Nuisance tripping - faulty RCD?

[I Trippet]

Had a real headache of a fault finding job which has left me scratching my head - RCCB on a split load board, supplying 3 circuits (shower, immersion and downstairs lights) constantly tripping. The frustrating thing about this was while it would sometimes trip within seconds, mostly it would take anything from ten minutes to several hours before tripping, which made it very difficult to pin down to anything in particular. Insulation resistance readings for those circuits were fine, RCD not tripping at 15mA. Checked every light switch and fitting, shower and isolation switch, immmersion heater switch, no sign of anything that would cause it to trip. Tried leaving shower and immersion heater MCBs off, still tripped. Switched shower off at the isolator, still tripped (immersion heater was hardly ever used so that was already switched off). Disconnected the shower and immersion heater circuits at the board leaving only downstairs lights running off it, still tripped. To take a closer look at the downstairs lighting obviously involved lifting parts of the floor upstairs which was easier said than done. There were 2 or 3 junction boxes on the circuit which I opened and were fine, and all cables were in good condition. Only thing I could think left to try was swapping the RCCB with a spare one I had. Reconnected everything, tested the new RCD and it hasn't tripped since! All the fault finding I've ever done it's been because of the wiring or a faulty appliance, never come across a faulty one RCD before and wondered how common they were. I took the old one back with me and did an experiment to see if it tripped with no load connected and it didn't, so maybe it wasn't faulty. Either way, replacing it cured the nuisance tripping.

 

[ipf]

Did you ramp test the RCD? They are pretty reliable, these days.
Could be a bit of e-n leakage, a bit of a screw scratch at a switch somewhere, who knows?
Maybe time for an earth leakage tester.
Whatever, time may hold the key.

 

[Pretty Mouth]

I have only had this once. The RCD had been randomly tripping and had gotten gradually worse over time. No IR issues when tested. Replaced it, and all was well.

 

[timhoward]

In a funny way the failure mode of RCDs could do with inverting. It’s very uncommon for them to trip without cause, and much more common for main switch RCDs to not trip at all through lack of exercise.

 

[littlespark]

I’ve seen them loads of times on the caravan park.

Identical RCDs, same age, same environmental conditions… one would fail before the other just because the customer had faults and tripped it over and over….

Constant tripping through faults can damage the rcd so it either (occasionally) trips well within threshold, or, more dangerously, doesn’t trip at all.

 

[UNG]

Did you ramp test the RCD? They are pretty reliable, these days.
Could be a bit of e-n leakage, a bit of a screw scratch at a switch somewhere, who knows?
Maybe time for an earth leakage tester.
Whatever, time may hold the key.

The ramp test would be my go to on faults like this as it gives a good indication of the state of the RCD
Finding and identifying the leakage needs a clampmeter if the cause isn't readily identifiable, one of the intermittant favourites for me is the auto defrost fridge and freezer with a faulty heating element

 

[I Trippet]

Did you ramp test the RCD? They are pretty reliable, these days.
Could be a bit of e-n leakage, a bit of a screw scratch at a switch somewhere, who knows?
Maybe time for an earth leakage tester.
Whatever, time may hold the key.

Yes sorry forgot to say 21mA exactly the same for the one that new one that replaced it.

As I say it was a real headache because whatever caused it to trip was very brief and random making it and really difficult to pin down. After narrowed it down as much as I could, in the end I temporarily put the downstairs lights on the RCBO for the upstairs lights (no borrowed neutrals anywhere btw) and the RCCB still tripped with nothing connected to the load, the RCBO didn't trip. Was a bit peed off after wasting so much time but that's my fault.

What I don't understand is why it would trip with no load at this property but not when I tried it at home. The client did say he thinks it started happening not long after the smart meter was installed so could it possibly be to do with that?

 

[Pretty Mouth]

The client did say he thinks it started happening not long after the smart meter was installed so could it possibly be to do with that?

What brand of RCCB? and how far away fro the CU is the smart meter located?

 

[I Trippet]

What brand of RCCB? and how far away fro the CU is the smart meter located?

Proteus, and right next to each other!

 

[Pretty Mouth]

Proteus, and right next to each other!

Have a look at this old thread. It's usually old Memera RCDs that seem to be a problem with smart meters. I haven't heard of it happening with Proteus, but that's not to say it doesn't happen.

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[I Trippet]

Have a look at this old thread. It's usually old Memera RCDs that seem to be a problem with smart meters. I haven't heard of it happening with Proteus, but that's not to say it doesn't happen.

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Ah, thanks for that. It's not something I've come across before but it would make sense if that was the cause.

Not sure how I'm going to charge for this job, having wasted so much time going down dead ends!

 

[littlespark]

Ah, thanks for that. It's not something I've come across before but it would make sense if that was the cause.

Not sure how I'm going to charge for this job, having wasted so much time going down dead ends!

That’s the nature of fault finding.
You have to charge the time it takes to find the fault…. Not just the end repair.

 

[loz2754]

I always make it clear before starting a fault finding job that it's charged at an hourly rate, as it's impossible to say what the the fault is until, well, you've actually spent time looking for it!
I usually give an estimation of how long I think it's likely to take though...

 

[I Trippet]

That’s the nature of fault finding.
You have to charge the time it takes to find the fault…. Not just the end repair.

I get that, just thinking that I could have saved myself a lot of time if I'd tried putting the downstairs lights onto the upstairs lights RCBO before lifting the floor and inspecting the circuit.

 

[LastManOnline]

What I don't understand is why it would trip with no load at this property but not when I tried it at home.

If an rcd is tripping with no load connected then you have an N to E fault somewhere in the installation.