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Rcd Box Tripping for no Apparant Reason

Postby Crawfy » Fri Feb 03, 2017 10:04 pm

Over the last three years, the "socket" circuit in my rcd box has tripped many times, for no apparent reason. It may trip 3 times in a day or once every 4 months. It's appears random. I can't detect a pattern!! Each time, I have disconnected all plugs in the sockets to isolate the problem. Even with all plugs disconnected, I am still unable to reset the rcd box. It may take 30 minutes until I can reset the box or 3 days ( the maximum so far). My fridge/freezer is on this circuit, so defrosting has caused expensive food waste on 2 occasions. I have called an electrician out, but by the time he arrived, I had been able to reset the rcd box. He checked the box and could not find any fault there. He said that the only way he could definitely diagnose the problem was if he could visit during a trip. Due to the timing of trips since then, that has not been possible. My question is, is it not possible for an electrician to identify and underlying problem, without waiting for a trip condition?
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Postby ericmark » Sun Feb 05, 2017 6:59 pm

Not all RCD's are equal, although we want a RCD to trip at 30 mA in 40 mS some are very basic, others have electronics to help reduce unwanted tripping. There was one called X-Pole which was claimed to be very good, but internet hunts seem to be full of pole dancing. Try this link if left in ... htm#tabs-1
I have a very old RCD and like you I find every so often I will get a series of trips, then nothing for a year or two. However there are some items which do cause problems.

The freezer is the big one, with frost free freezers they have a heater which comes in say every 4 hours, once tripped the time is reset, but likely if that was the fault there would be a pattern.

Neutral is the next one, many items don't switch the neutral, so even when switched off they can cause the RCD to trip. Here the voltage between neutral and earth is critical. With a TN-C-S supply the neutral and earth are connected together under the road, but are split when they reach your house. As more and more load is put on the circuit the neutral starts to move towards the line voltage wise. So with a bit of bread stuck in a toaster which has become a little damp and no load the RCD is fine. However put the kettle on and the neutral voltage raises and so enough current goes neutral to earth for the RCD to trip. I bet many a kettle blamed for toaster fault?

So how a RCD works is it measures the power in the line and neutral if both the same then all OK, if they vary by between 15 and 30 mA then the RCD will trip. It assumes the difference must be going to earth. The X-pole works however between 90 ~ 100% tripping current not the standard 50 ~ 100% tripping current, it also has an indicator to show when approaching tripping current.

Every circuit will have some leakage, so the more we split the supply into circuits the less likely each circuit is to trip, also the less we lose when it does trip. The definition of a circuit has become something electricians argue about, "Circuit. An assembly of electrical equipment supplied from the same origin and protected against overcurrent by the same protective device(s)." now the question is if 30 mA to earth is overcurrent? If it is then fitting two RCD's means you have formed two circuits. Then fitting say 4 MCB's to each RCD produces 4 sub-circuits.

314.1 Every installation shall be divided into circuits, as necessary, to:
(i) avoid hazards and minimize inconvenience in the event of a fault
(ii) facilitate safe inspection, testing and maintenance (see also Section 537)
(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit
(iv) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation
(v) mitigate the effects of electromagnetic interferences (EMI)
(vi) prevent the indirect energizing of a circuit intended to be isolated.

Looking at (iv) it would seem the RCD is considered as making a circuit, and if having just 2 RCD's they trip, then we should have more. This can be done by using what we call RCBO (Residual current breaker with overload) these are more expensive than MCB's but means we don't need a general RCD and when a RCBO trips you lose less and because it controls less it is less likely to trip.

All well and good with a TN-C-S supply, but with a TT supply we really should regard the neutral as live, so we need double pole switching of the RCBO some makes have double pole RCBO's although the overload bit only works in the line.

RCBO's are longer than MCB's so will not fit in old boards, I am not a domestic electrician so can't tell you which boards will fit what.

With a TT supply (means you have an earth rod) we have had RCD's this would have been around 100 mA S type (means delayed) today this would not be allowed but nothing to stop using old system.
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