wiring. 

You have a "neutral" wire connected to earth ground somewhere. It is always 0V. 

There is a "hot" wire. 

The voltage on the hot wire goes from 

0V 
+170V
0V
-170V

Over a time period of 1/60 second. 

If you run it through a full-wave rectifier, you will get a peak voltage of 170V. If you put a capacitor on it to get DC, you'll get 170V. 

It's called "120V" because that's the effective power in terms of a resistive load like an incandescent light bulb. The 120V refers to RMS (Root Mean Square) voltage. A 120V RMS AC voltage will provide the same amount of power to an incandescent bulb as a 120 V DC supply. 

The maximum voltage present on a US household circuit is 170V. Even though the voltage goes from +170 to -170, there's no way to connect yourself between the + and - parts of the waveform because they occur at different times. 

The common US household 240 VAC circuit has one neutral wire and two hot wires. The hot wires are 180 degrees out of phase. When hot wire A is at +170, hot wire B is at -170 V and vice versa. A 240V load will be wired between the two hot wires and will see + and - 340V peak voltage for an effective 240 VAC RMS voltage. 

If you touch one conductor of a standard US household 240 VAC circuit, you only see 120 VAC. You have to touch both hot wires to see 240 VAC.


----------



## Apollo Cree

Will those of you who don't understand electrical safety stop posting things like "XYZ" is safe? 



You are scaring this old engineer. 

It's one thing to post something incorrect and cause someone to buy the wrong flashlight. It's much worse to post incorrect info and kill someone.


----------



## Apollo Cree

LuxLuthor said:


> Theoretically then, there should be no problem touching a 9V transistor battery to your tongue. First up to the plate?



Been there, done that, got the T-shirt. 

I've always stopped doing it really quickly. I don't recommend the experience to anyone.


----------



## wyager

Quension said:


> If you're speaking of US 120V household electricity, that isn't correct. It's hard to tell since you're directing your comment to someone in the UK who never mentioned 120V domestic, and I don't know where you are located...


 
Sorry, I'm here in the USA. I have no idea about the UK system other than it's referred to as 240V.



Mr Happy said:


> Volts don't pass through things, current passes through things. Volts sit across things. As it happens, if you had a billion volts sitting across you then several million amps would be flowing through you and you would be vaporized to atoms.
> 
> Likewise, the AC mains is called 120 V because if you connect it to a resistor (or person) it behaves like 120 V (not 240 V). Even though the voltage is alternating, you can never get +120 and -120 at the same time (though you can get a peak voltage of 170 V).


 
Sorry, I gave a pretty bad explanation. If you had a billion volts "sitting across" you and a trillion ohm resistor in series then the amperage flowing through you should be safe.... less than 1mA, right? 1000000000/1000000000000 or more=.001 amps or less. And yeah, that's what I was trying to say about AC. It deviates so that the maximum voltage difference at any one time is <170V. I mistakenly wrote 120V.


Apollo Cree said:


> You still need to be careful. Even if the device will only generate 1 mA long term, there will always be some capacitance in the device and things it's connected to. You will get a higher current when you first touch the device until the capacitance in the circuit is discharged.


I was talking about a theoretical circuit-I'm aware that current will build up in something left alone. 


Apollo Cree said:


> Common US household 120VAC wiring.
> You have a "neutral" wire connected to earth ground somewhere. It is always 0V.
> There is a "hot" wire.
> The voltage on the hot wire goes from
> 0V
> +170V
> 0V
> -170V
> Over a time period of 1/60 second.
> If you run it through a full-wave rectifier, you will get a peak voltage of 170V. If you put a capacitor on it to get DC, you'll get 170V.
> It's called "120V" because that's the effective power in terms of a resistive load like an incandescent light bulb. The 120V refers to RMS (Root Mean Square) voltage. A 120V RMS AC voltage will provide the same amount of power to an incandescent bulb as a 120 V DC supply.
> The maximum voltage present on a US household circuit is 170V. Even though the voltage goes from +170 to -170, there's no way to connect yourself between the + and - parts of the waveform because they occur at different times.
> The common US household 240 VAC circuit has one neutral wire and two hot wires. The hot wires are 180 degrees out of phase. When hot wire A is at +170, hot wire B is at -170 V and vice versa. A 240V load will be wired between the two hot wires and will see + and - 340V peak voltage for an effective 240 VAC RMS voltage.
> If you touch one conductor of a standard US household 240 VAC circuit, you only see 120 VAC. You have to touch both hot wires to see 240 VAC.


Ah, I see. Sorry about that, I thought each wire alternated. I was aware, however, that you could not get the max positive and negative voltage at the same time. And I have to do some more research on RMS, it's still fuzzy for me but thanks for the help!



Apollo Cree said:


> Been there, done that, got the T-shirt.
> 
> I've always stopped doing it really quickly. I don't recommend the experience to anyone.



this is probably just me, but I love the experience! Probably a bad idea, but when I was a little kid I touched my tongue so much with the 9V battery I got a big brown mark in the shape of battery terminals and a line between them my parents didn't let me have any batteries for a while after that....

will


----------



## Apollo Cree

wyager said:


> Sorry, I gave a pretty bad explanation. If you had a billion volts "sitting across" you and a trillion ohm resistor in series then the amperage flowing through you should be safe.... less than 1mA, right? 1000000000/1000000000000 or more=.001 amps or less.



999,999,999 V would be sitting across the resistor and 1V would be sitting across you. 

If you're standing under a 384 KV power line, 383,999.9 volts are standing across the air between you and the powerline overhead and 0.1 V is standing between your head and your feet. 

Also, when you first reach up and touch the bare terminal on the billion volt resistor, the capacitance between the wire and the ground might carry enough joules to electrocute you. 

BTW, I'd like to see a billion volt 1 trillion ohm resistor. It would need to be 500 feet long or the voltage would simply arc between the terminals through the air.


----------



## wyager

Apollo Cree said:


> 999,999,999 V would be sitting across the resistor and 1V would be sitting across you.
> 
> If you're standing under a 384 KV power line, 383,999.9 volts are standing across the air between you and the powerline overhead and 0.1 V is standing between your head and your feet.
> 
> Also, when you first reach up and touch the bare terminal on the billion volt resistor, the capacitance between the wire and the ground might carry enough joules to electrocute you.
> 
> BTW, I'd like to see a billion volt 1 trillion ohm resistor. It would need to be 500 feet long or the voltage would simply arc between the terminals through the air.



My DMM says I'm about 18MΩ.... so 18MΩ + 1 Trillion Ω = 1000018000000, and 18000000/1000018000000=1.7999676 × 10^-5, and 1000000000V*1.7999676 × 10^-5=17999.676V. So there would still be significant voltage going through me, right? And once again, theoretical circuit... no capacitance, no dielectric breakdown. Also the resistor would have to be about 850M long in dry air, if there were dielectric breakdown.

will


----------



## jtr1962

Apollo Cree said:


> This capacitance is, for instance, the reason you get a small zap when you walk across a carpet and touch a doorknob. It's lethal voltage and current, but it only lasts a tiny fraction of a second. If you have a high voltage, low current device and it gets connected to something with a lot of capacitance, it CAN kill you.


That's exactly what I meant in an earlier post when I said it's really the total number of joules which kill you, not voltage or current. As a general rule, always be cautious around large capacitors, especially those charged to ~50 V or more ( although as I mentioned in my earlier post a large enough 12V capacitor can still store enough energy to kill you, given a low impedance path through your body ).


----------



## Apollo Cree

jtr1962 said:


> That's exactly what I meant in an earlier post when I said it's really the total number of joules which kill you, not voltage or current. As a general rule, always be cautious around large capacitors, especially those charged to ~50 V or more ( although as I mentioned in my earlier post a large enough 12V capacitor can still store enough energy to kill you, given a low impedance path through your body ).



No, "joules" is still absolutely wrong in terms of what it takes to kill you. High current, short duration without that many joules can kill you. Low current, long duration with lots of joules may be entirely safe.


----------



## griff

touch a 9v battery to your tongue


----------



## Apollo Cree

wyager said:


> My DMM says I'm about 18MΩ.... so 18MΩ + 1 Trillion Ω = 1000018000000, and 18000000/1000018000000=1.7999676 × 10^-5, and 1000000000V*1.7999676 × 10^-5=17999.676V. So there would still be significant voltage going through me, right? And once again, theoretical circuit... no capacitance, no dielectric breakdown. Also the resistor would have to be about 850M long in dry air, if there were dielectric breakdown.
> 
> will



There would be no voltage "going through" you. Voltage exists across you, not through you. It's like saying there's 6 feet of height "flowing though" your body from your head down to your feet. 

In theory, the current would only be 1 mA flowing through you. If your body's resistance was really 18MΩ, you'd have 18 KV across your body, but only 1 mA so you wouldn't be electrocuted. 

However, if you body resistance was really 18MΩ, you wouldn't be able to electrocute yourself with household current. 120 V would only produce 0.006 mA and you probably wouldn't even feel it. 

Measuring the body's resistance is very tricky. It varies so widely with varying conditions that measuring it with a home voltmeter is almost meaningless. I think the human body is also not really a "pure" resistor and the measured resistance varies with the applied voltage.


----------



## TorchBoy

jtr1962 said:


> I said it's really the total number of joules which kill you, not voltage or current.


Everything I can recall on the matter says it *is* current that kills, and maybe not very much of it. I'll side with Apollo Cree on that.


----------



## Mr Happy

TorchBoy said:


> Everything I can recall on the matter says it *is* current that kills, and maybe not very much of it. I'll side with Apollo Cree on that.


It's actually sort of complicated. Your heart is like an oscillator, with waves of electricity flowing across it in a regular cycle. Think of it like a pendulum. If you push a pendulum at the right time during its swing it just swings a bit higher and carries on. But if you push a pendulum at the wrong time you can stop it dead so it's hardly swinging at all.

So it is with your heart. If an electric shock from outside the body hits the heart at the right time in the cycle nothing may happen. But if it hits the heart at the wrong time it can kick the heart out of its normal cycle, leaving it effectively stopped. Whether or not this happens depends on when the shock arrives and how long it lasts for.

The defibrillators that they have in hospitals deliver a carefully measured shock to the heart of a size and duration to kick it back into its normal rhythm. It's like giving a pendulum a big push in the right direction to get it swinging again.

But if there is one thing to take away from this thread, it is that electricity is dangerous. There are unpredictable circumstances that mean no voltage is guaranteed safe, and no current is guaranteed harmless. If you want to avoid the risk of death, don't get casual when working with electrical equipment, and don't assume that it won't happen to you.


----------



## LuxLuthor

wyager said:


> No. Your tongue is saturated with water, and has no insulating layer. Skin is a decent insulator.
> will



Oh come one. Don't be a wuss. Apollo Cree did it, and got a free t-shirt.


----------



## Apollo Cree

LuxLuthor said:


> Oh come one. Don't be a wuss. Apollo Cree did it, and got a free t-shirt.



Yup!








By the way, notice they got the apostrophe wrong. :devil:

Yes, it was free. I only had to pay $199 processing to Billy Mays.


----------



## jtr1962

Apollo Cree said:


> No, "joules" is still absolutely wrong in terms of what it takes to kill you. High current, short duration without that many joules can kill you. Low current, long duration with lots of joules may be entirely safe.


Yes, I agree there's a minimum threshold of current it takes to be harmful, no arguing that part. It's when you exceed this threshold which interests me. Due to the resistance of your body it takes a certain minimum voltage to make that current flow. But that's not the entire story. The lethal current needs to flow for a certain length of time in order for your body to respond to it. That's where the joules part comes in. Joules = volts x amperes x seconds. Now here's the caveat. I've often heard that 100 - 200 joule figure I mentioned earlier thrown around but that's for a shock on dry skin. I would imagine the number of joules would be quite a bit lower if you had probes inside the body near the heart. The current needed would still be the same, but there would be fewer volts needed to initiate that current.

Anyway, strictly speaking it's the product of current and time which kills, provided the current exceeds a minimum threshold. A huge current for picoseconds or nanoseconds may do no harm, whereas 100 mA for half a second could kill. But in any case you need voltage to make that current flow, and that's where joules come in. An interesting fact not mentioned here is once voltage is high enough to exceed the breakdown threshold of the skin's insulating layer, the effect is essentially the same as if you had stuck probes directly into soft tissue. IIRC it takes kilovolts for that to happen.

I'm only pushing the issue because most of my teachers told me you needed a certain amount of total energy to be lethal, provided the current exceeded a certain threshold ( i.e. microamps might be unpleasant, but can flow through you all day long with little harm unless applied directly to the heart ). There was/is disagreement on exactly what this amount is, but not on the concept. Even medical defibrillator equipment is calibrated in joules. I would imagine there's a reason for that.

I found some interesting reading on the subject.

According to one post in the link, internal tissue has a resistance of 50 or so ohms, while skin is anywhere from 500 to 20,000 ohms. 20 to 150 _microamps_ is considered medically lethal if currents are induced directly in the body. For probes stuck in soft tissue all you need then is 1mV to 7.5 mV. Yes, a single AA Eneloop can kill some people under ideal circumstances if probes are inserted directly into soft tissue near the heart. However, you usually need at least 100 mA to kill because the current isn't directly applied to the heart ( most of it ends up shunting around the heart ). Therefore, in more realistic circumstances where shock occurs through the skin, you would need at least 50 volts ( assuming minimum skin resistance ). If you want to go with the highest resistance numbers, then you end up with 2 kV. But the lethal current still needs to be flowing for a certain amount of time for the heart to respond to it. No idea exactly what this length of time is. Note that 50V*100 mA = 5 watts, and 2kV*100 mA = 200 watts. That's the range of _power_ it takes to electrocute. If we assume that earlier 100 joule figure has some basis in reality, then that's 0.5 seconds of 200 watts. From a biological standpoint that probably makes sense. I'd guess the minimum length of time current needs to flow in order to be fatal would be on the order of some tenths of seconds. This of course assumes that the current isn't high enough to cause burns or other secondary effects. I would imagine thousands of amperes flowing from a lightning strike can kill by burning tissue, even if the strike doesn't last long enough to stop the heart via defibrillation ( although the heart still stops in this instance by virtue of being physically damaged by burns ).

*As always, take all my numbers above with a huge grain of salt. While 50 volts has been thrown around this thread as the threshold when DC voltages start to become dangerous, in reality you can be killed with much lower voltages under ideal circumstances.*

Oh, and I did the 9V battery trick myself. Not pleasant at all. Do I get a T-shirt also? :nana:


----------



## Mr Happy

I'm not entirely sure the minimum length of time is necessarily as long as tenths of seconds either. For instance it is often mentioned that if you play with electrostatic generators like Wimshurst machines you should be extremely careful if you ever attach a Leyden jar to its output terminals. The risk of death from accidentally touching such a charged capacitor is evidently quite high, even though I think it might take mere microseconds to discharge through your body.


----------



## jtr1962

Mr Happy said:


> I'm not entirely sure the minimum length of time is necessarily as long as tenths of seconds either. For instance it is often mentioned that if you play with electrostatic generators like Wimshurst machines you should be extremely careful if you ever attach a Leyden jar to its output terminals. The risk of death from accidentally touching such a charged capacitor is evidently quite high, even though I think it might take mere microseconds to discharge through your body.


I did mention that in the case of currents high enough to cause severe burns there is no minimum safe time. The minimum time reference has to do with death via defibrillation. Also, the whole issue is a lot more complex than it seems on the surface. DC and low-frequency AC voltages are more dangerous volt for volt than high-frequency AC which doesn't penetrate as far into the body. That's the principal stun guns work on. They generate very high voltages, but at a high enough frequency so as not to penetrate to the heart.


----------



## Nos

german industial norm for "self systems" (safty extra low voltage) is

50V AC / and 120V DC ..... at this voltage you are still able to pull your hands/body away from the source, and your heardbeat wont be disturbed


----------



## Popsiclestix

Apollo Cree said:


> No, "joules" is still absolutely wrong in terms of what it takes to kill you. High current, short duration without that many joules can kill you. Low current, long duration with lots of joules may be entirely safe.



Joule is the correct term to use when talking about periods of contact that is shorter than the duration of your heartbeat (typically less than 300-1000 milliseconds).

Too see why this is, consider your average static electricity shock. The potential difference between you and the object is typically 5000V or more. Your body has come so close to the object that the you have exceeded the breakdown voltage of air, so electricity is literally jumping with no resistance from your body to the object. Furthermore this charge is being transferred very quickly. Since current is charge over time (and here the time factor is very small), it is a very large current that is transferred. And suprise, it doesn't kill you!

Also, when the doctors get ready to defibrillate you they yell (CHARGE TO 300)

That 300 is joules my friend.

However, in interest of our discussion of accidental contact with DC voltages, you are quite correct in the simplication that it is the current that kills. Simply because the periods of contact are long enough so that significant amounts of joules are delivered to the heart.


----------



## Popsiclestix

jtr1962 said:


> DC and low-frequency AC voltages are more dangerous volt for volt than high-frequency AC which doesn't penetrate as far into the body.



Be careful with that. People have gotten fried by getting too close to a radio tower, and they didn't even touch the antenna. :naughty:


----------



## HarryN

Hi, perhaps a slightly different approach to the question would be helpful.

In the US national electrical code, there is a specific voltage level for separation of "signal / extra low voltage" and "low voltage" (household type AC). The difference is quite significant from a code perspective, as "low voltage" items do not need nearly the level of inspection and compliance as "high voltage".

Further, in the US, "extra low voltage" and " low or high voltage" wires cannot be in the same cable or conduit. The logic behind this is that you don't want 200 volt power to short into a 5 volt signal line. At least in the past, the EU code was different and allowed this type of mixing of signal and power wires, as EU code is more "current" driven vs the US "voltage" driven approach.

Please note that I am not saying that a particular voltage is "safe or not", just that there is a "NEC code" for this.

I wish I remembered the exact transition level in the NEC code, but I think it is around 40 - 45 volts. Maybe someone remembers this number correctly?

BTW - I think the telephone voltages are allowed because there are special protections for those circuits, and they are grandfathered in, even though they are higher than the low voltage code.

edit - I did some more searching on this area, especially in wikipedia. Like most electrical code, it is about as clear as mud, but in some cases extra low voltage ratings ranged from 32 - 70 volts. The desire by the telcos to keep their nominal 48 volt supplies as ELV even though it is not really compliant with the concept is what makes this complicated.


----------



## HKJ

According to ISO60950-1, it is safe to touch 42.4 volt peak or 60 V DC in a SELV circuit.
This standard is used in both USA and Europe.


----------



## TorchBoy

Popsiclestix said:


> Your body has come so close to the object that the you have exceeded the breakdown voltage of air, so electricity is literally jumping with no resistance from your body to the object.


I know plasma has a very low resistance, especially compared to air, but I hadn't heard that it's a superconductor, with no resistance at all. Is that what you mean?



Popsiclestix said:


> Furthermore this charge is being transferred very quickly. Since current is charge over time (and here the time factor is very small), it is a very large current that is transferred.


Which is why those tiny little sparks look like really massive fat bolts of lightning. Or not. :thinking: That doesn't sound right either.


----------



## Popsiclestix

TorchBoy said:


> I know plasma has a very low resistance, especially compared to air, but I hadn't heard that it's a superconductor, with no resistance at all. Is that what you mean?



For all intents and purposes, you could pretty much model it as a superconductor. It is far below the resistance of any metal that I know.

See Comparison of plasma and gas phases: http://en.wikipedia.org/wiki/Plasma_%28physics%29

Conductivity is the inverse of resistivity.





> Which is why those tiny little sparks look like really massive fat bolts of lightning. Or not. :thinking: That doesn't sound right either.


They look like lightning bolts when you pump up the voltage to millions or billions of volts. It is the exact same concept, except the distance between the cloud and the ground allows the potentials to build much much higher.

See also Petawatt laser:

https://www.llnl.gov/str/MPerry.html 

10 mil A, only 680 joules delivered all because the time of contact is 440 femtoseconds. By comparison, 680 joules power many hotwires on this forum for 6-10 seconds.


----------



## Apollo Cree

Popsiclestix said:


> Joule is the correct term to use when talking about periods of contact that is shorter than the duration of your heartbeat (typically less than 300-1000 milliseconds).
> 
> Too see why this is, consider your average static electricity shock. The potential difference between you and the object is typically 5000V or more. Your body has come so close to the object that the you have exceeded the breakdown voltage of air, so electricity is literally jumping with no resistance from your body to the object. Furthermore this charge is being transferred very quickly. Since current is charge over time (and here the time factor is very small), it is a very large current that is transferred. And suprise, it doesn't kill you!
> 
> Also, when the doctors get ready to defibrillate you they yell (CHARGE TO 300)
> 
> That 300 is joules my friend.
> 
> However, in interest of our discussion of accidental contact with DC voltages, you are quite correct in the simplication that it is the current that kills. Simply because the periods of contact are long enough so that significant amounts of joules are delivered to the heart.



No!

Joules as a measure of lethality of electrocution is simply wrong. This IS my area of professional expertise. 

Anyone thinking limitations of "joules" will keep them safe is risking their lives.


----------



## Popsiclestix

Apollo Cree said:


> No!
> 
> Joules as a measure of lethality of electrocution is simply wrong. This IS my area of professional expertise.
> 
> Anyone thinking limitations of "joules" will keep them safe is risking their lives.



I'm new to this place, and im not sure of what you do for a profession. But an argument based on your qualifications alone is not much of an argument at all.

But just because you said it, I'm an Electrical Engineer working in Power Electronics (while my college area of expertise was embedded systems). So I have a bit of experience that ranges all the way from milliamps and volts to kiloamps and megavolts. Safety is a big part of what I do, and I regularly deal with electric devices in the ranges of 100V-10K V and several hundred A. 

While I'm not claiming to be an expert on human physiology, I've yet to see a situation where a small amount of energy delivered across dry skin has killed anyone, regardless of the amperage. 

Perhaps you can provide evidence to the contrary?


----------



## Apollo Cree

HKJ said:


> According to ISO60950-1, it is safe to touch 42.4 volt peak or 60 V DC in a SELF circuit.
> This standard is used in both USA and Europe.



What is a "SELF" circuit?

In general, you do use rules of thumb for what's a "safe" circuit. You consider 110 VAC household current as "dangerous." You don't normally think of 12 VDC car circuitry as dangerous. 

It's a useful generalization. I automatically use more caution when dealing with household 110 V wiring. I generally don't worry about electrical hazards on 12V car wiring. You can still hurt yourself with car wiring.



jtr1962 said:


> DC and low-frequency AC voltages are more dangerous volt for volt than high-frequency AC which doesn't penetrate as far into the body. That's the principal stun guns work on. They generate very high voltages, but at a high enough frequency so as not to penetrate to the heart.



A high enough frequency AC current will flow along the surface of a conductor instead of through the bulk of the conductor due to inductive effects. This is called the "skin" effect since the current travels on the "skin" of the conductor. I think in the human body, the "skin" effect is most important in frequencies much higher than 60 Hz. For instance, when working with radio equipment, RF burns can cause nasty burns along and just below the skin. 

The stun guns use a high open circuit voltage, but the current is limited to low levels. The high voltage will help the current reach your skin by, for instance, arcing through the clothing, or penetrating the surface resistance of dry skin, but the current available is low. Once a conduction path is established, the voltage will drop to whatever level is necessary to limit the current. This isn't necessarily any smart circuity, it can be as simple as a high value resistor to limit the current. 

I don't know if the skin effect is significant in stun gun safety. Stun guns tend to use a series of pulses, not a steady AC or DC current. A little reading indicated that some of them claim to design the waveforms involved to lock up the muscles without affecting the heart. 

By the way, be careful with any "stunning" devices. There are lots of idiots out there building them and publishing plans. I heard of a story recently where someone made an electric fence around his property by simply hooking up a 110 VAC household circuit to the fence wire. Someone died. The description of what the person "should have done" in the local media was entirely wrong and the "safe" way being discussed was just as dangerous as the "dangerous" way.


----------



## AnAppleSnail

You both seem to agree that small electrical incidents in ideal conditions do little, but working with more interesting situations removes the guarantee of safety. I'm not sure whether voltage or amps are more likely to stop my heart, so I follow my rule of thumb for most things:

If you wouldn't lick it, be careful with it.

It's served me well so far, as long as I know what 'careful' is. I'd suggest that once you start getting more volts than fingers, or more amps than hands, that more caution is needed. Not that volts and amperages below that are Safe, but that is probably going to ensure that I'm reasonably unlikely to die for hobby electronics.


----------



## Apollo Cree

Popsiclestix said:


> I'm new to this place, and im not sure of what you do for a profession. But an argument based on your qualifications alone is not much of an argument at all.
> 
> But just because you said it, I'm an Electrical Engineer working in Power Electronics (while my college area of expertise was embedded systems). So I have a bit of experience that ranges all the way from milliamps and volts to kiloamps and megavolts. Safety is a big part of what I do, and I regularly deal with electric devices in the ranges of 100V-10K V and several hundred A.
> 
> While I'm not claiming to be an expert on human physiology, I've yet to see a situation where a small amount of energy delivered across dry skin has killed anyone, regardless of the amperage.
> 
> Perhaps you can provide evidence to the contrary?



The guy claiming something is safe in conflict with generally accepted safety standards is the person who needs to provide proof. 

By the way, who said anything about dry skin? 

As I've posted before, I'm an electrical engineer who has designed equipment to meet US and international safety qualification of electrical equipment. I'm familiar with US and international safety standards, and with my company's internal safety standards, and the detailed medical reasons behind the standards.

Note that I'm not claiming "XYZ" is safe. I'm stating that "ABC" has been shown to be dangerous under some circumstances. Knowing that "ABC" can kill you is more important than knowing that "XYZ" didn't kill the last guy who did it.


----------



## HKJ

Apollo Cree said:


> What is a "SELF" circuit?



Misspelling of SELV circuit, that is a circuit where it is possible for normal users to touch parts with voltage.

There are more about it here, but the voltages from Wiki does not match the actual values in the standard.


----------



## Popsiclestix

Apollo Cree said:


> The guy claiming something is safe in conflict with generally accepted safety standards is the person who needs to provide proof.



One cannot prove anything. All we can do is disprove something. What you meant was probably evidence. Philosophy aside, there are two parts to my arguments. The first is to disprove your assertion that it is the the current kills, I will attempt this by providing a counterexample. Then I will attempt to explain why Joules kills (and let you try to disprove it)

Your post said that a high current, low duration, low joule contact can kill. I provided an example of a very common everyday occurrence (static electricity) where very high current, very low duration, and very low energy contact that has never killed anyone as far as I know. Thus I have disproven that *currents alone kill*.

Now for the Joules part.

There are two modes of action to cardiac failure. 

One is in which the electrical activity in the heart is disrupted due to application of an electrical signal which throws the electrical mechanisms of the heart out of sync. To overcome the signals output by the the heart's own natural pacemaker, you need to provide a signal which will overpower that. This requires expenditure of energy, and thus, joules.

The other mode of failure is through physical burning of the cardiac tissues. One can see this mode of damage, requires orders of magnitude of more energy than the previous case, and thus can be safely ignored when calculating the upper bounds for a "safe" energy exposure.

Furthermore, consider GFCI sockets (such as those found in your bathroom, which are designed to save lives). When you touch a fault, the current flows through you into the ground. Whether or not this GFCI device trips, the SAME amount of current flows through you at the same potentials. However, because the device has shortened the duration you are in contact with the current and voltage, it limited the amount of energy delivered to you. 

Joules = Watts * seconds = Voltage * Current * seconds.


----------



## TorchBoy

Popsiclestix said:


> I provided an example of a very common everyday occurrence (static electricity) where very high current, very low duration, and very low energy contact that has never killed anyone as far as I know. Thus I have disproven that *currents alone kill*.


Unless I missed something you didn't prove that static electricity involves high current. And... the "static" bit generally means it *doesn't* involve a flow of current. Just because a spark caused by static electricity looks like a little lightning bolt doesn't mean that little spark is a huge current flow like a lightning strike is.

Is this off topic?


----------



## jtr1962

TorchBoy said:


> Unless I missed something you didn't prove that static electricity involves high current. And... the "static" bit generally means it *doesn't* involve a flow of current. Just because a spark caused by static electricity looks like a little lightning bolt doesn't mean that little spark is a huge current flow like a lightning strike is.


Actually, "static electricity" is a bad term to describe the phenomenom, and yes, it does involve currents flowing. What really happens when you walk across a carpet wearing rubber shoes is that your body becomes charged ( acquires a potential ) relative to ground. In effect, your body acts as a capacitor. Here is a great description of the phenomenom. Basically, the body can acquire a potential in excess of 10kV, enough to jump right through the air. The typical model used for the human body is a 100 pF capacitor in series with a 1.5 kΩ resistor. Doing the math, that's a peak discharge current of 6.67 amps if the body acquires a potential of 10 kV. This is _well in excess_ of what is considered lethal. So is the 10 kV potential. The reason these static discharges aren't fatal is because the amount of energy involved is small. A 100 pF capacitor charged to 10 kV has a stored energy of only 0.005 joules. Moreover, the time involved for the discharge is very short ( ~3 RC time constants, or ~0.5 microseconds ). Interesting to note here is that the current and voltage levels, and hence the average power, during the discharge is enormous, on the order of tens of kilowatts, and yet the effects are merely unpleasant. Nevertheless, whenever I walk on carpet in stores which make the mistake of having carpets, I periodically discharge myself on any metal available. The longer I let the static charge build, the more unpleasant the inevitable discharge will eventually be.


----------



## Popsiclestix

TorchBoy said:


> Unless I missed something you didn't prove that static electricity involves high current. And... the "static" bit generally means it *doesn't* involve a flow of current. Just because a spark caused by static electricity looks like a little lightning bolt doesn't mean that little spark is a huge current flow like a lightning strike is.
> 
> Is this off topic?



Current is defined as the rate of flow of electric charges. If the electric charges did not flow across the gap, what caused the spark and what caused the potentials to be equalized?

Static electricity may be slightly-off topic, since it is getting into AC circuit analysis, which jtr explains quite well. I couldn't have said it better myself :thumbsup:


----------



## TorchBoy

jtr1962 said:


> Doing the math, that's a peak discharge current of 6.67 amps if the body acquires a potential of 10 kV.


6.67 amps from walking across a carpet, even if you scuff your feet, sounds like quite a lot. Would you mind showing your working?


----------



## Apollo Cree

Popsiclestix said:


> One cannot prove anything. All we can do is disprove something. What you meant was probably evidence....



OK, give me a high quality, low resistance, 10,000 microfarad capacitor and put the correct voltage on it to provide me with 50 Joules. I'll design a circuit with no power source other than the above capacitor and the appropriate means of connecting it to your body. This may involve probes that pierce your skin, or perhaps catheters. We'll experiment with it, I'll tweak the circuitry and the connections and see if we can cause your heart to stop. 

Are you ready to take me up on this challenge? 

Note: I'm not really willing to do this. 

If anyone else wants "proof" that xx joules are inherently safe, you can assume that it's true. You can use electrical devices assuming that it's true. If you do so, you're essentially volunteering to be the lab rat in the test above. 

Once again, I'm not saying 50 joules will kill you. I'm saying I've seen no recognized source claiming it WON'T kill you.


----------



## Apollo Cree

jtr1962 said:


> The typical model used for the human body is a 100 pF capacitor in series with a 1.5 kΩ resistor. Doing the math, that's a peak discharge current of 6.67 amps if the body acquires a potential of 10 kV.



There's probably an inductive component as well that may limit the peak current to a lower value. A microsecond time constant equates to frequency components in the MHz range, and the human body will behave as a distributed network of capacitors, inductors and resistors at those frequencies. 

Also, the skin effect would be very important in the microsecond time range and current flow would tend to concentrate on the outer surface of your body instead of flowing through the heart. 

I will agree that the duration of a current through the heart is relevant to the risk of electrocution. I remain unconvinced that joules are a useful way to quantify the risk.


----------



## Popsiclestix

TorchBoy said:


> 6.67 amps from walking across a carpet, even if you scuff your feet, sounds like quite a lot. Would you mind showing your working?



The easiest way to explain this would be that capacitors act like short-circuits at initial time conditions:

10000V / 1.5 kOhm = 6.67A





Apollo Cree said:


> OK, give me a high quality, low resistance, 10,000 microfarad capacitor and put the correct voltage on it to provide me with 50 Joules. I'll design a circuit with no power source other than the above capacitor and the appropriate means of connecting it to your body. This may involve probes that pierce your skin, or perhaps catheters. We'll experiment with it, I'll tweak the circuitry and the connections and see if we can cause your heart to stop.
> 
> Are you ready to take me up on this challenge?
> 
> Note: I'm not really willing to do this.
> 
> If anyone else wants "proof" that xx joules are inherently safe, you can assume that it's true. You can use electrical devices assuming that it's true. If you do so, you're essentially volunteering to be the lab rat in the test above.
> 
> Once again, I'm not saying 50 joules will kill you. I'm saying I've seen no recognized source claiming it WON'T kill you.



I'd be willing to take you up on that. But not at 50 joules if piercing the skin is involved. Doctors routinely use energies in that amount when performing open-heart surgery. I'd be willing to bet on 5 joules though.



Apollo Cree said:


> There's probably an inductive component as well that may limit the peak current to a lower value. A microsecond time constant equates to frequency components in the MHz range, and the human body will behave as a distributed network of capacitors, inductors and resistors at those frequencies.
> 
> Also, the skin effect would be very important in the microsecond time range and current flow would tend to concentrate on the outer surface of your body instead of flowing through the heart.



That model came out of the paper as (what I guess to be) a first-order approximation of the current flowing from you through the device from an ESD discharges. ESD discharges almost always happen in the microsecond range. If there was some inductance limiting the amount of current, electronic devices wouldn't be so easily destroyed. There's no significant inductance component here.

Most devices I've come across have these things called clamp diodes which shunt the current into the ground, saving the device.



Apollo Cree said:


> I will agree that the duration of a current through the heart is relevant to the risk of electrocution. I remain unconvinced that joules are a useful way to quantify the risk.



Consider other forms of trauma:

A car hitting you. It is the kinetic energy of the car that you absorbed that kills you. Usually measured in kJ.

How about radiation poisoning? You were close to an atomic bomb when it went off. The amount of radiation you received is measured in Sievert. This is J/kg. Again, a measure of energy.

What about burning yourself with hot water?
It is the thermal energy that disrupts your cell membranes. Which can be calculated from the temperature of the water, the specific heats involved and results in Joules.

What makes electricity different?


----------



## TorchBoy

Popsiclestix said:


> The easiest way to explain this would be that capacitors act like short-circuits at initial time conditions:
> 
> 10000V / 1.5 kOhm = 6.67A


Which post was that 1.5 kΩ mentioned in? And where did that come from?

And capacitors discharge according to a time constant, anyway.


----------



## Apollo Cree

Popsiclestix said:


> What makes electricity different?



If I have to explain why electricity is different from mechanical impact, thermal burns, or radiation poisoning, I'm probably wasting my time. Stimulating the electrical circuitry of the heart is different from burning flesh with heat, breaking bones with impact, or breaking down chemical bonds with radiation. 

We've been accidentally electrocuting people for over 100 years. The accepted safety standards usually refer to safe levels of current, not joules. 

If anyone wants to take the word of an anonymous internet poster that something is SAFE based on joules instead of amps, go ahead and put yourself at risk. Personally, I will advise people to go with NFPA, CDC, and other recognized experts before they decide something is safe.


----------



## ElectricRS93

JohnR66 said:


> I heard somewhere that low DC voltages that you may not notice can do electrolysis damage to cells. Supposedly, a person was killed while working on a vehicle with a 24 volt battery and some how manged to get pinned with part of his body in contact with one of the terminals and another to the body (ground) of the vehicle. He was trapped that way for a while (hours?) and died later due to the damage to his cells from the current.
> 
> I can't attest to the validity of this story, but it sounds plausible as electric currents passing through chemicals can break down the molecules or form new ones.




i am curious as to why in the world would someone want a 24V battery in their car, Hybrid vehicles have over 600V from my understanding...but to get a 24V battery set up, wouldn't you cross wire (attatch the positive to the negative and the negative to the positive of the two batterys making a 24V battery set up with two batteries) the batteries for more power such as for bigger speakers or something of that nature? and reguarding being pinned to the vehicle, why wouldnt someone have been around to notice something for hours? and farthermore, if one part of his body was pinned against a terminal of the battery (my guess is it would have to be the positive side because electricity flows in the Electron Theory of positive to load to negative) and another part of his body pinned to the body of the vehicle, creating a grounded electrical system using his body as a conductor, wouldnt the electricy pass thru his body without harm? i'm sorry for the long message, but i'm just curious and play with Electrical stuff, which is why my username is my nickname haha (no clue why i said that but okay)


----------



## Mr Happy

ElectricRS93 said:


> i am curious as to why in the world would someone want a 24V battery in their car, Hybrid vehicles have over 600V from my understanding...but to get a 24V battery set up, wouldn't you cross wire (attatch the positive to the negative and the negative to the positive of the two batterys making a 24V battery set up with two batteries) the batteries for more power such as for bigger speakers or something of that nature? and reguarding being pinned to the vehicle, why wouldnt someone have been around to notice something for hours? and farthermore, if one part of his body was pinned against a terminal of the battery (my guess is it would have to be the positive side because electricity flows in the Electron Theory of positive to load to negative) and another part of his body pinned to the body of the vehicle, creating a grounded electrical system using his body as a conductor, wouldnt the electricy pass thru his body without harm? i'm sorry for the long message, but i'm just curious and play with Electrical stuff, which is why my username is my nickname haha (no clue why i said that but okay)



Hi there.

Trucks and heavy vehicles have 24 V battery systems in most parts of the world outside North America as it allows more power for heavy loads like starter motors without needing such thick wires. NATO military vehicles also have 24 V batteries.

You could get 24 V by putting two 12 V batteries in series, but they make dedicated 24 V batteries for this purpose. It is safer and more reliable that way.

You can't normally assume electricity passes through your body without harm. Electricity is potentially dangerous and you should make every attempt to avoid it passing through your body. You don't have to feel an electric shock either. Some people have been seriously injured by swallowing button cell batteries when the current caused damage to their insides.


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## uk_caver

I thought with button cells it was often the chemicals that cause damage - I remember watching a TV programme where a doctor retrieved a cell a child had swallowed, and it was scary how corroded it had become in a fairly short time due to stomach acid action (maybe assisted by the cell causing some electrolysis).


----------



## BobBarker

Mr. Happy & ElectricRS93, Most large tractors (both highway and farm and aircraft) are still 24v systems in the U.S for exactly the reason that Mr. Happy gave.

Popsiclestix, defibrillators are DC devices. The do not try to "give the heart a little push" they in fact stop the heart completely. The idea is that the heart naturally wants to start beating at a certain rhythm. When the doctors stop a persons heart it interrupts the unnatural rhythm that it is beating at, and allows it to restart at a normal rate.
To answer your question as to why a doctor calls out the charge in joules is because you are right in that they are time based devices. They operate at 1000v +_1v and you adjust the charge based on the individual heart you are treating. And that is with an almost ideal external contact of over 8 square inches.

As to what is "safe" there is no truely "Right" answer. I will point you all to a reserch study done in the late '60s that may give you a better idea...
http://www.highvoltageconnection.com/articles/ElectricShockQuestions.htm

As with all things, there will be wide variations. That is the way of nature.

But my general guideline is this; If it is less than 36v DC (or 25v AC) I just practice general grounding safety (i.e. no metal jewelry and watching where i let wires or tools touch). Above 36v DC (25v AC)to around 70v DC (50v AC) I will wear rubber exam gloves (neoprene or nitrile). Above that I will only work on deactivated systems or will have "real" insulation work gloves on.
This is no guarantee that I won't get into trouble... But in my experience (making custom amplifiers) it has worked.

Also, just for general information... If you attach a full-wave bridge rectifier to your ac line (or transformer)... you WILL NOT get 120v RMS... You WILL get a rectified 169v (you multiply the input by 1.41 to get your output)


----------



## HKJ

BobBarker said:


> Also, just for general information... If you attach a full-wave bridge rectifier to your ac line (or transformer)... you WILL NOT get 120v RMS... You WILL get a rectified 169v (you multiply the input by 1.41 to get your output)



That is not correct, you will still have the exactly same RMS voltage minus the drop in the rectifier.
But add a capacitor and you get the a DC voltage with a RMS value close to the peak voltage of the original AC voltage (i.e. 1.41 times more voltage).


----------



## flashflood

I've definitely gotten a charge out of this thread, which has sparked several observations.

Resistance is futile.
Protect the family Joules.
Stay current to maximize your potential.
Watts your problem? More power to you!

Oy -- Thank God it's Faraday.
(I know, I know -- revolting!)


----------



## Shadowww

Just wondering, if I'd touch + and - of, let's say, 1000V DC with two of fingers on same hand, would the damage be limited to hand, or current will in some way go through whole body?
Sorry if this has been asked here, just too much posts to check all of them.


----------



## FRITZHID

99% of the time, you will only lose those fingers and the meat in between... but that 1% is still there looking to kill you. so basically, don't go around touching 1000v DC or AC systems unless you really don't value your life.


----------



## InHisName

flashflood said:


> I've definitely gotten a charge out of this thread, which has sparked several observations.
> 
> Resistance is futile.
> Protect the family Joules.
> Stay current to maximize your potential.
> Watts your problem? More power to you!
> 
> Oy -- Thank God it's Faraday.
> (I know, I know -- revolting!)


I appoint you to be in charge of the daily humor.
Balancing your d(humor)/dt will be a daily delicate balancing act.
I know you can do it, as long as you don't get overcharged.


----------



## ToyTank

The most damage would be done where the current leaves your body. You need to isolate yourself from any ground. Use isolated tools. People in my line of work die from -48V unfused power. One guy was sweaty and bald bent down to get his wrench was touching a bus and his head hit the frame(which is bonded to ground). That room still smells funky and I get the chills when I work in there. 

Amperage is what does the damage. 

Voltage is just the pressure behind the current. Voltage needs to be high enough and your body's resistance needs to be low enough for the current to flow through you. Without any amperage though it is just like a static shock from the carpet. High voltage but harmless. Just fun to tease the cat.


----------



## BVH

To add a theoretical example to Mr Happy's post. Lets say you had an old 350 Cubic Inch V-8 Chevy engine. A few decades ago, the starter motors for these engines pulled about 220 Amps at 12 Volts - OR 2,640 Watts of power were used to accomplish the work of turning over the engine if the starter was in good shape. Fairly thick cables are needed to move 220 Amps of current. IIRC something like #1's or aut or 2/0. In theory, (lots of other complications would occur) if you could find a 24 volt starter for that same engine, the starter would pull only 110 Amps at 24 Volts but you still are using the same 2,640 Watts of power (24 x's 110 = 2,640 Watts) to accomplish the same work. So higher voltage allows you to move less current while maintaining the same power level. Saves a lot of copper!

I wouldn't say that "Amperage is what does the damage". How many times have we touched a leaking 40KV spark plug wire only to quickly pull your arm away and have your elbow strike something and hurt like heck, if not draw blood.


----------



## e1sbaer

Back at school they taught me that from 40V, DC voltage can become deadly.


----------



## CKOD

http://www.darwinawards.com/darwin/darwin1999-50.html

anything 9v and over is potentially lethal in the right circumstances


----------



## flashflood

InHisName said:


> I appoint you to be in charge of the daily humor.
> Balancing your d(humor)/dt will be a daily delicate balancing act.
> I know you can do it, as long as you don't get overcharged.



I am honored by this induction.


----------



## ToyTank

BVH said:


> I wouldn't say that "Amperage is what does the damage". How many times have we touched a leaking 40KV spark plug wire only to quickly pull your arm away and have your elbow strike something and hurt like heck, if not draw blood.



The current itself did no damage, it was your nerves reacting causing the reflex. If that had been a 200 amp you'd lost your arm.

I stand by my statement that amperage is what does the damage, but yes you need enough voltage to overcome your resistance and complete the circuit.

Current = Voltage / resistance. I can't find what the average resistance of a human is though. I'm sure there would be variation among people and individuals sweaty and not etc. But if your resistance is higher than the voltage than current= Zero and there is no circuit. Wikipedia... electric shock

 A person can feel at least 1 mA (rms) of AC at 60 Hz, while at least 5 mA for DC. The current may, if it is high enough, cause tissue damage or fibrillation which leads to cardiac arrest. 60 mA of AC (rms, 60 Hz) or 300–500 mA of DC can cause fibrillation.[2]​[3]
​*
Body resistance*

The voltage necessary for electrocution depends on the current through the body and the duration of the current. Ohm's law states that the current drawn depends on the resistance of the body. The resistance of human skin varies from person to person and fluctuates between different times of day. The NIOSH states "Under dry conditions, the resistance offered by the human body may be as high as 100,000 Ohms. Wet or broken skin may drop the body's resistance to 1,000 Ohms," adding that "high-voltage electrical energy quickly breaks down human skin, reducing the human body's resistance to 500 Ohms."[9]​
The International Electrotechnical Commission gives the following values for the total body impedance of a hand to hand circuit for dry skin, large contact areas, 50 Hz AC currents (the columns contain the distribution of the impedance in the population percentile; for example at 100 V 50% of the population had an impedance of 1875Ω or less):[1
0]​

End Quote

conductors/fuses are rated for amperage not by voltage or wattage.

Voltage is rated by the insulating properties of the sheath. 

I always like the water analogy. For lower resistance and more current you need bigger ID pipes or wires. If you want to push the water/electricity further you need higher PSI/voltage and thicker walls on your pipe/wire. Over simplified but I still like it.
You need enough voltage to get the amperage to pass through your body for the amps to be dangerous


----------



## mto123

Apollo Cree said:


> Common US household 120VAC wiring.
> 
> You have a "neutral" wire connected to earth ground somewhere. It is always 0V.
> 
> There is a "hot" wire.
> 
> The voltage on the hot wire goes from
> 
> 0V
> +170V
> 0V
> -170V
> 
> Over a time period of 1/60 second.
> 
> If you run it through a full-wave rectifier, you will get a peak voltage of 170V. If you put a capacitor on it to get DC, you'll get 170V.
> 
> It's called "120V" because that's the effective power in terms of a resistive load like an incandescent light bulb. The 120V refers to RMS (Root Mean Square) voltage. A 120V RMS AC voltage will provide the same amount of power to an incandescent bulb as a 120 V DC supply.
> 
> The maximum voltage present on a US household circuit is 170V. Even though the voltage goes from +170 to -170, there's no way to connect yourself between the + and - parts of the waveform because they occur at different times.
> 
> The common US household 240 VAC circuit has one neutral wire and two hot wires. The hot wires are 180 degrees out of phase. When hot wire A is at +170, hot wire B is at -170 V and vice versa. A 240V load will be wired between the two hot wires and will see + and - 340V peak voltage for an effective 240 VAC RMS voltage.
> 
> If you touch one conductor of a standard US household 240 VAC circuit, you only see 120 VAC. You have to touch both hot wires to see 240 VAC.



I had to create an account on CPForums just so I could post this comment: THANK YOU Apollo Cree for your fantastic explanation of household AC in the US. I am very interested in electrical / electronics and I've done a lot of reading on the subject over the past few years. Your comment is one of the clearest descriptions of that specific topic I have ever seen. I don't know if you are an educator by profession, but you certainly have a gift for clear communication. Cheers!


----------



## Dr. Tweedbucket

Yeah, it's not the voltage, but the current across your heart that will kill you. Think about scuffing your feet on the carpet and touching a door knob .... that's 20,000 volts or so, but barely any current. I've touched a car 12V battery and can feel the tingle from 12VDC ... several amps roaming around inside. At school some kid had a electrical plug in the 115AC socket and two bare wires. A bunch of us took turns grabbing both wires and shocking the crap out of ourselves. I hung on for about 2 seconds and then let go, it was intense!!! So figure 115VAC and 15amps .... BUT that's AC voltage, it's DC that you have to worry about. 

We've all tested a 9VDC battery against our tongue, right? Gets a little zappy if it's a fresh alkaline!

Too, as someone mentioned, it depends on your body's chemistry and resistance, how sweaty your palms are and all that jazz. Stay safe!


----------



## uk_caver

The thing about *most* shocks from static is that they are *typically* of very little _energy.

_There might effectively be a high _instantaneous_ current due to the high voltage, but because there's very little energy stored in the first place, the current flows for next to no time, and no damage is done.
Find a system which could store static electricity of the same voltage but with much more energy stored, and you really wouldn't want to touch it.


----------



## magellan

My first electronics class instructor once said all it takes is 30V and 1/10th of an amp to kill you.

Doesn't seem like much with flashlight batteries that can put out 30A pulses these days.

I realize we're talking DC, but I had an uncle who was sorta famous for being able to hold onto the wires from a 120V AC wall outlet, although I don't think he was taking it across the chest, but I never witnessed it myself.


----------



## FRITZHID

In my youth, I accidentally took 12kv 30mA thru my hands. Scared the ever loving hell outta me. Left a bruise on my chest the size of a soft ball and 2 cauterized holes, one on each hand. I lived...... Idk how close to not living I was tho.


----------



## SemiMan

-----


----------



## FRITZHID

SemiMan said:


> In general things get "dangerous" at 30V wet, 60V dry.
> 
> 
> Fritz, you were probably pretty lucky, but you would not be the first to get hit with something in that order .... like old TV flybacks, etc.
> 
> It's not just current (or the voltage needed to generate that current given contact points, skin surface resistance, etc.) but what happens w.r.t. your heart. Usually it's extended contact stopping the heart and a significant jolt stopping the heart that causes electrocution. I used to work with electricians that said 277/347 was scarier than 480/600V at times, because 480/600V would seriously hurt .... likely when it threw you across the room. On the other hand, touch 277/347 and it is hard to let go. Often when you do touch something "high voltage", the muscle contractions are severe enough to "throw" you away from the contact. If you happened to "grab" it though, then likely you are toast, as you may not be able to open your hand.
> 
> I am often working around 277VAC, and 500VDC, but in a lab environment. Isolation transformers are a great tool ... rubber gloves too ... and Crocs.



That's exactly what I was. Lucky.

Was a NST. The juice went right thru the insulation into my pointer finger on my right hand and into my left palm. Still have the scars. 
Scariest thing was I couldn't let go. Was juiced for approx 10-15s..... Until I could get up enough.... "mental energy" to throw the wires down and at which point I literally jumped across the room, sweating and panting. Unplugged the NST, wrapped the cord around it and put it in the basement until I found a buyer for it. Lol
I've long since learned.


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## SemiMan

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## FRITZHID

SemiMan said:


> Was that a new neon sign transformer or one of the newer (past 20 years) electronic ones. The new ones are high frequency which ends up being not as dangerous.
> 
> Old magnetic ones are quite dangerous.



It was one of the really old, iron core, tar potted, ceramic terminal units that breaks several bones if you ever dropped it on your foot.

I have a few of the newer ones, they have a spark gap that triggers a safety system so are almost useless for Jacob's ladders and the like. 
I also have some of the old iron core as well.... Now that I know more about the safety aspects.


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## flashflood

When I was a kid, I learned how to make an electromagnet by winding bell wire around a nail and connecting the two ends to a dry cell battery. 30 years later, I thought it would be fun to show my son the same thing. I didn't have a dry cell battery, but I did have a nickel metal hydride D cell battery on hand. I figured I'd just hold the wires in place with my thumb and index finger. OUCH! Instantly boiled the skin. I hadn't accounted for the vastly lower internal resistance of NiMH. It's only 1.2 volts, but with almost no resistance, a huge amount of current.


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## snakebite

best post here! ASSume nothing and treat each device as lethal.under the right conditions your heart can be stopped by a watch battery.
and those that think led circuits are safe are in for a rude awakening!some led backlit tv's have 300+ volt dc strings.led bulbs in 240v areas can have 380v dc in them.same in 120 areas where the bulb uses a doubler.
some folks who should know better are the worst for doing stupid things.i was in a factory fixing a motor drive unit when the "i know everything"jerk of a maintainence man of the place argued with me when i told him a machine was live.he turned off the wrong switch and i had noticed that.he argued till i tripped the cabinet interlock switch so he could see it power up.
same guy poking his fingers in stuff asking "whats this do" while i am testing stuff.he pointed at an led in the cap bank.this is a 600v dc bus with huge caps.i told him that light means the cap bank is charged and he would be dead before he hit the floor if he touched it.he left.


Apollo Cree said:


> Will those of you who don't understand electrical safety stop posting things like "XYZ" is safe?
> 
> 
> 
> You are scaring this old engineer.
> 
> It's one thing to post something incorrect and cause someone to buy the wrong flashlight. It's much worse to post incorrect info and kill someone.


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## NoNotAgain

snakebite said:


> best post here! ASSume nothing and treat each device as lethal.under the right conditions your heart can be stopped by a watch battery.
> and those that think led circuits are safe are in for a rude awakening!some led backlit tv's have 300+ volt dc strings.led bulbs in 240v areas can have 380v dc in them.same in 120 areas where the bulb uses a doubler.
> some folks who should know better are the worst for doing stupid things.i was in a factory fixing a motor drive unit when the "i know everything"jerk of a maintainence man of the place argued with me when i told him a machine was live.he turned off the wrong switch and i had noticed that.he argued till i tripped the cabinet interlock switch so he could see it power up.
> same guy poking his fingers in stuff asking "whats this do" while i am testing stuff.he pointed at an led in the cap bank.this is a 600v dc bus with huge caps.i told him that light means the cap bank is charged and he would be dead before he hit the floor if he touched it.he left.


I guess mister Dillwadd never heard of Lock out Tag Out?

I wasn't involved in the Maintenance department, but worked in testing of materials and introduction into service. I never too for granted that a circuit was dead until I verified. Once verified, MY LOTO lock went on.

I had a shop worker attempt tightening a hydraulic line on a test machine at was energized with 4000 psi of Skydrol. The tube broke and injected his hand with an ounce or two of Skydrol after ripping a gaping hole in his hand.

Energy in any form can kill you when released quickly.


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## smokinbasser

This is factual info, I don't suggest trying it to satisfy your curiosity. While in the USAF a few biker buddies and I went on a country ride and hit several bars to sate our thirst, As time and liquid made it southward we had to make a whizz stop. We made sure no traffic or farmers were present and proceeded to drain our surplus on to the fence that kept the cattle in, (if you observe white insulators on the top wire DO NOT- I repeat DO NOT whizz on that fence!! it uses 48 or higher voltages to keep the cattle contained. we wetted our pants, boots and the other bikers when we made a connection with the power wire. one time will suffice for you to know to NEVER whizz on a farmers fence ever again.


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## Minimoog

Very useful info here. I will add that no matter how careful you are, others will not be. I was asked to repair a standard lamp that was flickering - so I unplugged it and started undoing it. The screw clamping the wires were loose in the inline cord switch and I was holding it in while clamping the screw down. Suddenly I get the most painful jolt - REALLY hurt from the 240V AC mains - my aunt had plugged in the light to help me see what I was doing. Not really helpful.

Also even when working on completely isolated items, a dangerous shock can still be had from capacitance holding a DC voltage in the wires. I was thrown from a stepladder from this - the jolt made my muscles contract.

Now I always remove the fuse from the consumer box and bleed by energizing the circuit with no power which is more than my electronics installation recommended but it is worthwhile.


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## LichtAn!

When measuring current draw of a flashlight, I often see people holding the probes with their bare hands. I'm assuming this is no problem then? I mean we have 16V battery packs nowadays and current draw over 30A possibly in some modded flashlights. So is the voltage just too low to cause any effect on humans?


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## sbj

Yes, exactly. Nothing serious should happen up to 60V.
But if you hold both contacts of a 9V battery to the tongue, you will already feel it.


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## LichtAn!

The reason is that the voltage is too low to pass the (dry) skin barrier, right?


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## broadgage

As many others have posted, there is no defined limit up to which DC voltage is safe and over which it is dangerous.
In practice, I feel that the following is a guide.

Up to 12 volts, almost no risk except under most improbable conditions.
Up to 24 volts, very low risk under normal conditions.

50 volts up to 120 volts, increasing risk, but generally survivable unless doing something very silly and being unlucky.

220 volts and up, not always fatal but undeniably dangerous.

A lot depends on the circumstances, other factors being equal a utility supply with a grounded neutral is a lot more dangerous than an isolated battery. To get a fatal shock from utility supply only needs one to touch the live wire, a fatal current can then pass through the body to earth. A high voltage battery is only dangerous if BOTH connections re touched at the same time.
A fixed battery with one pole grounded is a similar risk to a utility supply.

Whilst it is often said that "the current kills, not the voltage" This refers to the current THROUGH THE BODY and not he current in the circuit. A 12 volt lithium battery able to supply 100 amps is no more of a shock risk than a 12 volt zinc/carbon battery that can only supply a couple of amps.
(the lithium battery is a greater fire and explosion risk, but no worse as regards electric shock)

A tiny fraction of an amp through the body is likely fatal, it makes no difference to the electric shock risk if the circuit can supply one amp or hundreds of amps. A higher current circuit is more dangerous WRT burns and other damage.


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## tripplec

If you are getting zapped across you hand, arm etc its a discomfort with DC at best. As the voltage goes up its arcing intensity and starts to burn and cauterize size and flesh if maintained long enough.

IT DEPENDS !!!

Getting it from one are to the other where the current flows through you chest is where concern ramps up a lot since your heart can be affect.

I worked in various service and electrical repair trades and have been nailed with full powered AC voltage up to 550VAC on a 3 phase transformer leg, DC volts hard to say but around 10000VDC off corona arc rods in duplicating machines where current is not too high and my times lower around 400-500VDC across fingers. IMO AC is the worst as it disables you from moving since the body trembles from the AC current and you can't always move or pull yourself away.

To some degree you get used to it and shrug it off after working with live circuits of all type for many decades. 

Bottom line is across hands, finger and arms you might get some point of contact burns if the current was high enough as its searing like bug in bug killer until you remove yourself. THEN YOU HEAL for a while and you do. I have


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## wweiss

My electrician told me to always check 120v with one hand and keep the other behind your back...


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