ECN Forum Forums General Topics of the Trade General Discussion Area Volts, Amps, & Ohms

pauluk,
Good example on the 12 volts. A car battery is 12 volts and capable of hundreds of amps, but the voltage is too low to draw it. Unless you lay a metal bar across it (0 resistence).

added the "low"

GETELECTRIC,
Yes, thats exactly what I'm saying. If your resistence was the same when touching either one, you will draw the same amount of current on either one. Amps are not just there, they are "drawn" depending on the resistence and voltage. As the resistence goes down, or voltage up, you will draw more current. Of course the 15 amp circuit will reach tripping much sooner than the 400 amp, because it will trip at 15 amps, and the 400 amp will trip at whatever it is fused at.

And, irrespective of the volts, if 500ma passes through you, somebody else will be doing the math, because you will be ....
.......dead!

So what kills voltage or current,

Volts gives you the jolts, but Amps puts out your lamps!!

Alan

so what is your answer

Your body's resistance, which can vary widely, is around 10Kohms. Much lower if you're in the bathtub taking a bath and touch a wire with wet hands. GFI's are set to trip at about 7ma of imbalance. 7ma of course will not trip a regular 15 amp circuit breaker or a 400 amp breaker. A major variable if you live or not is the path the current takes thru your body. One hand touching the 120V and the other hand touching a ground will cause the current to pass thru the area of your body where your heart is. That ain't good, might goof up the heart beat.

So the answer is: the fault current is the killer, but because of Ohm's law what voltage source you happen to touch while part of you is also touching another voltage source or ground will determine what fault current you'll pass thru you. A 12V car battery won't create enough current to harm you (but do be careful of metallic rings on fingers don't create a dead short from the + terminal to the car's frame and get very hot and burn your finger off).

Higher voltages will create higher fault currents thru your body given the same resistance. Also a poor enough contact that would let you survive a contact of 120V to ground can kill you at 277V.

If you touch 120V or even 277V and nothing else (because you are standing on insulation) you are an open circuit and no current flows. That's the reason for the ham radio safety rule of placing the other hand in your pocket while poking around high voltage in a radio transmitter. Not that you purposely touch high voltage, but if you slip with the voltmeter probe this precaution offers redundancy to prevent shock.

[This message has been edited by wa2ise (edited 04-13-2005).]

'So what kills voltage or current,"

Neither.

It's a combination of factors:
- length of time "hung up"
- amps passed through the body
- path current takes through body

Here's a 13 minute long video: http://www.electrician.com/safety/electricalsafety_01.htm

*****WARNING****
The video contains graphic images of the effect of electric on the body...not for the faint of heart ~ if you can't bear to watch the video, you have NO BUSINESS being IN BUSINESS.

"True, thats because the #12 circuit trips the circuit when it reaches 20 amps."

I have seem 20A breakers hold 20 -30 amps for a long long time.

Also during direct or partial shorts this not exactly true either..... a short, acts slightly different than an over-load, and has more to do with the AIC rating of the breaker/fuse and size of transformer, and distance to it via the conductors than simply "20A". The main reason you'll get that fireball/arch-flash effect on a service short, as opposed to a feeder or branch circuit is that circuit breaker or fuse has an AIC rating determined by many varying factors, and it should clear below those ratings safely. Exceed the ratings, as many people dont even think about them during most installs, and you could have a breaker or fuse that fails to clear a fault in time, explodes in the proccess, or worse not at all.

Most residential ditribution systems the idea is to limit to be 10,000 AIC or below, and likewise some comercial, but some 22,000, 30,000, and upwards for some types of industrial work.

Theres kind of an explaination here: http://www.currenttechnology.com/pdf/Current_Terminology.pdf
http://www.mikeholt.com/mojonewsarchive/EES-HTML/HTML/ElectricalCircuitBreakers~20030621.htm

And, I'm sure some of the other guys will have sometime else to add.

Its really the current thats gets you. But you have to have either enough voltage to push it, or a low enough resistence to allow the current to flow. All 3 relate to each other.
Again, a car battery is a good example. Go out to your car, touch the pos & neg terminals with your hands and you will not get shocked, yet a car battery is capable of putting out hundreds of amps. So why don't you get shocked? Because the voltage is to low. But on the other hand, if you lay a metal bar across the 2 terminals (don't do it), you will get high current flow through the metal bar (arking, heat, etc) because the resistence is almost zero in the metal bar. If you could gradually lower your resistence while touching the post, or gradually raise the battery voltage, you would eventually start feeling a tingle increase more and more.