MCB trips out for no reason


Postby dr martin » Thu Jul 09, 2009 3:46 pm

I was replacing the cooker (with socket) switch unit last night having decorated the kitchen.

I had the cooker circuit mcb switched off but the rest of the mcbs were on. All the wires (incoming and feed) in the wall box were exposed and I had touched them all and they were not live (or I wouldn't be writing this today :D )

The main central mcb in the unit tripped out. Why?

My domestic supervisor (wife) has reminded me that we used to have the same problem occasionally with the old electric cooker when we switched too many things on too quickly but I just thought it was the cooker being ancient. This cooker has now gone and we have a new gas/electric one instead. All wires on the new cooker are new.
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Postby sparx » Thu Jul 09, 2009 8:39 pm

Hi, presume by main central MCB, you mean RCD, in which case if Neutral touches earthed metal or earth wire even with MCB for that circuit off the RCD will trip.
If other circuits are on, as the neutrals are common throughout you provide a parallel return path via earth which bypasses some current that would normally go via RCD, this causes it to (correctly) trip.
On of the many PITA's of rcd's!
regards SPARX
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Postby dr martin » Fri Jul 10, 2009 2:00 pm

RCD - that's the word I wanted!!!

Thank SPARX for the reassurance that it is doing nothing it shouldn't.

I can now get on with the rest of my life knowing it's not likely to catch fire immediately.

M
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