Electricity problem

[Broadback]

My electric is supplied by overhead cables, and power failures are not rare. The electric comes into the detached garage, though a mains breaker then fed to the house via an armoured cable, into another breaker box. When the power fails, every so often it will trip the mains breaker in the garage or the house. When it is in the house it is easy to reset, however as we have electric doors to the garage it is a bit of a pain having to manually open the garage doors, then go through the reset procedure to get them working again. What I would like to know is what is the likely cause of the mains trips operating, I don't know if it is when the electric cuts off or returns, as it usually happens overnight. Is there anything I can or should do to stop this PITA occurrence?

[Tim Watts]

On Sunday 09 June 2013 08:09 Broadback wrote in uk.d-i-y:

Photos of the breakers in question? (Please post to tinypic or somewhere and put a link here).

I *suspect* the "breaker" is a Type S RCD as would be required in a TT installation (which your's must be, as overhead supplies generally do not carry an earth wire).

In which case, RCDs can trip when the supply does weird things (spikes etc) as it might when it fails.

[Brian Gaff]

Sounds limke he needs a remote resetable breaker with battery back up. Never een such a device though. Brian

[Tim Watts]

On Sunday 09 June 2013 08:58 Brian Gaff wrote in uk.d-i-y:

Well, not in the domestic arena. I suspect there are industrial devices with full remote capability.

[Scott M]

In one REC area I've lived the whole system is company earth and for overhead supplies the poles are labelled accordingly (TN-C-S - think that's right) to show you can use it.

Scott

[Dave Liquorice]

Or possibly a voltage operated breaker, which would sort of explain trips on supplier outages.

Ish. In reasonably densly populated areas the distribution can be "5 wire", ie three phases plus neutral plus earth (TN-S) or "4 wire" (TN-C). On more isolated installation it'll be "2 wire" with the earth and neutral combined and the earth supplied by the DNO (TN-C-S).

TT is where the DNO does not supply an earth, the consumer does.

This does strike me as an "odd" installation we need to know where the head end and meter(s) are in relation to these whole supply breakers. Pictures will help immensly. Preferably just URLs direct to the images not some fancy front end.

[Tim Watts]

On Sunday 09 June 2013 11:26 Dave Liquorice wrote in uk.d-i-y:

True - I'd mentally assumed all those would have been replaced by now - but who knows...

[ARW]

It would be more likely that your electrical installation is the cause of the tripping.

[Broadback]

Thanks for the replies. I have photos here, not certain I have that correct as have never used this type of system before:

As regards other information. I am almost 100% certain that the earth is supplied, as there are three wires on the pole, and the middle one is not insulated. As regards the positioning the pole is adjacent to the garage. The meter is directly on the outside of the wall from the distribution box. If it is my electrical installation what checks are needed? The meter was moved at my request and the new distribution box needed installed by a reliable qualified electrition, the work was done

13 years ago.

[ARW]

Can you get photos of the cut out or any other switches or junction boxes? The sort of thing in the photos on this link

I am not asking you to remove any front covers from the cut outs:-)

[Tim Watts]

On Sunday 09 June 2013 16:59 ARW wrote in uk.d-i-y:

Yes - something seems missing from the photos.

The garage CU has a main isolator feeding:

1) Type B 20A MCB for the garage local supply. 2) Type C 63A MCB feeding the house as a distribution circuit.

The house CU has a 30mA main RCD feeding a number of MCBs for house circuits.

That all looks as expected - but something is missing and I suspect another RCD or voltage based breaker between the garage CU and the main supply.

[ARW]

A couple of extra questions? Does the garage have it's own CU - not just the Memera 2000 box that you have shown? A photo of what trips in the garage is also needed.

I suspect that you have RCDs in series and as you have a "whole house RCD" on the house CU I am still tempted to say that you are suffering from nuisance RCD trips and not power cuts.

HTH

[Broadback]

The RCDs which trip are the main ones (shown in picture) in the garage and the house. I will try to get some decent photos tomorrow, but alas I am no photographer.

[John Rumm]

Twas true at one time, although these days they have made many of the overhead supplies PME capable. Our place is wired for TT, but the actual supply is TN-C-S in spite of it being overhead.

Yup, could be an indication of a "sensitised" RCD.

If it is TT, and the upstream is not already a type S, then changing it for one would be a good move - they don't trip on transients.

[John Rumm]

Could you clarify which "main one" in the garage CU trips?

The Garage CU does not appear to have *any* RCD - just a normal main switch, and then two standard circuit breakers.

The 63A one that feeds the whole house, is notable in that it is a type C device, which means it will cope with a substantial switch on surge of current without tripping.

[Broadback]

I have now loaded additional photos. It is worth noting that we have several power fails a year, these we see during the day, in addition to those at night, which we only know as the clocks have reset. The garage distribution box never trips, only usually the main in and the house occasionally, on rare occasions both. None of them always trip on power fail and restore.

[Andy Burns]

Isn't the "saip" CU with 2xMCB and RCD in the garage immediately to the left of the Memera one, connected by a couple of inches of conduit?

[Broadback]

[Andy Burns]

Seems I must have looked just after broadback uploaded the extra photos...

[John Rumm]

ok (and sorry for labouring the point - but need to get the topology clear in my head)...

Your incoming cutout would appear to be a TN-C-S (PME) style (you can't tell directly since the same enclosure can be configured for TN-C-S or TN-S. However combined with the two wire drop from the overhead supply, it seems like a safe bet that the earth wire you see emerge is bonded to the combined neutral and earth from the pole.

Next I am guessing the feed goes into the Mem CU, and two submains come out. One to the small garage CU that is fitted with a normal 40A 30mA trip RCD, and the other feeds the house CU (a bigger Mem CU) which also has an RCD main switch.

So in the event of a transient, Either of those RCDs can trip - especially if there is already a fair amount of leakage on them. If the garage one goes, then it takes out the garage power and you can't open the door?

So far so good?

If all that makes sense, then add an extra MCB into the mem CU with the main switch. Use that to feed the garage door only. That way it will not trip off with the RCD.

You may find that replacing the RCD in the garage with a new one makes that less likely to nuisance trip on transients - some are better than others at rejecting these sources of apparent imbalance.

The "whole house" RCD is a deprecated way of doing things anyway, and you could certainly improve matters there as well, depending on what you want to spend.