Switched Emergency Bulkhead tripping RCD

I have a switched Emergency Bulkhead 500mm away from the CU, wired staight into the same MCB as the properties ground floor lighting circuit.

A normal light switch sits across the Live.
This light is not required to be an Emergency Light, it has been installed to provide light in the cupboard where the CU sits, in the case of a power failure.

When switching 'off' the live (to make the light come on) the RCD will sometimes trip.

There are 4 other Emergency Bulkheads throught the property spurred from the lighting loop circuits, with Test Switches - the RCD does not trip when these are isolated/tested from the test switch.

I have replaced the switch, but still get the tripping.

I had two RCD's and resetting one would often cause the other one to trip, no good reason why is should happen, but it did, and no faults found. Which is why new house is all RCBO.

ericmark said:

I had two RCD's and resetting one would often cause the other one to trip, no good reason why is should happen, but it did, and no faults found. Which is why new house is all RCBO.

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I hope that resetting one of your RCBOs doesn't make all the others trip

Kind Regards, John

My point is there is no reason why one RCD should trip when resetting the other, switching off all MCB's then resetting RCD worked OK, testing the RCD with RCD tester showed them as being within spec, and testing the installation with an installation tester also showed OK, but the early RCD fitted around 1992 would sit there for 12 months or more without tripping, then would have a bout of tripping maybe 10 times in a month, then again run 2 years or so without a trip.

Likely some spikes on the line caused it to trip, I think likely some one using a welding set near by, but never did find out why, would say likely over the years food loss in freezers when did not realise in time cost well over having RCBO's, but when fitted RCBO's were double modular width, and there was no consumer units made to take them in the UK. I fitted them because my 14 year old son had passed his RAE and with playing with ham radio, and wanted to protect him. At that time it was unusual to have RCD protection in domestic situation they were before two Wylex fuse boxes with fuses swapped for MCB's.

Mothers house 5 RCBO's in kitchen and two RCD's for rest of house never had one trip. This house RCBO's have tripped but for good reason with water leaks, which since only effected circuit tripped was not a problem no freezer supply lost.

However the only way is to test the circuit/s and RCD, and I realise now in my case I had never measured the leakage, my clamp on would only go down to 0.1 amp, fact that with 1/2 set with RCD tester it did not trip seems likely not a high back ground leakage, but never tested, the insulation tester uses DC so really no idea of total leakage. This is where the RCBO wins, I can have 15 mA per circuit leakage without a trip so 210 mA without tripping, (14 RCBO's) but only total of 30 mA across two RCD's, so RCD's are far more likely to trip for no good reason.

How new or old is the switch?

Although it seems strange I’ve found switches which have poor contacts and arc excessively when operated can sometimes cause RCDs to trip.

Do you have a spare switch you could try in place of the existing to rule it in or out?

The RCD is a MK 7860 63a unit, about 4 years old.
The Switch is 10+ years old, but had little use.

switches which have poor contacts and arc excessively when operated can sometimes cause RCDs to trip
yes I suspect this could be part of the problem, do light switches not all work the same way, could the copper contact have become scorched and the contact not be as immediate as you would wish ? I dont see any flickering - I did try a brand new switch - with no difference, still tripped. Stabbing at the switch makes it trip more regularly, as opposed to a more gentle flicking of the switch.

The same switch and light when wired through a plug top and extension lead didn't trip the RCD (which is how it was operated for several years - the spark who did the EICR noted this as one of the fails, and hence it is now wired direct to the CU (not through a plug top)- so only difference is 750mm less 1.5 T&E on the run, a plug top, and a 4 way 2mt extension (no SPD or anything fancy on it).

I had two RCD's and resetting one would often cause the other one to trip
Yes that happens to the other RCD in the CU.

I hope that resetting one of your RCBOs doesn't make all the others trip
Is there a reason if that is the case ?

Last job I specified all RCBO's for each circuit - to avoid this nuisance tripping - and a soggy freezer compartment.

frank999 said:

Last job I specified all RCBO's for each circuit - to avoid this nuisance tripping - and a soggy freezer compartment.

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That's fine if one has some sort of alarm on the freezer (which I do) but, if not, then failure of the supply to a freezer on a dedicated circuit (if that's what you're implying) can sometimes go unnoticed 'until it is too late'.

As for 'nuisance trips' (which I personally have hardly ever experienced, despite having lived for many years with a lot of RCDs), I suppose it depends upon what is causing those nuisance trips. If it's due to build up of leakage currents across multiple circuits, then 'all-RCBO' should obviously avoid that, but if it is truly some quirky 'of unknown cause' phenomenon (as it seems to be for some people) then having maybe 8+ (rather than just a couple of) residual current devices which can 'misbehave' might move the goalposts in the other direction

Kind Regards, John

They did look like tops so can understand the name.

The freezer is on the kitchen circuit, so imagine you would notice that - but take your point if it was on a totally dedicated circuit and it tripped.

residual current devices which can 'misbehave' might move the goalposts in the other direction - yes thats another way of looking at it.

The trips from this very short lighting circuit is still unresolved, will make a call to MK ... I understand they don't make CU's anymore, so hopefully they can help.

frank999 said:

The freezer is on the kitchen circuit, so imagine you would notice that - but take your point if it was on a totally dedicated circuit and it tripped.

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Indeed, that was my point.

frank999 said:

.... The trips from this very short lighting circuit is still unresolved, will make a call to MK ... I understand they don't make CU's anymore, so hopefully they can help.

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I'm not quite sure how they will be likely to be able to help.

Whilst it is true that "MK don't make CU's [or bits that go in them] anymore", "MK CUs" [and the bits that go in them] are still made, by Circpro, an "Authorized Licensee" of MK's.

Kind Regards, John