weird RCD fault

[andy]

a few years ago we had a problem with the electrics and the RCD. the power went off so we thought there was a power cut, but the lights still worked. turns out the RCD had tripped. reset the RCD but then it wudnt re-set. turnd off all MCBS, reset RCD and turned MCBs on one at a time. the RCD didnt trip. later all the power went off (everything, but the RCD hadnt tripped). since it was a council house we called them and they sent a sparky round. meanwhile, (sill w/o electricity) the RCD switch off. weird since there was no power to make it trip. i re-set it (dunno why - i jnew there was no power) but within a few mins it was off again. it turned out that the live incomin cable goin the the isolator had came lose, givin us no power. once tightened we have had no problems.

the part i dont understand was why the RCD kept trippin when there wasnt a earth fault and why the power worked one minute then not the next. ne one had this kinda problem before?

 

[Damocles]

RCD (modern ones) measure the difference between current on the N and L lines. Maybe the L has been disconnected, but it is still possible in some circumstances for current to flow on the N. Just 30mA through N will trip it.

Which doesn't exactly explain where the 30mA was coming from.

 

[dingbat]

Some designs of RCD will not reset or hold on if there is no power.

 

[ban-all-sheds]

RCD spurs and sockets yes, but I've never heard of an active RCD in a CU - why would you ever want one of those?

 

[securespark]

I had a neutral/earth problem today. I reinstated power after fitting a S/L board, and one of the rings tripped. I switched off the affected circuit, but the RCD refused to reset until ALL the circuits on that side had been isolated.

Then the RCD remained on until the same ring was switched on again.

The odd thing was that the RCD tripped when I switched off the main isolator for the CU.

I found a faulty fridge, unpluged it and all was fine.

The cause? N/E reversal in the plug top............