Electric Fault-Finding Question

[Ferdinand]

Can someone advise on how to approach a tripping RCBO.

Normally my electrics are great, the house was rewired about 15 years ago, and I had a new consumer unit in about 2017.

Recently I had an RCBO tripping on my part-of-downstairs lighting circuit. It had been repeatedly tripping. I removed what I thought was a questionable lightbulb (porch) and it improved.

Now it has tripped again, and is at immediate-retrip when switched to "on" stage.

So do I need to look first at

 

1 - Remove all the lightbulbs to see if it helps.

2 - Swap out the RCBO as possibly faulty (which will need my friendly lecky, as I normally would not venture inside the fuse box -- though I could do so).

3 -Something else.

 

Comments and advice are most welcome.

Ferdinand

[ProDave]

You won't get far with trial and error component substitution.

 

To make any meaningful progress you need an insulation tester.  It's likely you have a faulty fitting, damp or damaged wiring.

 

If you fancy a go yourself then you can buy a cheap old uncalibrated tester that will do what you want for not a lot on ebay, otherwise get your electrician to look at it.

 

I guess one thing you could try is remove every single lamp, and see if the tripping stops, then replace one at a time if it does.

[SteamyTea]

1 hour ago, Ferdinand said:

I removed what I thought was a questionable lightbulb (porch)

Try drying the fitting out with a hair dryer.

[Ferdinand]

2 hours ago, ProDave said:

You won't get far with trial and error component substitution.

 

To make any meaningful progress you need an insulation tester.  It's likely you have a faulty fitting, damp or damaged wiring.

 

If you fancy a go yourself then you can buy a cheap old uncalibrated tester that will do what you want for not a lot on ebay, otherwise get your electrician to look at it.

 

I guess one thing you could try is remove every single lamp, and see if the tripping stops, then replace one at a time if it does.

That's a thought.

I might have an insulation tester somewhere !

(It's been that long since I did anything.)

Thanks all.

[markc]

A few months ago a friend asked me to look at a new lighting circuit tripping problem. Turned out to be some replacement LED GU10 downlight bulbs (lamps) her dad had convinced her to put new ones in because they are more efficient now … I put the old ones back and hasn’t tripped since.

[Temp]

3 hours ago, Ferdinand said:

thought was a questionable lightbulb (porch) and it improved.

 

I would temporarily disconnect power to the porch light, ideally some distance away in the house. See if problem goes away.

 

PS Not just the live but neutral as well. 

Edited December 3 by Temp

[Alan Ambrose]

The sure fire way to debug this (well nearly, if it's not due to transient or intermittent behaviour) is to use a residual current / earth leakage meter. Modern day electronics (LED lighting 'transformers' etc) often leaks tiny currents (single digit mA) to ground. Not really a problem, but if you get a bunch adding up to >30mA, or a single one (like a damp porch light) then the RCD will trigger. So, clamp your meter on, observe residual current, switch devices on or off and see which one causes the high leakage current. This usually means poking inside the (live) CU so you may want to get a good spark (describe the fault and double check whether they've got the right test equipment for this first).

Edited December 3 by Alan Ambrose

[dpmiller]

5 hours ago, Alan Ambrose said:

Modern day electronics (LED lighting 'transformers' etc) often leaks tiny currents (single digit mA) to ground

 

how/ why? Considering very few have an earth wire...

[RobLe]

We had a trippy circuit for ages, it got worse and worse - the gap between tripping of 6 months, 3 months, 1 month, every week.  Tried a current clamp around the combined tails that some internet bod recommended, all the circuits had some, that one didn’t stand out.  By the time it was tripping every week I was switching off a few double sockets for a week at a time, trying to isolate what it was - nothing I did made any difference.  We had some stuff running on long extensions from sockets on other circuits, as part of this elimination process.  In the end I swapped out the RCBO with an identical one.  That fixed it, no trips in over a year now.  YMMV.

[ProDave]

Yes of course, test the RCD / RCBO, but then you need to buy an RCD tester if you don't have one........

[Ferdinand]

On 03/12/2024 at 22:31, ProDave said:

Yes of course, test the RCD / RCBO, but then you need to buy an RCD tester if you don't have one........

I actually have spare RCBOs in stock ... but I might prefer my normal chap to do it as a Saturday morning job rather than personally.

I have a small job at a tenant that needs a look. 

[Alan Ambrose]

@dpmiller

 

>>> how/ why? Considering very few have an earth wire...

 

Now that is a good question and I had to think for a sec.

 

It revolves around this:

 

(a) RCDs, RCBOs & most earth leakage testers measure the current imbalance between line & neutral, the balance is assumed to be from line to earth. This might be due to some leaky electronics, a damp or badly insulated line conductor or … a human touching something that’s at 230V and also earth. That’s why the RCD is there in the first place.

 

(b) LED assemblies generate a lot less heat than filament bulbs, but they often still get quite hot and need to have heat sinks. Now materials that conduct heat really well but don’t conduct electricity hardly at all are fairly unusual and they’re not cheap. So, as we’re talking about cheap consumer items, there are compromises and the insulators are not perfect and they leak a little electricity from line to ground. Similarly, LED ‘transformers’ are usually cheap capacitative droppers, having to cope with spiky ‘dimmed’ waveforms (which puts the cheap pcb insulator under dV/dt ‘pressure’j. So, they leak a little current from line to actual ground. Similar argument a lot of electronics having to deal with mains voltages. This tiny leakage isn’t much of a problem … except that the cumulative effect can cause RCDs to trigger.

 

(c) So, the small leakage currents bridge from line to actual ground (not the earth wire unless we have an earthed chassis).

 

Edited December 6 by Alan Ambrose

[Russdl]

Have you got two RCBO’s of the same rating? If so swap those around, that’ll test the RCBO.