RCD's tripping with no current?

Clark3169 · Oct 4, 2012

Hi guys,

I am helping a friend with extending the ring main and adding some lights etc... in his new bedroom. I have put in a few new sockets & some two way lighting etc... I have isolated the circuits by turning off the appropriate MCB's at the CU which is split and both sides are protected by RCD's I cant kill the lot as the Fire Alarm activates and my mate doesn't know how to deactivate the alarm completely. This isn't a problem until I cut a cable and even though the circuit is off (MCB off) the RCD trips. I am guessing that as I have only killed line, the neutral and earth are still connected to the CU when I cut through it is causing an imbalance but I thought there needed to be a current flowing for the RCD to operate and to detect the imbalance. Surely neutral & earth are both at 0 if I have broke the circuit at the line by switching off the MCB? I don't understand how it all works, any help would be greatly appreciated.

Thanks in advance.

Deleted member 26818 · Oct 4, 2012

Current will take any path available.
So the current on the circuits you have not de-energized is flowing through the RCD along the line conductor, then along the neutral conductor to the neutral bar, then back through the RCD.
By cutting through both the neutral and the earth conductor at the same time, you have created another path for the current to flow.
Some of the current when it reaches the neutral bar, will then flow along the neutral conductor you are cutting, along the earth conductor to the earth bar and then to the cut out where it will rejoin the rest of the current that is in the neutral conductor.
As the earth conductor does not pass through the RCD, the RCD will not register that current, as such it registers that X amount of current flowed up the line conductor, but only X minus the amount that flowed down the earth conductor returned along the neutral. As those two values are diferent, the RCD will operate.

Clark3169 · Oct 4, 2012

Thats a great reply, thanks! I was half way there in my head but now its much clearer. Brilliant! Thanks. Is this the same reason that when I test for voltage on a circuit that I have turned off at the MCB I get the phase indication light on my tester but no voltage?