Two RCBOs tripping together, no cross-connection. Need help…

[weevilward]

I was recently called to a domestic nuisance tripping job, dual-RCD Wylex board with one of the RCCBs tripping intermittently, mostly during the night. TNC-S, rewired couple of years ago (not by me).

Three circuits on this RCCB: Cooker, Downstairs sockets, Downstairs lights (I know - dumb layout…)

• all tested for insulation resistance to earth at 250V (all >99M), also tested ins res between these circuits, all >99M;
• earth leakage clamp revealed 7mA on cooker, 7mA on ring, and 2mA on lights. This was steady state. When first switched on, earth leakage spiked higher.

Couldn’t find a smoking gun so I put this down to either cumulative fault current, or an intermittent fault on an appliance. I reconfigured the board so these 3 circuits were on separate RCBOs, in the hope that either it would cure the problem or at least narrow down the search.
All fine for a week or so.

Then - phone call - both the cooker and downstairs sockets RCBOs had tripped simultaneously. And the lady thought the downstairs lights flickered a second or two before.

Any thoughts? I don’t know where to go next with this. I went back today and rechecked the insulation resistance and clamped for fault current - no change to the above.

It tripped again the same way this evening and this time they were 100% sure the lights went dim immediately before.

There’s no leakage between these circuits so how can two RCBOs be tripping simultaneously, and what is causing the lights to dim???

 

[Intoelectrics]

If the circuits are completely separated as you claim, yet both trip simultaneously then there has to be a common factor that you have not inspected. If the lights do indeed dim just before the RCBO's trip then this may point to an intermittent imbalance mains issue. Have you checked the incomer and associated connections?

 

[yorkieone]

If it worked before, then it’s a new fault developed. Wrongly connected would show up straight away.

anything new been added to any of the circuits?

Smart meter? Long shot, but it’s happened before.

Individually, we might think an outside light or socket taking in water, or rodent damage… but for all 3 at same time?
What is the likelihood of rodents? Near fields?

house mice don't need fields lol

 

[weevilward]

If the circuits are completely separated as you claim, yet both trip simultaneously then there has to be a common factor that you have not inspected. If the lights do indeed dim just before the RCBO's trip then this may point to an intermittent imbalance mains issue. Have you checked the incomer and associated connections?

I think you’re onto something there – I got the customer to ask around in the street last night, and one of his neighbours two doors down had exactly the same issue at exactly the same time, so I am thinking there is a problem with the supply in The Street. I’ve got him to raise the issue with the DNO. The only bit I have not checked is the switch fuse and sub main from my customers meter to his CU.

 

[timhoward]

All fine for a week or so.

This is possibly the biggest clue.
As well as pursuing investigating a supply issue....

I'd be thinking whether anything might be out of circuit when testing that is in circuit during the fault.
Do they have any outside lights or anything time clock controlled?
e.g. an outside light switched by a double pole switch on the ring that is also linked to a PIR fed by the lighting circuit with all neutrals joined up, or something similarly special.

I'd also take a look at the boiler wiring if it's on the downstairs sockets circuit as when combi's are fitted all sorts of delightful things are done by our friends who fit them.

Have you tested the cooker itself? A cooker with a slowly developing N-E fault touching a bonded gas pipe behind can be the source of interesting fault chasing as the thing triggering it can be anything that is earthed.

I'd also try to be thinking about whether there is anywhere that the circuits run together. I remember one very obscure fault where cables had been pulled hard and they'd cut into some metal trunking, and vibrations changed the IR from absolutely fine to practically zero.

Best of luck and do report back!

 

[littlespark]

house mice don't need fields lol

Of course they don’t. But there is a higher chance of rodents in an old cottage, on a farm, full of holes than in a new inner town development.

Looks like there’s a common problem for the whole street, so no point guessing now. Lol

 

[Intoelectrics]

I think you’re onto something there – I got the customer to ask around in the street last night, and one of his neighbours two doors down had exactly the same issue at exactly the same time, so I am thinking there is a problem with the supply in The Street. I’ve got him to raise the issue with the DNO. The only bit I have not checked is the switch fuse and sub main from my customers meter to his CU.

When you come across faults like these that are not blatantly obvious then you have to follow a process of elimination and commonality. The actual details in the facts matter so it almost becomes like a court of law situation where you need the actual details from the users. Then you can start to dismiss things like "It only ever happens when the X-factor comes on".

I remember upgrading a CCU many years back at a house to include a RCD. A couple days later I was called out to the RCD "nuisance" tripping. Long story short it turned out that there was a fault on the cooker and when a certain hob ring was switched on it would trip the RCD. It took me a while to figure it out and i had to interrogate the customer to find a commonality that fitted to the problem.
Though they did try to argue with me that the cooker was fine and had been ok until I coincidently fitted the new CCU. It took me a while to explain and get them to accept that the fault was most likely all ready there and that the new safety device I had fitted highlighted this, and that it had potentially saved them from a future nasty electric shock.

 

[yorkieone]

Of course they don’t. But there is a higher chance of rodents in an old cottage, on a farm, full of holes than in a new inner town development.

Looks like there’s a common problem for the whole street, so no point guessing now. Lol

I live in an 1890s house, mice arrive every autumn and I'm in the middle of a town

 

[yorkieone]

When you come across faults like these that are not blatantly obvious then you have to follow a process of elimination and commonality. The actual details in the facts matter so it almost becomes like a court of law situation where you need the actual details from the users. Then you can start to dismiss things like "It only ever happens when the X-factor comes on".

I remember upgrading a CCU many years back at a house to include a RCD. A couple days later I was called out to the RCD "nuisance" tripping. Long story short it turned out that there was a fault on the cooker and when a certain hob ring was switched on it would trip the RCD. It took me a while to figure it out and i had to interrogate the customer to find a commonality that fitted to the problem.
Though they did try to argue with me that the cooker was fine and had been ok until I coincidently fitted the new CCU. It took me a while to explain and get them to accept that the fault was most likely all ready there and that the new safety device I had fitted highlighted this, and that it had potentially saved them from a future nasty electric shock.

when mcbs where newish i kept getting call to a house where the mcb tripped every morning it turned out to be his transistor radio being turned on lol

 

[Lucien Nunes]

Simultaneous tripping of multiple power circuits that are proven not to be interconnected, plus flickering lights on a lighting circuit that is not tripping, suggest to me an intermittent connection upstream, possibly in the DNO supply, as is now suspected.

When an intermittent connection arcs, it causes rapid fluctuations in voltage which momentarily increases the leakage current through interference suppression capacitors connected L-E within class I appliances. These normally pass a small amount of functional leakage current which depends on the supply voltage and frequency. Xc = 1/(2*Π*50*C) and I = V/Xc. But the fast risetime spikes caused by the series arcing subject the capacitors to frequencies much higher than 50Hz, at which their reactance is lower, causing impulses of leakage current sufficient to trip the RCD / RCBO.

So, if the upstream connection arcs, the lights flicker but as there are no L-N suppression capacitors on the lighting circuit, its own RCD / RCBO doesn't trip. But the power circuits are likely to be feeding appliances with suppressors, so these do trip. I bet they wouldn't trip if the appliances were all disconnected and/or if the cooker switch is turned off. Does the cooker circuit feed an induction hob?

Will be interested to hear the resolution.

If I were to guess id say there is a low resitance between both L&N to earth on one of the circuits.. quite often the circuit effected wont trip as the imbalance is shared (both L&N) so the coil on the rcd wont trip.

I can't follow this logic when applied to a 230V single-phase system. You are implying that the leakage L-E is cancelled out by the leakage N-E resulting in no net imbalance, however this is almost impossible for two reasons. Under normal conditions, at most points along the circuit, the voltage between L -E (which is the supply voltage, around 230V) will be approximately in-phase with the voltage between N-E (due to the voltage drop in the neutral conductor, typically a few volts). This means that two similar resistances or reactances, connected L-E and N-E, will pass currents approximately in-phase that add instead of cancelling. There will likely be a slight phase difference between the voltages due to below-unity power-factor in the loads, as the N-E voltage drop is in phase with the load current. But obviously, not so much as to enable cancellation.

We might be able to conjure up a peculiar situation where there is an N-E voltage opposite in phase to the supply voltage, perhaps due to circulating currents in the the earthing system. But even in that peculiar situation we must now also contrive to make the resistances L-E and N-E lie within a specific range of ratios to make the leakage currents cancel. So if there's 230V L-E and say 2V N-E, the resistances must be in the ratio 230/2 to cancel completely. Taking 23mA as a typical threshold for an RCD, a standing leakage of 30mA via 7.7kΩ L-E would need to be cancelled by a leakage N-E of between 7 and 53mA so the resistance N-E would need to lie in the range 38-285Ω

Theoretically possible but most unlikely, so I would tend to discount 'symmetrical' leakage situations on SP+N wiring. Where it is more likely to occur is from conductors with symmetrical voltages. A 3-phase system could leak symmetrically to earth and not trip its RCD, if one were unlucky enough to have three identical low insulation resistances, but that too is pretty unlikely.

 

[weevilward]

Simultaneous tripping of multiple power circuits that are proven not to be interconnected, plus flickering lights on a lighting circuit that is not tripping, suggest to me an intermittent connection upstream, possibly in the DNO supply, as is now suspected.

When an intermittent connection arcs, it causes rapid fluctuations in voltage which momentarily increases the leakage current through interference suppression capacitors connected L-E within class I appliances. These normally pass a small amount of functional leakage current which depends on the supply voltage and frequency. Xc = 1/(2*Π*50*C) and I = V/Xc. But the fast risetime spikes caused by the series arcing subject the capacitors to frequencies much higher than 50Hz, at which their reactance is lower, causing impulses of leakage current sufficient to trip the RCD / RCBO.

So, if the upstream connection arcs, the lights flicker but as there are no L-N suppression capacitors on the lighting circuit, its own RCD / RCBO doesn't trip. But the power circuits are likely to be feeding appliances with suppressors, so these do trip. I bet they wouldn't trip if the appliances were all disconnected and/or if the cooker switch is turned off. Does the cooker circuit feed an induction hob?

Will be interested to hear the resolution.

Thank you for this post Lucien Nunes - it explains something I knew could happen, but could never explain how it could happen! The basic description of how an RCD works leads to the conclusion that nothing upstream should be able to affect an RCD. Thanks for taking the time to explain it.
It's in the hands of the DNO now to check out the supply. Seeing as one set of neighbours (two doors down) is experiencing a similar phenomenon, at exactly the same times as my customer, it seems likely that it's a supply issue on an individual phase? Hope so.

I will report back.

Thank you everyone for your input on this.

 

[weevilward]

So, an update - UK power networks came out that night at 9pm (very impressed with that) - checked everything at the customer's cutout, the neighbour's cutout, and also checked the 'sub'??? (Transformer?) This is second hand info from the customer, so I'm not sure exactly what was checked. UKPN guy said "no fault found", but requested a data logger be fitted on the customer's intake point, to monitor it.
Oddly, since he came, nothing has tripped out, no flickering lights, no problems... Neighbours all report nothing too. So I don't know if he wiggled something or tightenned something but for now this fault has magically gone away.

I will let you know if the datalogger reveals anything.