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The more serious electrocution and shock hazards occur above the let go limits. 99% of the female population have an let go limit above 6 milliamps, with an average of 10.5 milliamps. 99% of the male population have an let go limit above 9 milliamps, with an average of 15.5 milliamps. Prolonged exposure to 60 Hz. currents greater than 18 milliamps, across the chest causes the diaphragm to contract which prevents breathing and causes the victim to suffocate. No data is available for females or children but suffocation is presumed to occur at a lower current level.

The frequency of the electrical current is as important as magnitude when evaluating electrocution and electrical shock injuries. Humans and animals are most susceptible to frequencies at 50 to 60 hertz. The internal frequency of the nerve signals controlling the heart is approximately 60 hertz. Ventricular fibrillation occurs when 60 hertz current from the electric shock interferes with the natural rhythm of the heart. The heart loses its ability to pump and death quickly follows. Ventricular fibrillation can occur at current levels as low as 30 milliamps for a two year old child and 60 milliamps for adults. Most adults will go into ventricular fibrillation at hand to hand currents below 100 milliamps (0.1 Amp).

Humans are able to withstand 10 times more current at DC and at 1000 hertz than at 50 or 60 Hz.. Electro-Surgical equipment operating above 100,000 Hertz pass high currents through the body with effect on the heart or breathing of a patient. All of the current limits referred to in this article are based on power line frequencies of 50 or 60 hertz.

July 30, 1888 - Brown and his assistant Dr. Fred Peterson of Columbia show experimental results at the School of Mines at Columbia University by administering a series of DC shocks to a large Newfoundland mix dog. By 1,000 volts DC, the dog is agonized but not killed. Finally, Brown finishes the off with a charge of 330 volts AC. On a follow-up demonstration, SPCA steps in and second dog becomes first creature ever publicly reprieved from execution by electrocution (although it was later killed at another demonstration).

RCD Residual Current Device
GFCIs Ground Fault Circuit Interrupters
Measures difference between current in phase and neutral leads. Turns off power QUICKLY when the difference exceeds some pre-set value. A simple way to do this is to wind a few turns on a toroid from each leg so that the net currents sum to zero when they are balanced.

When you get a return path to ground the go and return currents differ and the system shuts down. You get a short sharp kick and then the power is gone.

Defibrillation (or more properly here, fibrillation) depends not only on current but also on duration. RCDs are designed to disconnect power within a critical period. I think it's within 50mS (which fortunately makes sense in the context of 50Hz (or higher) mains frequencies) and corresponds to a critical period during the heart's cycle. In fibrillation the normal voltage patterns of the heart are disturbed and while it may still oscillate it may subsequently do so in a mode which is not conducive to pumping blood. The television (and real life) "300, joules, clear, zap" routine is designed to polarise the muscle, clear all oscillation patterns and let the oscillator try and start clean again. (Master Reset).

As you'll well appreciate if you've "field tested" one, an RCD does not stop you receiving a shock (the kick is not necessarily pleasant) but strictly limits the duration.

Many technicians are shocked as Management had the workbenches built of metal, for extreme durability, and for "convenience" decided to ground the entire bench with 12AWG wire - That way there's always a ground handy when you need one.

If you are near the underside of the bench, your leg is GUARANTEED to be grounded or at best *near* ground, if you are wearing heavy long pants. NOT recommended!

Get in the habit to not reach for *anything* that falls off of a workbench. Your brain isn't fast enough to evaluate whether it is safe to catch or not. Instead watch yourself and jump back if needed so it doesn't land on you. You may see a few expensive objects fall to the floor and break and wonder if you could have caught them. But that's a lot better than getting hurt by something that is too heavy, too hot, electrified, caustic, etc. to handle. The way to prevent things from falling to the floor is to not let them get close to the edge in the first place. One habit for keeping you from grabbing a falling charged object is to cram the unused hand into a rear pants pocket, then make a fist, this tends to stop most people from whipping that hand out in time to catch that falling HV wire or whatever.

Worst current paths are across the center of the chest - from left arm to right leg, etc. From arm to arm doesn't hit the heart AS badly as across the center of the chest, and of course neither's recommended

See also:

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