An Intermittent RCD Tripping Issue Question

[Sparky54]

Our very elderly neighbour has recently started having irregular RCD tripping on her ring mains ccts. Usually with a few days in-between. There are two (Steeple) MCBs in the fuse box with their circuits supplied through the RCD. She had her side utility room rebuilt and new LED lights fitted recently. These are supplied by a 2-pole switched fused spur on one ring. I changed the sfs for her at the weekend as the old one was buzzing a bit… As both live and neutral are switched in the box and the lights are off when a trip happens I have discounted this new wiring. Nothing else has changed electrically.

I spent many years on the installation and testing of DC, control and RF wiring but have no real experience of testing domestic ac mains wiring. My question is, if I knock the main breaker off and completely isolate the two ring main circuits in the fuse box by disconnecting the lives from the two MCBs and their neutrals from the bar is a DC resistance test of lives and neutrals to earth going to show me anything useful which would help identify the cct with the potential high leakage? Or does it need some sort of an ac resistance test using a higher voltage?

Thanks.

 

[securespark]

It most probably is, but just check the Fused connection unit controlling the lighting is actually double pole.

How have you narrowed the problem down to the ring final circuits?

Have you disconnected all appliances from the circuits to eliminate them?

You will be best to do insulation resistance tests on the wiring at 500V.

 

[ericmark]

My last house I would get a batch of trips, maybe it would trip 5 times in a month, then go for 2 years without a trip, I tested the RCD with a RCD tester, and I tested the installation with a 500 volt insulation tester, and never found a fault.

The RCD's were old, fitted around 1990, and each one fed 4 MCB's, I had all sorts of ideas as to why they tripped, from neighbours welding, to thunder storms, but never did find out why.

What I never did check was the AC leakage, the insulation tester uses DC so will not show any capacitive or inductive linking, and clamp on ammeter will, but until about a month ago my clamp on only measured 0.01 or 10 mA so not really a good indication of leakage, new one will measure 0.001 or 1 mA and on measuring whole house, showed 19 mA the pass mark for a single 30 mA RCD is considered to be 9 mA, so with 14 RCBO's unlikely any single one exceeds the 9 mA although never measured, but had I used just 2 RCD's one would have had too much leakage.

But I am measuring live, with all appliances plugged in, only did it out of interest, we are told when testing a RCD to unplug all appliances and to test with no load, which seems to me rather unhelpful as we want to know if OK with all the load.

If we used the RCD tester ½ test with system under load, then we would know it can have a further 15 mA leakage before it trips, but the way we test it could be just on the edge of tripping, which was the case with old house, as using the non trip loop impedance tester would trip the power, they typically use 9 mA so my house would not take 9 mA extra leakage most days, some times it would work.

But without a clamp on meter which can measure 0.001 amp how can you find the problem appliance or circuit? My PAT tester showed mA leakage, so I could test items which plugged in, but to PAT test every item in the house would take a long time. And we are allowed a little leakage, so even if just 1 mA many items together could cause the RCD to trip.

When we look at the insulation resistance this is recorded on the EICR so we can see if there is an increase from last time tested, but we don't record the earth leakage, so we have no idea what is normal. Now retired so only this house I can test, I tried to get people on this forum to post typical results, but seems few electricians can measure down to 0.001 mA. The same applies with DC on the supply, we are told type A will work with up to 6 mA leakage, but we never seem to measure it, and don't record it if we do. And my attempts to measure it I found zeroing the clamp on was a problem and 6 mA could be just an error in reading.

 

[ericmark]

You will be best to do insulation resistance tests on the wiring at 500V.

I normally use 250 volt unless certain there are no USB sockets, or filter sockets etc in the house, I know my house would show a low insulation resistance at 500 volt due to the SPD fitted.

 

[Sparky54]

All noted thanks. The 96-year-old lady’s daughter was staying with her this week so I have only been back in once. I have bought a known double-pole switched fused spur just in case the spare one I had wasn’t. I just presumed the separate L and N load terminals were both switched. Good point securespark!

I have suggested she unplugs everything not being used, see if that helps stop her trips to the garage to reset the ‘green switch’. I don’t have an insulation tester working at any voltage or a sensitive clamp-on meter, just a bog standard DVM, so if a physical inspection doesn’t throw anything up I will have to hand this over to a local electrician. The randomness of the tripping needs a wiser AC brain than my RF one and proper test gear on the job…

As and when the reason is found I will try to remember to update this thread.

Thanks again.

 

[EFLImpudence]

. I just presumed the separate L and N load terminals were both switched.

They will be otherwise the two Neutrals would be in one terminal.

 

[ericmark]

I hope you have some luck, I never did find out why my old house had a problem, now my sons problem, he bought my house, John on this forum says how rare it is his house trips, and I often wonder why?

I know when my daughters boy friend left with all his computers my problem reduced, I will assume the filters on the power supplies caused some leakage?

I paid £35 for my insulation tester, and £35 for my clamp on ammeter, for me a good investment, but for some one who rarely needs them, £70 is a lot of money. The cost of a replacement RCD tester or loop impedance tester means I have never bothered to replace them. My sons went faulty so he had mine, now it seems they have gone faulty.

I take the attitude, may be wrong, that the RCBO RCD function with a TN supply is secondary protection, so not that worried if it fails to work, be it due to DC or any other factor. I volunteer at local railway and I am sure I could borrow the test gear, but don't really want them to realise I have electrical skills, I enjoy myself doing less technical things, like parking cars.