Electronic – Fault-current circuit breaker breaking on grounding neutral conductor

mainssafety

I just put some lamps on and got the fault-current circuit breaker triggered. While skinning the earth conductor the knife touched the neutral.

I was a bit puzzled: I thought that happens only under the following conditions:

  1. Live goes to ground and
  2. Live is actually live (the light switch was turned off).

Apparently those assumptions were wrong. So my questions:

  1. What kind of fault current can there be from neutral to ground?
  2. If neutral has tension relative to ground, just out of curiosity, how much tension is it usually?

I'm in Germany in case this matters.

Best Answer

As you are in Europe, the breaker is most likely an RCD ("Residual Current Device") triggered by approx 20ma imbalance between Live and Neutral.

As the Neutral wire has some impedance, it will show some small voltage due to currents returned from other circuits in the house back to the substation, even though the circuit you were working on was disconnected (It was, wasn't it? :-)

So a momentary short from neutral to earth would divert some neutral current to earth, introducing the imbalance which tripped the breaker.

It seems unlikely that you actually shorted a phase to earth, as you did check for "live" voltages. However as the neutral current could be quite large and the earth wire has a low impedance, that imbalance could have been several amps despite the low voltages involved. So it is worth taking the whole house supply off before doing this sort of work.