Fault finding head scratcher.

Quick drop in at a house on way home…. Customer couldn’t reset mcb for rfc.

Power off at board, outgoing cables disconnected… I was getting 200V between the neutral bar and the earth bar within the CU.

With the CU mainswitch on, I am getting a voltage on the disconnected live cables.
Have not disconnected the neutrals yet,

For now, I have left the cables disconnected, and the rest of the power on. Ran an extension lead to power the customers freezer. Will return to finish off tomorrow.

Looking at the picture below, the only other live circuit is the smoke detectors from the Wylex heating board to the right.
The rest of the heating board, and the all the off peak side of the teleswitch goes to redundant boards. It is so much more complicated than it should be.

What looks like a 3 phase head is a neutral, a live fuse, and a looped fuse which powers an advertising sign for a neighbouring hotel. Not metered, apparently.





Cross connection somewhere introducing a voltage onto the neutrals? Can only be from the smoke detector circuit.

However, this doesn’t explain the sudden cause of the breaker tripping.
Expect to find a smoking rodent in this one.

Do you have voltage on both legs of the disconnected RFC

yep.... and it is an rfc... could continuity check end to end..... was just when the IR tester started squealing at me when i did L-E that i realised there was a voltage... not FULL supply voltage... thats actually sitting at 250V... but 200?

maybe appliances in circuit giving funny readings... I'm going to have to strip it down a bit to find the corresponding neutrals.

Yep you need to disconnect the whole circuit away from the CU and test again IR and ring continuity check if voltage is still on it first.

Almost certainly interconnected circuits. Switch off the other boards until it disappears then switch individual circuits from the offending boards. The tripping mcb is probably totally unrelated.

westward10 said:

Almost certainly interconnected circuits. Switch off the other boards until it disappears then switch individual circuits from the offending boards. The tripping mcb is probably totally unrelated.

Click to expand...

I would have said with an interconnection the OP would get full nominal voltage on the disconnected conductors. 200v or less could be lost neutral.

Found the cause of the tripping….



Just jointed them through… all clear now.

Removed the smoke detectors completely. They were due to be replaced in 2013!

They had battery ones that were operational anyway.