Intermittent tripping MCB

[MarkyofftheMark]

I’ve a fuseboard as pictured, which has been nuisance tripping on the downstairs ring final circuit. It started off with 32a breaker tripping which is RCD protected, and now it’s tripping the Main RCD switch.

This is a relative's property and he’s already had an electrician in, who’s ran a series of tests (unsure of which) and replaced the 32a MCB but it hasn’t resolved the problem.

The longest its gone without tripping was 3 weeks and it seems to trip most between 9-10am and the last time it tripped there was no appliances being used. Now that the main RCD switch is tripping, it only stays on if the previously mentioned 32amp breaker is switched off and this is with everything on it unplugged.

I’m going to take a look to put some fresh perspective on it. I’m going to open up the affected sockets to see if there’s anything obvious, and if nothing is visible carry out an IR test on the cables to determine their condition. I don’t do much fault finding nowadays so any further advice would be appreciated.

 

[pc1966]

It is unusual for a MCB to trip fast enough on a L-E fault that a 30mA RCD would not also trip, but theoretically possible. So I suspect some damage somewhere that has got worse.

An intermittent open on some heavy load like a fridge motor might just trip a MCB before it progressed to cable damage shorting to E, but again it seems unlikely to me.

Do it systematically, disconnect the suspect RFC and do IR testing on it (L+N to E) and also on the rest of the board (to see if any dodgy N-E faults separate from the MCB tripping).

If you have got definite insulation problems on the RFC, and not on the rest of the board, remove all accessories and see if it is still there. Also the end-end to check (L-L, N-N, E-E) for bad joints. If the RFC tests out fine without anything plugged in, then it is the more tedious step of trying to check each accessory for a fault.

Doing L+N to E is always safe at 500V so easy to do. But beware of doing L-N above 250V as that can over-volt low power electronics that only needs ~1mA so the Megger can charge it well above the ~340V peak AC it normally expects. Sockets with USB charges built-on come under that category, as do many light accessories and low power LED lamps.

 

[i=p/u]

Fridge, wm , kettle , something that kicks in

 

[Mike Johnson]

Fridge, wm , kettle , something that kicks in

Are you trying to say they have not done any washing or boiled a kettle for three weeks. ?

 

[MarkyofftheMark]

This is the downstairs ring that feeds the sitting and dining room; no Fridges, Kettles or Washing machines supplied.
Mike, No. The MCB has been tripping for about 3 months but up until yesterday it hadn't tripped for 3 weeks with all the accessories/appliances connected.

 

[Mike Johnson]

MarkyofftheMark I was addressing my remark to i=p/u.

Have you tested the RCD to see if it works? As pc1966 has suggested, suspect some damage to the cables, it may simple be one has fallen out of a socket terminal, or it may be rodent damage, difficult to tell at a distance, best get in an electrician to have a look.

 

[Timbo]

And while you're there, that missing blank.....

 

[pc1966]

And while you're there, that missing blank.....

Well spotted, there should be two (or double-width) blanking plates in the CU not one hovering half-way!

 

[MarkyofftheMark]

Thanks for all your responses. My father in law has had an electrician out three times and he's been unable to solve it. I'm also a qualified Electrician and was just looking to pick some brains, so I'm sure I'll be able to resolve the issue. I'll keep you posted.

 

[Mike Johnson]

Please do keep us posted, its not often we get feed back, but would be appreciated.

 

[LastManOnline]

I’ve a fuseboard as pictured, which has been nuisance tripping on the downstairs ring final circuit. It started off with 32a breaker tripping

That it started off with the 32a mcb tripping first(and rcd not tripping as you describe) would mean the initial fault was L to N and then gradually became an L to E fault as well. As the electrician who initially investigated appears to have found nothing wrong when he came it starts to sound a little like a "water fault". This kind of fault usually vaporises the liquid at the same time as the fault occurs meaning there is nothing for the electrician to find when he comes along.(until the next rain falls) However, as you do not appear to have anything from the garden connected to the circuit you will be looking at something like a potential leak

 

[LastManOnline]

you will be looking at something like a potential leak

Apologies. Should have said you "COULD" be looking at a potential leak. There could of course be a number of causes of foreign matter getting in somewhere it should, nt and then being vaporised by the short

 

[loz2754]

Apologies. Should have said you "COULD" be looking at a potential leak. There could of course be a number of causes of foreign matter getting in somewhere it should, nt and then being vaporised by the short

Indeed.
I once traced an RCD tripping fault to a small caterpillar draped elegantly across the L and E terminals of a socket!

 

[i=p/u]

I once saw pig fly and the rcd went loz

 

[MarkyofftheMark]

Went to look at the job and on the very last socket I opened up, the socket was indeed tripping from a leak. The cause of this was a heating pipe which I think must be weeping from an elbow. This validated the intermittent faults as the heating was only put on every now and again this causing the issue. Removed cables from socket outlet, placed them in wago connector and securely cable tied them in cellophane bag to keep dry and will leave the rest to plumber. Mystery solved, thanks fellas.

 

[LastManOnline]

Went to look at the job and on the very last socket I opened up, the socket was indeed tripping from a leak. The cause of this was a heating pipe which I think must be weeping from an elbow. This validated the intermittent faults as the heating was only put on every now and again this causing the issue. Removed cables from socket outlet, placed them in wago connector and securely cable tied them in cellophane bag to keep dry and will leave the rest to plumber. Mystery solved, thanks fellas.

Interesting one. With intermittent faults one of the golden rules is to note the circumstances present when the fault occurs. A pattern will always develope.With intermittent faults the homeowner is thus key to finding the fault, rather than the spark who ends up confirming it. Vast majority of water faults are weather related This is the first time I have come across central heating as being the culprit. I shall add it to my "potential intermittent faults" list?