How variable DC power supplies prevent electrocution?

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I’ve played around with a power supplies in the past and each one seems to have a safety feature preventing electrocution even when touching something which has a current running through it. How does this work? How does the power supply know that’s it’s touching skin as opposed to another conductive material.

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What DC supplies are you talking about? If it is a low voltage supply electrocution is not possible if you do not price your body with electrodes.

50V and below is called Extra-low voltage (ELV) and has a very low risk of dangerous electric shocks. If you go down to the more common 12V system it is practically no risk.

The reason is your body resistance is so high that the current through it will me minimal. This means the energy transferred is minimal too and no damage is done.

A 12V car battery can deliver hundreds of amps but when you touch it you only get a small fraction of an amp. If you would attach it to an electrode close together and penetrate your skin or touch your tongue the resistance is a lot lower and the current can do damage or at least be felt,

Take a 9V battery, You only feel the current if you touch the poles of you lick on it not if you touch it with your skin.

It’s just a low voltage isolated power supply. There is simply not enough voltage to overcome your skin resistance. You would find it to sting if you licked a 5-12V power supply’s terminals (don’t do that, that’s putting more trust in the isolation quality of a 2$ made in China wonder than comfortable) much like a 9V battery does, and when fiddling with circuits on a breadboard I sometimes accidentally brush the stretched skin of my finger joint against a terminal with 12V on it which feels like I just poked something sharp.

Human skin varies in resistance from about 1kohm (under wet conditions) to 100kohm (under dry conditions). With DC voltage about 5mA is the threshold of sensation and 300-500mA can cause heart fibrillation (AC is much lower 1mA for sensation and 60-100mA for fibrillation). You can still be significantly harmed (internally burned) by current levels below fibrillation levels, you skin may break down under higher voltages decreasing your resistance, you may not have a good idea of your skin resistance, etc. In short DO not go experimenting with this…

Under best (dry) conditions, it would take V = IR = 0.005 * 100E3 = 500V to reach a current that you could feel. It would take V = 0.3 * 100E3 = 30000V to reach fibrillation.

Under wet conditions, it would take V = 0.005*1E3 = 5V for sensation level and V = 0.3 * 1E3 = 300V for fibrillation levels.

Normal benchtop supplies typically max out around 60-100V, but usually well under 50V which is relatively safe. They probably provide a few safety warnings, but do not provide specific protection against electrocution. In short, you should not be messing around with any power supply that can produce significant voltage and current unless you know what you’re doing.

Two common safety procedures that are recommend when dealing with high current/voltage supplies are:

1) Remove ALL jewelry before working. No fun to have you fingers burned because your ring touched two points in the circuit, or your necklace dangles onto something it shouldn’t.

2) Only use one hand when dealing with the circuit. This prevents creating an easy path for the electricity to flow across your heart (your heart is located between your two arms).

With DC if your body’s resistance is much much higher than the device it’s trying to power then very little current will actually go through your body. This is especially true for something like a 12V power supply. The amount of power being sent out is just too small to affect your body.

That does NOT mean you should go around touching electric circuits. But DC is safer in this regard as opposed to AC because in general your body isn’t going to be as good at conducting electricity as the wire.

This is NOT true for AC circuits. AC circuits are dangerous at almost any voltage and power level because we don’t measure resistance in the same way as we do in DC. It turns out that with AC your body can act as a capacitor or an inductor and carry current and power through your body pretty easily.