2 Way Lighting Circuit Tripping RCD


Postby jamesl » Wed Oct 06, 2010 12:50 pm

I have recently had an upgrade from a rewireable consumer unit to a twin rcd unit.Since the swap over if the landing lights are switched on the main rcd trips and will not reset.With no lamps in the pendants the rcd does not trip any ideas.Never had a problem when on the fused consumer unit.
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Postby ericmark » Wed Oct 06, 2010 4:28 pm

This is a common problem. Years ago it was common to use just two wires between the two "Two way switches" and the third wire required was borrowed from another switch. This was all well and good if both lights were fed from the same protective device. Although it did cause a mains hum with some equipment as is now no longer done. If the lights are switched on with one combination they work OK (i.e. both switches up) but in other combination (i.e. both switches down) if the upper and lower floors have been separated then it will trip.

There are two ways to cure. One is replace the twin cable with a triple cable between both switches. The second is to combine both lighting circuits onto one protective device. Today with energy saving lamps there is no need to separate the upper and lower lights as far as load goes. The only problem is safety as if anything trips the RCD you lose ALL lights. The easy way around this is to fit an emergency lamp so should there be a power cut of any kind you can still see to get down the stairs. In fact I think all houses should have that as in the case of a fire very likely the RCD will trip.

However it is up to the electrician who did the work. He has to issue the certificates to say it complies with BS7671:2008 and so he needs to do a risk assessment and decide with consultation with owners the best way around the problem.

Third cure would be to do away with two way switching of course but can't see how one can justify that with a risk assessment.

I use an emergency light in my house as all lights have always been on same circuit. The regulations just say it must be split into circuits and does not say exactly how.
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Postby BLAKEY1963 » Wed Oct 06, 2010 9:23 pm

[quote="jamesl"]I have recently had an upgrade from a rewireable consumer unit to a twin rcd unit.Since the swap over if the landing lights are switched on the main rcd trips and will not reset.With no lamps in the pendants the rcd does not trip any ideas.Never had a problem when on the fused consumer unit.[/quote]

JAMES1
Get your electrician bac 2 check that each lighting neutral is on the rite rcd neutral bar
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Postby ericmark » Tue Oct 12, 2010 10:24 am

I hate fitting RCD's to circuits which have been in use many years without them. There are so many things that can trip an RCD and 30ma is such a low figure it is so easy to miss something on PIR before fitting.

Although I have already written about the two way lighting problem there are of course other things which can go wrong.

One of the hardest I had to trace was a fridge/freezer fault where when it went onto de-frost cycle the faulty element caused the RCD to trip. Since the defrost cycle was not activated when I tested I missed it. I did finally realise the fault when I used a 10ma plug in RCD and rotated it around all appliances until the faulty one was found.

The problem is when found who pays for the time? If an electrician finds he has made an error then OK he does not charge. However if he finds someone else has made an error then of course he should not be expected to work for free.

If you paid for a PIR (periodic inspection report) before the electrician changed the consumer unit and he had missed incorrectly wired two way lighting then clearly his mistake and one would consider he would have to either correct free of charge or and some reduced rate. But had he found the fault you would have had to pay for correction so to reduce fee by cost of PIR could be considered as fair?

However if a PIR was not paid for then you can hardly blame him for what some other electrician has done in the past. Although what I described is a borrowed live it results in what is refereed to as a borrowed neutral which has never been allowed and breaks the health and safety rules.

When one tests a property one can't of course pull out every wire and inspect it and so of course one will never find all the faults. In the main we are looking for deterioration although we do test for looped ring mains and some other installation faults like swapped line and neutral but when I was taught to test we were never taught to test for the borrowed neutral fault which plagues so many houses when RCD's are added due to the old method of two way wiring. It was just not on our check list. Today I would guess it has been added to what is taught but one can still understand how it can be missed.

If one employs a gardener who hits a water pipe which is only 9" deep we do not blame gardener but the guy who only dug down 9" to bury pipe and did not cap it with tiles. It is the same for the electrician who fits a RCD consumer unit he has to assume that in the main cables are fitted correctly. OK in same way as gardener could have scanned the garden with metal detector first to locate and pipes or cables the electrician can test the installation for faults first. But just as metal detector can miss the plastic pipe so can the electrician miss faults which only appear when switches are arranged in one set way.

If as BLAKEY1963 suggests the electrician has made a mistake in which neutral bar he has used then it is of course his fault but it also could be the problem found with so many houses of as we went for bigger and better lights splitting the upper and lower floors to take the increased load without realising that the stairs two way lighting used the supply from both upper and lower radial supplies.

The blame is really down to lack of "as built" schematic diagrams showing how the house is wired. And these were very rarely produced in the 20th Century. Only as Part P came in did we see paper work raised detailing what has been done and even then it is rarely kept next to the consumer unit so reference and up-dates can be made. Time costs money and in the main all we want is for the items to work and we are not really interested in the paperwork and really don't want to pay for it.

Yes call back the electrician but be aware it may not be his fault. Please say how you get on.
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