MCBs on.. But still power?

[enpassant]

Hello all,

I've searched the forum before asking but coming up empty handed??

I have a power cut from earlier. Live in a flat with a joined kitchen / living room and heard a bang from around the boiler area, then darkness.

Checked the CB's and had 2 tripped which were "B circuits" and lighting. The main 100A MCB was still in, but no power anywhere in the flat.

I've put both CB's back in and still nothing. I have no CB's off now but still the flat is dead. Checked with neighbours as I thought maybe it was the supply and we were all down but they're good. So MCB in and subsequent CB's in but still nothing... Can anyone please advise?

 

[Andrew B]

I bet you they have a primary distribution board feeding all the flats, and that it has a 30mA RCD or RCBO protecting the flat.
Something in the lighting circuit of your flat has a ground fault which is causing both your RCD and the upstream RCD at the primary distribution board to trip.

Next time they reset the supply press your RCD test button, I bet it will cause the whole flat to go dead.
If that is the case all you need to do is find the ground fault in your lighting circuit.

 

[Strima]

I bet you they have a primary distribution board feeding all the flats, and that it has a 30mA RCD or RCBO protecting the flat.
Something in the lighting circuit of your flat has a ground fault which is causing both your RCD and the upstream RCD at the primary distribution board to trip.

Next time they reset the supply press your RCD test button, I bet it will cause the whole flat to go dead.
If that is the case all you need to do is find the ground fault in your lighting circuit.

That would be an extremely poor design if they had placed one 30mA RCD covering the whole flat supply in a locked cupboard.

 

[Andrew B]

Not really, electrician wouldnt know it will be locked...
And the electrician that did this CU assumed it was a standard residential install, he probably didn't have access too or knowledge off the upstream RCD at the main distribution panel.
Although I'd question how he could have tested his RCDs without finding out about the upstream one.
Or the primary distribution board was altered after the flats were built to add RCD protection - not realising the flats already had this covered.

This happens more than you would think. At my site we have two 30mA RCDs and a 10mA RCD on the socket. A test of the 10mA will trip all three!

 

[Strima]

Not really, electrician wouldnt know it will be locked...
And the electrician that did this CU assumed it was a standard residential install, he probably didn't have access too or knowledge off the upstream RCD at the main distribution panel.
Although I'd question how he could have tested his RCDs without finding out about the upstream one.
Or the primary distribution board was altered after the flats were built to add RCD protection - not realising the flats already had this covered.

This happens more than you would think. At my site we have two 30mA RCDs and a 10mA RCD on the socket. A test of the 10mA will trip all three!

Then the installing electrician must have either been blind or a complete ----wit. Basic electrical design takes discrimination between devices into consideration.

10mA RCD on the socket sounds very much like an American set up?

 

[Andrew B]

Yes American company, they haven't worked out we use a higher voltage so should expect higher leakage current!

 

[Strima]

Yes American company, they haven't worked out we use a higher voltage so should expect higher leakage current!

So they installed to American standards and not British standards?

 

[Andrew B]

Strima, out of interest what would you have done differently? If you turned up to fit a CU in a flat which included circuits needing protection - but suspected an upstream 30mA RCD in a locked area which you and the resident didn't have access too.

I suspect most would just get on with the job and fit the CU with RCD as they can't verify the upstream RCD. Doesn't make it unsafe just more likely to be inconvenient when it does trip - as the OP is finding out

 

[Andrew B]

Actually British standards do now cover 10mA RCDs. They are hard to find and expensive but they are there.

A few years ago however we were having to import American inline RCDs ...which were only rated for 115Vac ... We tried to explain how stupid that was. They would get quite warm under continued use.

 

[Strima]

I would have consulted the building owner to confirm the supply perameters and also the isolation point for the flat supply. Are you saying the spark swapped the flat DB over whilst the supply cables were live?

 

[Deleted member 26818]

I’ve used 10mA RCD sockets in the UK before now.