This fault has me confused!

[HappyHippyDad]

I rather foolishly booked in a fault finding job in a 2 hour slot thinking I could solve it quickly, I couldn't!

It has me confused, although I haven't let myself slow down yet and focus so you guys may point out something obvious.

It is an intermittent tripping MCB on a 32A ring ciruit (sometimes the RCD trips) on a dual RCD board. It trips a number of times each day. This has been going on for perhaps a week. Over the last 2 days 5 sockets have actually stopped working completely even with the power on. The ring feeds the whole house.

r1 = 0.14 Mohm (140,000 ohms)
r2 = 28K ohms (28,000 ohms)
rn = 0.7 ohms

With all sockets connected and the ring split in the CU I get :

Leg 1 = L/N - E 0.01Mohms
Leg 2 = I didn't write down the results but they were >1Mohm.

When I split the ring at one socket the IR results were all >1Mohm when tested in the CU, I didn't test at the split socket.

I think I'm getting confused because of the 5 non working sockets. If a ring is broken (as this is) all sockets should still work. Am I right in thinking there HAS to be 2 faults either side of those non working sockets (assuming they are not all spurred from the same socket) or am I missing something?

Usually, in order to find the break I would split the ring. Reconnect the leg that has continuity with the relevant leg in the CU liven them up and eliminate the working ones, but if there are 2 breaks this wont work, i don't think!

Update... the customer has literally just rang saying he removed a socket and just plastered in the cables a while ago and when he bangs that part of the wall it trips! However, this would be just one fault and would still not stop 5 sockets from working!!

 

[Pete999]

I rather foolishly booked in a fault finding job in a 2 hour slot thinking I could solve it quickly, I couldn't!

It has me confused, although I haven't let myself slow down yet and focus so you guys may point out something obvious.

It is an intermittent tripping MCB on a 32A ring ciruit (sometimes the RCD trips) on a dual RCD board. It trips a number of times each day. This has been going on for perhaps a week. Over the last 2 days 5 sockets have actually stopped working completely even with the power on. The ring feeds the whole house.

r1 = 0.14 Mohm (140,000 ohms)
r2 = 28K ohms (28,000 ohms)
rn = 0.7 ohms

With all sockets connected and the ring split in the CU I get :

Leg 1 = L/N - E 0.01Mohms
Leg 2 = I didn't write down the results but they were >1Mohm.

When I split the ring at one socket the IR results were all >1Mohm when tested in the CU, I didn't test at the split socket.

I think I'm getting confused because of the 5 non working sockets. If a ring is broken (as this is) all sockets should still work. Am I right in thinking there HAS to be 2 faults either side of those non working sockets (assuming they are not all spurred from the same socket) or am I missing something?

Usually, in order to find the break I would split the ring. Reconnect the leg that has continuity with the relevant leg in the CU liven them up and eliminate the working ones, but if there are 2 breaks this wont work, i don't think!

Update... the customer has literally just rang saying he plastered in some cables a while ago and when he bangs that part of the wall it trips! However, this would be just one fault and would still not stop 5 sockets from working!!

You need to start from scratch in your testing procedure. let nothing distract you, finish testing and analyse the results.

 

[littlespark]

and when he bangs that part of the wall it trips!

oh my .... good luck with this one. Rather you than me.

seriously though. Remove every socket. Check what’s in the rfc and what’s spurred off.
buzz every leg so you know what goes where.
I had one fault finding mission turned out to be a faulty socket. Ie, all my cable testing was clear, but tripped rcd when I put the faceplates back on.

 

[Strima]

Beak the RFC down to the point where you can test where he plastered, might have nicked a cable. Sounds like two faults.

 

[telectrix]

first dig out his bodge and clear that fault. thenlogically test for the other fault/s. split ring and get wander lead out. i like to go round with wander lead on the socket screws before lifting sockets. increasing values as you get further from CU. once the layout is found, then start on the fault/s.

 

[buzzlightyear]

ring a ring of roses ,start one end and till you have found it .split it half and a quarter and see what you get ,a bright spark for doing the job right .I had one the other day a faulty rfc it was the first leg that went to the first socket ,other sockets where ok .

 

[Lucien Nunes]

Weird duplicate post, see below!

 

[shaun1]

One break could well be a badly made joint where he has plastered the cables into the wall (assuming there must have been some attempt to join them to give the rn continuity). If the failed sockets had some significant loading between them, then another weaker joint may have since failed due to increased loading after the socket removal.

If the plaster is still wet, that could also be the source of the low IR. Sounds like thats a good place to start, but as others have said you will probably need to map the whole ring to check for extra spurs and interconnects etc.

 

[Lucien Nunes]

There must be:
No breaks in N (you have end-end continuity)
At least 1 break in CPC (no end-end continuity, but might be present at all sockets)
At least 2 breaks in L (not present at all sockets)
By break, I'm talking about a defective connection or severed conductor that might have very variable resistance.

The intermittent short / flashover that is responsible for the MCB and RCD tripping is presumably under the bit of wall he bangs, and it is probably also the site of one of the breaks. You can read the resulting defective insulation on leg 1, as 0.01MΩ, and should be able to read some kind of defective IR on leg 2 albeit with up to 0.14MΩ end-end resistance in series. However, mashed cable faults and loose terminals are sufficiently unstable that I would not set too much store by the fact that you got a reading >1MΩ on leg 2 instead of 0.14MΩ at the moment you tested it.

Crack on...

 

[HappyHippyDad]

faulty edit being fixed now

Thats a pity as I was about to quote you...

In your final paragraph you wrote something along the lines of... "whilst on one leg the fault cannot be seen". My 11 year old son happened to be reading it and decided to stand on one leg, he fell over crashing into the cat which sprang up onto a shelf sending everything flying, he then fell on to the floor in a laughing frenzy!

Thankyou Lucien

 

[HappyHippyDad]

Thanks for the replies everyone. I think (as most of you have said) that I will find one of the faults clear when I break into the wall and fix his plastered in socket. I can then hopefully start some more 'standard' fault finding with only one fault left to find.

 

[Zerax]

Thanks for the replies everyone. I think (as most of you have said) that I will find one of the faults clear when I break into the wall and fix his plastered in socket. I can then hopefully start some more 'standard' fault finding with only one fault left to find.

Looking forward to the updates...(with pictures....no pressure !)

 

[HappyHippyDad]

Looking forward to the updates...(with pictures....no pressure !)

I'm 'usually' pretty good with updates, although not that speedy with them so it may not be for a few days!

 

[Dave OCD]

Isn't it great after hours of head scratching when the customer just happens to mention/remember things like this....

 

[123]

Isn't it great after hours of head scratching when the customer just happens to mention/remember things like this....

On fault finding I listen to what they say but never believe them to be giving me all of the information...
Me: "What where you doing when the RCD tripped? Any cooking, put the kettle on, ironing etc"
Customer: "Nothing just watching TV"
Me: Plugs kettle in, switch on, trips. Tests kettle, L-E short.
Customer: "Oh yeah forgot it happened as I turned kettle on"


More difficult fault here but the most important bit of information needed was left out.