How earthing actually works in a fault

ulfr · May 13, 2015

Hi everyone,

Finally I got it.

1) In this circuit it can operate without the cpc, because there are no metal exposed parts nor in the ceiling rose, nor in the switch and a plastic is good insulator. The only fault that can occur is L-N, which can lead to an over current.

2) The cpc is needed because of regulations, and for a future use. If for example someone will install a metal spotlights or something like that, then it should be earthed.

3) Earth fault can occur if the cpc and the line are shortened. For example if one of the cables are melted, or a nail was banged into the wall shortening them both, or something like that.

telectrix · May 13, 2015

ulfr said:
Hi everyone,

Finally I got it.

1) In this circuit it can operate without the cpc, because there are no metal exposed parts nor in the ceiling rose, nor in the switch and a plastic is good insulator. The only fault that can occur is L-N, which can lead to an over current.

2) The cpc is needed because of regulations, and for a future use. If for example someone will install a metal spotlights or something like that, then it should be earthed.

3) Earth fault can occur if the cpc and the line are shortened. For example if one of the cables are melted, or a nail was banged into the wall shortening them both, or something like that.

you're getting there. :yes:. don't forget that an earth fault can also occur if the N and cpc are shorted or have a finite low en ough resistance between them. a 30mA RCD willl trip at a leakage resistance of 8000 ohms ( R=V/I, or R=240/0.03 = 8000. Where 0.03 is thr 30mA tripping threshold)).

ulfr · May 13, 2015

telectrix said:
you're getting there. :yes:. don't forget that an earth fault can also occur if the N and cpc are shorted or have a finite low en ough resistance between them. a 30mA RCD willl trip at a leakage resistance of 8000 ohms ( R=V/I, or R=240/0.03 = 8000. Where 0.03 is thr 30mA tripping threshold)).

Thanks.

Sure.. because current passes through N as well, it makes a continuity with L.

Thanks telectrix, much appreciated.

telectrix · May 13, 2015

The RCD monitors the L and N currents. if no leakage from either to E, then it holds. a difference between the L and N currents in excess of the tripping threshold, and it will let go. so a RCD will still work if there's no cpc. as long as there is another path to earth. e.g. through your body.