Quote:
Originally posted by altis
For at least ten years, UK mains has been nominally 230VAC. However, the tolerances are asymetric. I can't remember the actual numbers but it's something like +10% and -6%. If you stick a meter in a socket it will often measure 240V. The nominal voltage has been changed as part of harmonisation with Europe - which is on 220V.
The phase wire carries the current from the substation. The neutral carries it back again. Because the neutral wire has a small resistance, the potential at the appliance not be the same as earth. Remember Ohm's Law (V = IR). This is why we need an extra earth wire.
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OK, time to get technical..............
The voltage in the UK is 240 Volts AC + or - 6% tolerance
Assuming the tolerance is the same in europe we still come into line with the euro voltages anyway, that has always been the case.
As for neutral having a smaller resistance than the phase conductor is incorrect, they are both identical, resistance in a circuit is based upon the length of the cable, so as the phase conductor is run alongside the neutral conductor, they must be identical in resistance values as far as socket outlets (ring main)are concerned, and as for the earth wire, if the installation is what we call a "PME" installation, the neutral will be at the same potential as the earth wire, which also means that ANY extraneous fixed metalwork within the confines of a dwelling need to be "bonded" to earth electrically. eg copper pipes, rsj,s if exposed etc, to eliminate the potential difference between the general earth and the electrical earth.
On an installation where the incoming earth is connected to the sheath of the incoming cable is relied upon for fault current situations, there is normally a slight difference in potential between neutral and earth at the incoming point at the mains intake position in the house, as the neutral is connected to earth back at the suppliers sub-station, (or on a circuit within the dwelling, because the phase conductor has further to run than the neutral i.e. "the switching of the circuit")
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Fault currents these days are normally dealt with by a residual current device (RCD), this piece of equipment operates via the mains voltage running through it via phase and neutral, whereby a flux running around a coil within the unit is equal to each pole i.e. phase and neutral, if the flux running around either pole is upset (and I don't mean it cries), then it automatically trips out, then you realise you have a problem.
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Apologies to the powers that be if this is going slightly off topic, but I needed to present my case as some people are guessing about most of their answers.