Rcd tripping?? when MCB is switched on?

kevin1234 · 16 Nov 2009

Hello, i have a question for you all,

I have narrowed it down to 1MCB that everytime it is switched on, it trips the RCD,

I unplugged everything on the circuit what else could be causing that RCD to trip?

Thanks.

Spark123 · 16 Nov 2009

Fixed appliance fed via FCU, faulty wiring, damaged wiring, water ingress to name a few

kevin1234 · 16 Nov 2009

so can a live to earth or live to neutral or neutral to earth trip an RCD?

and for fixed wiring off appliances would you have to disconnect them to see what the problem may be as well?

wingcoax · 16 Nov 2009

What kind of CU do you have? If twin RCD then have you got the neutral in the wrong terminal bar. If single RCD then is the neutral in the terminals for the non RCD?

kevin1234 · 16 Nov 2009

wingcoax said:
What kind of CU do you have? If twin RCD then have you got the neutral in the wrong terminal bar. If single RCD then is the neutral in the terminals for the non RCD?

It is your normal split load CU, 1RCD, and the rest NON RCD protected

well the lady said it has never tripped before so if the Neutrals were connected wouldnt it off tripped ages ago?

Spark123 · 16 Nov 2009

Sounds like the fault is live to earth, assuming of course that this problem has just surfaced and no work has been carried out on the board.

A neutral to earth fault will only trip if there is enough potential difference between the two for enough current to flow, if the circuit isn't energized this may not show up.
Either way, an Insulation Resistance test will show the fault up.

kevin1234 · 16 Nov 2009

Spark123 said:
Sounds like the fault is live to earth, assuming of course that this problem has just surfaced and no work has been carried out on the board.

A neutral to earth fault will only trip if there is enough potential difference between the two for enough current to flow, if the circuit isn't energized this may not show up.
Either way, an Insulation Resistance test will show the fault up.

what makes you think its a live to earth fault? and yes no work has been done,

what you mean by the neutral to earth fault having enough potential?

what exactley does the RCD detect and the MCB detect?

Spark123 · 16 Nov 2009

It only occurs when your circuit is energized hence imo more likely a L-E fault.
N-E faults can trip randomly depending on your supply type, (TN-S, TN-CS or TT), in the case of TN-CS the RCD may trip when a heavy load is applied on another circuit. In TN-S you'd be lucky to get the RCD to hold in at all. I'd imagine TT will be similar to TN-S in that respect too.
An insulation resistance test will show what sort of fault you are looking for.
As for the MCB/RCD, see your other thread.

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Rcd tripping?? when MCB is switched on?

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