RCD - Why does this Happen?

Switching off the MCB will *not* isolate the neutral

If the house neutral is even at only a few mV with respect to earth - and it usually is - then any short will cause the RCD to trip. Indeed, this is one of the more common causes of RCD misbehaviour.

No such thing as voltage flow. Step away from the crude tester. Someone might get hurt.

It goes from the live of the "on" circuit, through an appliance to neutral on the "on" circuit. This is connected directly to the neutral of the "off" circuit, through an appliance on the "off" circuit into the live of the "off" circuit and into the earth of the "off" circuit and into the ground. There are two appliances worth of resistance, but it can be enough to trip an RCD.

It is more likely on a neutralearth leak on the "off" circuit, as there is no need to get through an appliance to get to the shorted live.

Christian.

"Christian McArdle" wrote

...........snip................

Thanks Christian

Just to put your mind at rest, the tester is a pukka mains voltage device with LED voltage indicators and self test feature. The word "crude" was used, perhaps incorrectly, to differentiate this from a high spec multi-meter.

Phil

If you were to check with a decent multimeter you would probably see a small voltage difference between your neutral and earth. Especially if you have a TN-S earth (i.e. typically provided via the sheath of the incoming cable).

To be expected - more so with the neutral to earth connection.

A voltage present on the neutral can also find a route through a connected appliance to the live when the MCB is open.

Check the archives (e.g. groups.google.com) before posting as this is a FAQ (even if it hasn't made the formal list)... :-)

Mathew