# Amperage and voltage... what's dangerous?



## fixorater (Apr 25, 2007)

I purchased a beam scanner from a surplus store recently. It requires 24v DC power. Since I work at a computer repair place I looked around for some sort of power adapter that puts out 24v. I found a power adapter that puts out 24v at 1.875 amps- to use it I'll have to splice the end of the cable and there is a potential for shock.

What I'm unclear on is what constitutes a dangerous level of amperage... The impression I have is that this shouldn't be dangerous since its such a low voltage- but I've been told that amperage is generally what kills you. 

So should I worry, would a shock just be a little jolt or something real bad?


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## robk (Apr 25, 2007)

24V isn't going to hurt you, regardless of the amperage. Because of the resistance of your skin (dry), it would require somewhere from 90 V and up to even feel it. Even at line voltage, 117VAC, it would take more than a casual brushing against a conductor to do any damage - maybe grabbing a ground like a water pipe and cutting into a hot wire with uninsulated cutters. Don't get so worried about it. It takes a lot more voltage than 24V to overcome your skin resistance, then the amperage comes into play. 
Rob


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## fixorater (Apr 25, 2007)

Thx for the quick response. Thats what I thought but wanted to confirm before my monkeying around forces the girlfriend call 911.


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## rdh226 (Apr 25, 2007)

Agree -- 24V is quite safe. You can probably get a decent spark with 2A available,
but virtually no danger to wetware forms like us, externally at least (skin being a
fairly poor conductor).

Disagree, 90V *is* dangerous. Generally speaking, DC power up to about 44-48V
is considered "safe", over that starts getting "dangerous" (that's where various
gummint bodies start requiring warning labels and the like warning idiots to keep
out). If you grab a 90V line, YOU WILL KNOW IT, and it will be unpleasant.

-RDH


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## Ragnarok (Apr 25, 2007)

Do your splicing and other work first, *THEN* plug it in. No need to work on it while it's live.


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## fixorater (Apr 26, 2007)

Yeah figured I would be working on it then plug it it- but didn't wanna do a crappy job splicing it and have the thing come apart while live and fry the cat.


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## mikeage (Apr 26, 2007)

Treat it as if it's dangerous.

Don't get me wrong... it's not.

If, however, you need to ask the question, then you don't yet have the experience necessary to decide these things (it's ok... takes time). Use the chance now to develop good habits.

I've worked on everything from 1.5V to 15kV, both live and disconnected. Why not practice doing things the right way, every time?

It won't take you all that long to apply principals of basic common sense (only one hand in the circuit at a time, never leave exposed terminals where they can be touched, etc), and the habits you'll develop will come in handy if you decide to work on something dangerous (house wiring, etc.)

But don't sweat it. You'd be hard pressed to hurt yourself with 24V (the biggest danger is a burn from a spark, not a shock).


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## robk (Apr 26, 2007)

rdh226 said:


> Agree -- 24V is quite safe. You can probably get a decent spark with 2A available,
> but virtually no danger to wetware forms like us, externally at least (skin being a
> fairly poor conductor).
> 
> ...



Bah, that's for sissies. I know electricians who test for live wires by touching them! Not to say you should do it, but the skin is not that conductive to low voltages.
Rob

PS- I've gotten shocks that literally threw me across the room. When I was a kid and building ham radio transmitters, I touched the plate cap on a 807 tube (about 450VDC) and I saw stars. But I'm still here. The rule that has always saved me was to keep one hand in your pocket when dealing with dangerous voltages - no direct path from one hand to the other through your heart!
Rob


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## gadget_lover (Apr 26, 2007)

Please, please be careful with advice about voltage. 110 volts IS dangerous even if you are not grounded by grabbing anything external. The second wire in a power cord IS the equivalent path back to ground.

Lower voltages can be dangerous, especially if you do something dumb like stripping wire with your teeth. Stop laughing. It's been done. 

It's the amps that kill. 20ma can be fatal. 60ma can be quickly fatal.

Dry skin has a fairly high resistance (1 - 2 million ohms) but sweat, blood, spit can drop it to the thousands. At that point 60 ma is not out of the question with 110 volts.

So just use caution, and always turn things off and unplug before you start working.


Daniel


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## Ashton (Apr 26, 2007)

This, to me, begs the question why is high-frequency pwoer safe then? toys like van-d-graph gennerators and Tesla Coils emit electicity that, IIRC literally passes over your skin without ever entering your body.


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## carbine15 (Apr 26, 2007)

yes and stun guns and TASERs use hundreds of thousands of volts to stun and not kill... High voltage is fun until you get more amps behind it; then it becomes deadly.


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## mikeage (Apr 26, 2007)

robk said:


> The rule that has always saved me was to keep one hand in your pocket when dealing with dangerous voltages - no direct path from one hand to the other through your heart!




Good advice -- but not quite enough to allow one to touch a live wire. If one's wearing thin shoes on a concrete (or metal!) floor, there's a pretty good path to ground...

That said, many accidents happen from too many hands in too small an area. Keeping one hand out of the way (behind one's back, pocket, etc) is an excellent safety precausion.


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## SenKat (Apr 26, 2007)

Like Rob - I have learned the hard way - have a few minor scars to prove my battle success as well as knowing I have learned my lesson ! I am cautious (mostly) and will undoubtably be zapped again and again, working on live power outlets at 115v, I have blown a few circuit breakers in my time, especially in my old house - that house was 50+ years old, and the circuits were nilly-willy all over the place as far as the breaker box was concerned. Thankfully the new owner is completely reworking the entire electrical system - as NOTHING was grounded - with the exception of two rooms in the entire house - that I personally banged the 8 foot grounding rod outside, into the ground, ran the wire to the socket.

Always just be careful, and always have someone around if you are working on live circuits - OR DEAD. Always assume it is live - even after you have certified the circuit is off - it'll save you quite a few tingles in the long run !


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## The_LED_Museum (Apr 26, 2007)

It takes at least 48 volts before you can receive an electric shock on reasonably dry skin that would be considered the least bit hazardous.
24 volts is safe to work with, regardless of the available current (amperage).


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## Enigmafied (Apr 26, 2007)

Amperage is the dangerous one.

Cattleprods put out ~50,000 volts. Causes pain on the skin, to get the animal moving. Very safe because the electricity moves between the 2 prods.

Stunguns are basically the same thing as a cattle prod, except the voltage is ~20,000 to 150,000 volts, and it puts out more amperage. Relatively safe, as the electricity also only moves between the 2 prods.

Tasers have the same power as a stungun, except the prods are not in a fixed position, making them dangerous, as the route the electricity will take depends on where the "hooks" land. If that path is across the heart, and there is enough mA, well, thats why we have people who have died from being shot with a taser.

The problem is the mA required to stop a person, is also enough to stop their heart.

Here is a description of what different amperages will do:

Less than 0.5 mA No Sensation
0.5 to 2 mA Threshold of perception
2 to 10 mA Muscular Contraction
5 to 25 mA Painful shock, inability to let go
over 25 mA Violent muscular contractions
50 to 200 mA Ventricular fibrillation
over 100 mA Paralysis of breathing

Now, with dry skin having a resistance of 500,000 ohms or so, which is not uncommon, at 208volts only about 0.4mA is expected to pass, which according to the above chart, should not give any sensation.

However with wet/dirty/sweaty/bloody hands, the skin's resistance could drop to 500ohms, which would let about 400mA to pass... and thats not good for anyone.


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## Stillphoto (Apr 26, 2007)

"5 to 25 mA Painful shock, inability to let go"

On film sets, we usually keep a "grip stick" nearby..also known as a 2x4 or a wooden broom stick. If one of the grips begins to get bbq'd you grab the stick and whack him/her out of contact with the circuit. That, or you can jackie chan them, but you must be flying in the air and kick with both legs...Significantly harder lol.


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## matrixshaman (Apr 26, 2007)

The frequency of the voltage is a BIG factor also in how dangerous it is. 60 cycle AC (standard U.S. house voltage frequency) is amoung the most deadly. And just remember voltage is the amount of force and current is the amount of volume so it takes some of both. Be safe - if you have ever had a shock you'll respect it. I think my first experience was from an unmarked electric fence. Not fun.


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## SenKat (Apr 26, 2007)

An unmarked electric fence ? That is just wrong....OW.


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## Hemlock Mike (Apr 26, 2007)

Robk --

You must be an old ham too - to remember the 807, 811 and 813 tubes !!
That was an old expression I got too - "keep one hand in your back pocket" !!
I too went flying but it was a 6146 tube cap !! 

Those stars are so bright  

Mike


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## David_Web (Apr 26, 2007)

Ashton said:


> This, to me, begs the question why is high-frequency pwoer safe then? toys like van-d-graph gennerators and Tesla Coils emit electicity that, IIRC literally passes over your skin without ever entering your body.



In this case it's not the voltage that is the problem but voltage-drop. It is no problem running 1000 volts through a led, just make sure you do something with the remaining 997 volts.
That is why a cable will survive but not you.
We are also talking about very low amperage here so the total energy will not get very high.
For high voltage it's actually better to be more conductive as the voltage-drop is a lot less than it would be if you where less conductive. Within reason that is.
But that require that there is something else to conduct the rest away, but there usually is when it comes to these "toys", mainly the ground (and sometimes air).

I am in no way certified or otherwise educated in high voltage so use these words with caution! (use high voltage with caution as well!!!)


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## picrthis (Apr 26, 2007)

fixorater said:


> I purchased a beam scanner from a surplus store recently. It requires 24v DC power. Since I work at a computer repair place I looked around for some sort of power adapter that puts out 24v. I found a power adapter that puts out 24v at 1.875 amps- to use it I'll have to splice the end of the cable and there is a potential for shock.
> 
> What I'm unclear on is what constitutes a dangerous level of amperage... The impression I have is that this shouldn't be dangerous since its such a low voltage- but I've been told that amperage is generally what kills you.
> 
> So should I worry, would a shock just be a little jolt or something real bad?


Not trying to be a smarty, BUT you do computer repair and don't know what dangerous amperage is? I'll assume then your not A+ certified.
You should learn something about voltage & amperage before you go monkeying inside a computer. :naughty:


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## fixorater (Apr 26, 2007)

At my shop we do mostly warranty repair and whole part swaps. IE... take out the whole logic board and put in another. Very little soldering or other monkeying around with PCB, ICs. Our interaction with CRTs is limited to very few instances- and in those cases we're well versed in how to discharge the anode cap and all. We rarely are exposed to anything that provides a dangerous voltage. So basically- very few times are there any dangers of electrocution.

I think you'll find that the vast majority of computer repair technicians were never given the basics on electrical engineering. Its funny- it seems most people think that repair technicians went to school for EE or CS, I've rarely met someone doing a repair job who has had much college training in a related field. Personally I studied art in school. The truth is most of those that are well versed in the theory of electronics and computer science would find such a tedious job such as computer repair demeaning- there is low pay and little intellectual challenge.

All that being said- I do want to learn more about electricity etc... though it has virtually no real life impact on how I do my job.


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## Tek465 (Apr 26, 2007)

Ashton said:


> This, to me, begs the question why is high-frequency pwoer safe then? toys like van-d-graph gennerators and Tesla Coils emit electicity that, IIRC literally passes over your skin without ever entering your body.


 
I would hardly call Tesla Coils and Van deGraaff generators toys.

Tesla coils can give you a nasty RF burn. And those are like lasers where the damage can occur before you even feel it. They ask you a bunch of questions about RF safety if you ever get a Ham License.

The Skin Effect and Another Reason Why Tesla Coils Don't Shock

Van deGraaff Generators operate on Static electricity and can give you a nice shock but no current (microamps). However, that changes if you start playing with Leyden jars and human chains that can store more current.

(VandeGraaff) Electrostatic Machine Safety

But I think the most dangerous part is not shock hazards but the secondary injuries. Like knee jerk reactions when you see parts arcing or bursting into flames. (Or exploding, like placing an electrolytic cap in backwards!!)

Play it safe and respect the electron!!


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## robk (Apr 26, 2007)

Hemlock Mike said:


> Robk --
> 
> You must be an old ham too - to remember the 807, 811 and 813 tubes !!
> That was an old expression I got too - "keep one hand in your back pocket" !!
> ...



Yup, used to visit "Radio Row" with my Dad - Vesey St, Courtland St downtown Manhattan as a kid, 45 years ago. It was torn down to build the WTC towers, some stores moved to Canal St. We built all our own stuff, I'm surprised anyone remembers the 807, or the cheaper 12V heater version 1625! I certainly remember the 811's (massive triode!) from the old standard Tesla coils back then - the plates used to run dull red, and the plate caps were at the full potential of the power supply - if I recall we used 5V4 or 5U4 rectifier tubes up to 700 V or so. What memories - not too many young'uns know that tubes were used for rectifiers - now easily replaced with cheap solid state diodes. 

I'm surprised I'm still alive with all the shocks I've gotten just on hobby stuff. I never experienced the real dangers of working on big power alternators, high voltage power lines, etc. Just all hobby stuff. I still keep one hand in a pocket and wear rubber soled sneakers when I work on stuff like HeNe lasers, Tesla coils, etc - that's probably why I'm still here!

It's good to hear someone else remembers the pre-solid state era, and has lived to tell about a brief encounter with a plate cap on a 6146 (power tetrode used in TV's, wasn't it? or did they use 6BG6? )

Rob


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## Hemlock Mike (Apr 26, 2007)

Rob --

I don't think the 6146 was ever used in a TV. Mostly transmitters. I ran my 811's at 1500 volts and bright RED !!! 800 watts PEP output. How about the old mecury vapor rectifier tubes ??? THere was blue light but only when warmed up. I think it was 816 and 836 MV tubes. 
There's a lot of HV knowledge lost for people getting into HV CO2 tubes for deep IR. 
Mike -- WA0BMP !!!


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## TorchBoy (Apr 26, 2007)

fixorater said:


> Yeah figured I would be working on it then plug it it- but didn't wanna do a crappy job splicing it and have the thing come apart while live and fry the cat.


I know a young lady who has (present tense, amazingly enough) a cat which decided to chew on the power cord of her hair dryer. 230V of flash and bang later, the moggy was left with one less of its nine lives, and a scorched mouth.



robk said:


> Bah, that's for sissies. I know electricians who test for live wires by touching them! Not to say you should do it, but the skin is not that conductive to low voltages.
> Rob


"Old electricians don't die - they just burn out."  

Why are people talking about "amperage" in this thread not "current"? Is that the fashion in the States?


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## Ragnarok (Apr 26, 2007)

Unless they are of the tube-driven (or other pure oscillator) type, Tesla coils can shock the daylights out of you if you are not careful, because their output is not pure RF The stunts you may have seen where someone takes the arc from a Tesla coil to a metal object held in the hand are done with the victim -er, experimenter _isolated from the ground_, standing on an insulated, elevated platform.

The RF from a Tesla coils is not the problem, though RF cannot be felt like 60 cycle AC , it can burn.

The top terminal of a Tesla coil has a very small capacitance, but at several hundred thousand volts potential it can store a significant amount of energy. Every time a spark from the coil hits that metal object in your hand, the terminal capacitance discharges into you, and it's not RF. This current feels more like a strong static electricity shock. If you are grounded it will be MUCH worse and may be fatal.

Most Tesla coils are driven by 60 cycle AC transformers and a spark gap. The spark gap usually fires one to several times per AC half-cycle, so there is a pulsed component present in the output that behaves more like low-frequency AC than RF. Also, 60 cycle current directly induced in the secondary coil from the primary circuit can show up in the output.

The insulated platform isolates the experimenter from the major effects of the low frequency/ capacitive currents.


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## Hemlock Mike (Apr 26, 2007)

Correct -- A cheap spark gap tesla coil can couple 60 Hz power line into the HV electrode. A true oscilator type is isolated by the transformer in the power supply. Beware the small cheap TC's marketed. Some use voltage multipliers for the spark. The high frequency is created by the LC ringing of the output circuit.

Mike


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## The_LED_Museum (Apr 26, 2007)

If the tesla coil is powered from the AC line via an isolation transformer, the chance of becoming part of the AC primary circuit is eliminated; though you can still receive a nasty shock from components on the "low voltage" side.


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## Aseras (Apr 26, 2007)

The_LED_Museum said:


> It takes at least 48 volts before you can receive an electric shock on reasonably dry skin that would be considered the least bit hazardous.
> 24 volts is safe to work with, regardless of the available current (amperage).



I've shocked myself from working on 6 volt golfcart batteries. then again they have 2000+ amps. Simply touched my leg to the frame while tightening a nut. Only on the first batteries in the series so only 6 volts, but still enough to have a bite.


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## gadget_lover (Apr 26, 2007)

I used to work at the phone company, where there's a lot of 48v wiring. One of my jobs was to solder the wires for live phone lines. I did this all day long. The normal voltage is 48v DC, but when the phone rings it's around 100v 20Hz AC. 

It was always a bummer to touch a ringing line when connecting a circuit. It stung like a bee sting. Then you'd start to sweat. That ensured the next one would hurt worse. Yuck. In many cases you were hurt much worse by jumping from the shock and hitting things with the back of your hand.

My electronics instructor told us that more people were injured jumping from the TV tube shock than were ever hurt by the shock itself. A wildly flailing screwdriver is dangerous. I still jump at any little noise when working on electrical components above 3.51 volts.

Daniel


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## SenKat (Apr 26, 2007)

hehehehehehehe - yeah - try stripping live phone wire with your teeth (don't say it - I DO know better, I swear !) as a call comes in ! OWWIE !

I only did that ONCE !  Okay...maybe twice - but that's it, I SWEAR !


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## Hemlock Mike (Apr 26, 2007)

I used to give my dog a 9V battery to play with !!! What a gasssss. That dog actually bit a hole through the steel case - she was so mad !!!

Mike


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## PlayboyJoeShmoe (Apr 26, 2007)

I didn't read the whole thread (slaps himself!) but here is what I know.

Pressure washers (oil heaters etc.) have a transformer that puts out over 10,000V but at milliamps. It will buzz the sh*t out of you but usually not kill you.

Of house current and up, all have enough amp to hurt/kill ya!

We use a lot of 24V in pressure washers and mostly because it's safer.


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## Illum (Apr 26, 2007)

10,000 volts with negletable current is basically a static shock
10,000 amps is another story
studies have shown if correctly applied 500 ma of current will stop your heart. I cant verify it and i dont want to try


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## TorchBoy (Apr 26, 2007)

Illum_the_nation said:


> studies have shown if correctly applied 500 ma of current will stop your heart.


Isn't it more like 5mA?


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## gadget_lover (Apr 27, 2007)

Yep, 5 ma AC will cause your heart to flutter. I'm not sure what the threshold is where it stops and stays that way after the current stops. No, I'm not about to experiment to find out.

Daniel


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## David_Web (Apr 27, 2007)

Stopping the heart is probably made during the t-wave. IIRC getting struck by lightning during that time would kill you. Even if you where not directly hit by it but got hit by the EM pulse. That is why you can survive getting hit by the lightning but your friend standing a few meter away might be killed.
I might be wrong as I faintly remember this from a TV-show long ago.


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