I've been wondering exactly what is meant by a poor quality mains voltage, specifically in places like Africa, where the mains voltage is considered worse than say the UK. Having done some research, one of the issues is the fact that power cuts off entirely, especially in the summer (presumably A/C usage/overheating of supply components?). Having said that, are there other factors to consider? For example, is the voltage supplied noisy with lots of small fluctuations, be it in amplitude or frequency, or is the only issue the fact that the it can cut out entirely?
Electronic – What exactly constitutes poor mains voltage
mainsnoise
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It is to standardise everything on a single fixed voltage. But in practice, mains voltage isn't 230V. It is standardised at 230V +10% -6%, or 216V to 253V. Here, mains voltage measures 249V, which is on the high end of the scale - but we're on a small farm road with not many other customers. Away from home I have measured it at 233V, due to resistive losses from greater demand. Countries have not changed the actual voltage - that would require changing all the generators or all the transformers in the country (at a substantial cost), merely, the allowable range has changed.
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:
(source: diyhowto.co.uk)
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.
Best Answer
Good mains voltage is a clean sinusoidal signal (or several phases thereof, as the case may be) free of dips, spikes and excessive distortion, whose frequency is stable over time, and whose amplitude stays within several percent (say, five) of the nominal value. Also, on good mains voltage, an appliance can draw the maximum rated current for the circuit it is plugged into, without experiencing a voltage drop which takes the voltage out of range.
Good mains voltage is a function of the ability of the grid, as well as the local circuit, to handle the load which is placed upon it.
It is also a function of the quality of the local wiring installation in a building, and also of the quality of the devices that are plugged in. Devices can degrade power quality seen by other devices by spewing noise into the wiring.