Electronic – What makes mains voltage safe in homes


When I started learning about electrical engineering, one of the most important safety guidelines that I learned was that voltages over 36V can be dangerous. As my circuits professor put it, 120VAC will sting, and 240VAC will seriously hurt.

Now that I know more about these voltages and their dangers, I'm confused about mains electricity and how safe it is. I often see people (un)plugging power cables into outlets that are behind a desk or a couch without a clear view of what they're touching – sometimes with two hands to get a better grip, a definite no-no in my electrical labs at school. How dangerous is the risk of, for example, touching a finger across both the hot and neutral lines? Do the circuit breakers in a home really guard against this, or is 120/240VAC not quite as dangerous as I think?

Best Answer

Circuit breakers are not enough to protect life. Circuit breakers are there to stop the cable in the walls of your house melting and possibly catching fire – circuit breakers and fuses perform the function of stopping a fire (which of course is also very dangerous to life).

For direct contact with a live AC part, in the UK we have residual current devices (RCDs) – these "trip" the supply if the current taken down one of the AC wires is different to the current down the other by ~20mA:

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(source: diyhowto.co.uk)

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Clearly a fuse wouldn't be useful because the normal current of the devices attached to the AC will be tens or more amps. So if you have an appliance taking 10 amps and you touched one of the AC conductors you'd draw an earth current of maybe 20mA and this would "imbalance" the RCD and trip the supply.

As for touching both terminals simultaneously a different scenario has to be envisaged. I'm talking about AC power systems where one conductor (sometimes called neutral) is "earthy" i.e. it may have a voltage of only a couple of volts to earth – if you touched only this wire then it is very unlikely to trip the RCD BUT who cares – it's only a couple of volts put across your body at best and hardly any current will flow. If instead you touched both AC wires (live and neutral) then there will be an earth current taken from the live that is still significantly greater than the earth current from the neutral and the RCD trips.

Having said all of this ~20mA is still going to sting even if it is only for sub 100 milliseconds. Will it be lethal – possibly to people with heart complaints but will those folk be rummaging under a desk to blindly push a connector into a socket?

For AC systems that are "isolated" from earth, touching any one wire will barely be noticeable, but touching both will not trip an RCD and you'll be in serious danger – the current flow will be directly through the body and from conductor to conductor. Luckily these sorts of installations are not very common but certainly not unheard of. Losing the neutral-earth bond can cause this problem.