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Staying current on the state of electricity

By Marcelle Dibrell

Every now and then, the question will come up, which is the real danger: high voltage or high amperage?  And many of us have heard, “It’s not voltage that kills, it’s current.” But if that were true, how come we’ve all seen signs that say: “Danger – High Voltage!”? The real answer is that they are both dangerous.

Here’s a few interesting facts: Scuff your shoes on a rug and you can generate voltage of about 10,000 between yourself and the nearest door knob and get a decent shock. The voltage is huge but the fact that there is no continuous current prevented your swift demise. A 1-amp current can kill you, but you can hold onto the bare wires of a 1-amp, 3-volt flashlight without any trouble. The three volts simply aren’t enough to pass the current through your skin.

Electricity is often described by using a waterfall analogy where voltage is the height of the waterfall, and current is the amount of water flowing over it. If the waterfall is very high, but only a trickle is flowing, it is not be very dangerous to stand underneath. On the other hand, it would be very dangerous to stand under a raging waterfall (both voltage and current are high).

This analogy has its limits, however, because it neglects the relationship between voltage and current, a relationship called Ohm’s law. According to Ohm’s law, voltage is equal to current multiplied by resistance. So even if voltage is high, if the resistance is too, the current is limited.

Many of us know that current is what kills, but current can’t exist on its own — it needs something to get it moving, and that is what voltage is. It’s impossible to have current without voltage. It is possible to have high voltage with no current. People are harmed by high current because the current can stop the heart. While the amount of current needed to affect people depends on body mass, in general the following is true:

For AC current, 1 mA is barely noticeable as a shock, 16 mA is the maximum amps before you can’t let go, and at 100 mA, ventricular fibrillation begins. For direct current, the numbers are a little higher, but the effect is the same: it takes about 15-88 mA before you can’t let go and from 300-500 for ventricular fibrillation. 

High current stimulates muscles to extend and flex and when the flexing dominates, you can no longer let go. At 100 mA, if the current runs through the heart, the hearts begins to quiver and it leads to death. How long the current flows also has an effect. At 50 mA, if you manage to let go within 2 seconds, you will probably be OK. But if the current is 500 mA, then you have just 0.2 seconds to let go. 

Luckily, human skin is a relatively poor conductor. A body with dry skin may have a resistance of up to 100,000 ohms. But this resistance drops significantly with wet skin, or if the skin is cut. In general, however, it takes fairly high voltage to produce a dangerous current within our bodies.  So which is the danger, voltage or current? Both.

The general rule is that anything above about 40 volts and 10 mA may be hazardous. But if the voltage is much lower than 40V, even much higher currents may not be a problem.

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