RCD Tripping

[chalkmycue]

Hello

Can somebody please help me, i replaced a Socket Front today i isolated the circuit by turning of the MCB decided to strip the wires back on the socket at the same time as they were a bit tatty.

When i cut through the cables i tripped the RCD by presumably similating a Earth Fault what i dont understand is how does this happen when there is no current going to the circuit?

Upon completing the job turned the RCD & MCB back on and works fine.

Help Please

Thank you R.

 

[rebuke]

You would have created a temporary neutral to earth fault when you cut the cable.

Basically, an RCD works by comparing the amount of current flowing 'out' through the live, with the amount coming 'back' on the neutral. If they differ (normally because current is flowing to earth somehow) by more than a certain amount (normally 30mA in domestic settings) then it will trip.

As an MCB only isolates the live conductor, when you linked neutral to earth while cutting the cable, it provided a path for some current that was in the neutral from other circuits to flow to earth, that may have been better than the path through the RCD. The RCD therefore detected the imbalance, and tripped exactly as it is supposed to.

 

[chalkmycue]

Thanks for the reply that makes sense now like you say as the MCB only isolates Live & not neutral.

Thank You.
R.