Electrical query

[xrayspecs]

Whilst tiling my kitchen wall I removed from the wall an emersion heater switch. I removed the fuse for the circuit from the consumer unit but noticed when I checked the voltage that there was still a residual current running through the switch (about 5 volts). I turned the power off completely and obviously the residual curent went.

Firstly, why was there residual current in the switch after removing the circuit fuse from the consumer unit and secondly, would it have been safe to work on even with this current running through the switch.

[stanleyman]

This is probably an inducted, capacitive or resistive leakage induced voltage. If you disconnected the fuse on the live side, the neutral can still have a potential to earth. It is also possible to get this effect across live switch contacts that are only 3 mm apart. It might be worth having your earthing system checked and possibly upgraded to a positive multiple earth system (PME) if it is available and a resistance test.

Without knowing exactly why or how you got this residual voltage or even if you had the right fuse, I can only say it would be safer to switch the main power off and insulate the wires with a terminal block or tape before switching on again just in case.

[xrayspecs]

Stanleyman - thanks for that, I have arranged for an electrician to come and look over the wiring in the house and check out the problem.

[qapmoc]

Good answer except that PME is protective multiple earthing.
Xrayspecs, due to things like cable resistance and volt drop on the supply lines it is possible that the neutral cable at some distance from the supply transformer can have some voltage above the potential of the earth itself, this is not dangerous but cannot be ignored, for this reason if you have an RCD circuit breaker at your place and you take out the circuit fuse and then ground the neutral the RCD will trip. I always use double pole circuit breakers on everything for this reason.

[xrayspecs]

Thank you both for your answers, If the spark gets here before I get too old I'll go through all the points you have raised. (actually I'll ask him the same question and see if he concurs - that way I'll know if he is any good or not!) Once again, Thanks.

[stanleyman]

Thank you quapmoc for pointing out my deliberate tongue tied mistake. Just checking that you were on the ball!