Help with fault

[MalcyB]

Hi all

Got a call this morning saying the ring main had gone off in the house. I went expecting a straight forward fault find. I got an IR of .89 on the ring.

When I split it down, one side of the ring (the last 4 skts) is where the problem is, I'm getting a low IR.

The other side is fine up to one particular skt just before the last four. I disconnected the last four (one being an FCU) so that all the legs were left loose. This is where it gets odd

I took the IR from the CU as so.

Phase/E 266Mohms, N/E 235Mohms, Phase/N 436Mohms.

I took the IR from the outgoing cable which goes to the last four skts. I got these readings.

Phase/E 84.7Mohms, N/E >1000Mohms, Phase/N >1000Mohms.

I connected this cable to the socket and retested at the CU and got this.

N/E 196.5Mohms, Phase/N 187.3Mohms, Phase/E 1.628Mohms.

Why would connecting this cable bring the reading down so far?

Sorry for the long winded post, I hope I've explained it well enough.

Thanks

Malc

 

[NozSpark]

Why are you reconnecting the fail side to the rest of the circuit??? you should continue splitting the circuit..

Maybe you've got a faulty socket or a faulty appliance connected to the FCU

 

[SPECIAL LOCATION]

What were they doing immediately before the fault occurred?

e.g. DIY? running any high power appliances.. wallpaper strippers etc..

Keep a close look out at the cables for the condition of the insulation and sheathing where they are stripped back...

any nicks/cuts from knives/cutters/squashed from accessory fixing screws...

You could have a fault that alters as you disturb the cables....

Are your 156% sure ALL accessories are unplugged/disconnected from the circuit..?

No hidden spurs in cupboards..?

:C

---------- Post Auto-Merged at 00:27 ---------- Previous post was made at 00:26 ----------

What tripped?

RCD / MCB ???

 

[NozSpark]

I've had a couple of faulty sockets (well not personally, but you know what I mean),,, usually the one that has the deep fat fryer or kettle in all the time!

 

[steptoe]

and your problem with those readings is?

please state what the issue is, not just I have a problem and a series of readings.

 

[MalcyB]

Nozspark: I did split down the rest of the circuit. That's why I ended up with the last four skts. I was getting odd readings so I disconnected all the last four (inc FCU) and left the legs loose. So there is nothing connected to the circuit.

Spec: The guy woke at 5.30am and switched his lamp on, so it was ok then. They got up at 6.30am and the power was of. I did wonder if it could have been the boiler coming on but since have discounted that. As far as I know everything is disconnected. Where these last four skts are is an extension that a builder put up, he brought his own spark in for the work.

Check the quality.

http://i296.photobucket.com/albums/mm195/malcolm29858/29042011114.jpg

http://i296.photobucket.com/albums/mm195/malcolm29858/29042011115.jpg

http://i296.photobucket.com/albums/mm195/malcolm29858/29042011116.jpg

There are no grommets on any of the back boxes but I can't see any nicks or cuts.

The cables go into an almost flat roof so I'm taking my son during the week so I can squeeze him up there to trace the cable route!!

Steptoe: As I said the problem is the tripping RCD due to the fault on the ring circuit. It was getting late but I just can't get my head around why just attaching a small length of cable would bring the IR right down, when if I test the cable on it's own it is ok?

 

[NozSpark]

Surely if you have split the circuit right down then you should be able to identify just 1 faulty cable or socket,,,, unless it's been wired unconventionally that is

I had one similar to this a few weeks ago and could only get it down to the last 4 sockets as they had fitted wardrobes inn front of the old sockets and jointed out to new

My repair entailed rewiring that section directly to the newer positions from the accessible sockets (continuing the ring)

 

[MalcyB]

Surely if you have split the circuit right down then you should be able to identify just 1 faulty cable or socket,,,, unless it's been wired unconventionally that is

So did I, the readings I was getting just didn't make sense, but that's what I think might have happened?

I just wanted to know if anyone had come across a reason for why the Ir dropped so far

I left the good sockets working as a radial with a 15A fuse. So there is no rush to go back. But I do want to get it sorted.

I think there has got to be something in the roof void causing it to happen.

 

[NozSpark]

You'll probably find JBs galore in the loft!!

 

[binky]

Hi all I connected this cable to the socket and retested at the CU and got this.

N/E 196.5Mohms, Phase/N 187.3Mohms, Phase/E 1.628Mohms.

Why would connecting this cable bring the reading down so far?

Because that's where the fault is??? Any signs of damp in the back of socket????

You disconnected 4 accessories and re-connected the one, so what's happening at the other 3 points? When I get faults like this i test and re-test and bell out cables to ensure I've got the right leg and some idea as to how cables have been run. It's easy to get confused whilst tracing these sorts of faults. Go back with a fesh mind and double check everything.

 

[Doc Hudson]

Take a pen and paper when you go back to continue your investigations, sketch out the building layout and note your readings at the approximate accessory points as you work around the circuit. It can be very easy to forget exactly which readings you got where, once you start opening 3 or more accessories. 3 opened sockets could give you 6 ends of cable all going to different locations with 6 different sets of readings. Note pad and pen can save a lot of time.

Doc H.

 

[Zeespark]

I would concur with Doc, at times I have even run continuity tests to find out what leg exactly goes where, makes any repair a whole lot easier.

AndyGuinness

 

[MalcyB]

Binky: I am assuming that is where the fault is, but when I test just that leg the readings for that leg are ok. That's what I can't understand.

I know it was late and going back with a fresh mind will probably help. There wasn't any damp in this skt but there is in one of the other skts, but that is in the good part of the circuit.

I think like Noz says, I'll find things are not straight forward up in the loft, as they are not straight forward in the living area.

 

[NozSpark]

Make sure you check the accessories aswell!

 

[monkey5]

ive had poor IR readings due to the actually socket a few times, check between the teminals on the back.

wayne

 

[MalcyB]

Tried that monkey, took socket off completely and used terminal blocks. I see you are in Lincoln, same here.

 

[steptoe]

check all the cables first,

even block connector the cables if you must,

but dont connect any accessories until you have sorted out the cable first,

IR test the cable, NOT the circuit.

oh, one more thing , Im not sure why the readings you are stating would trip a good RCD anyway.

 

[nsbs]

you could try disconnecting the last 4 sockets and putting them into connectors, that will rule out the sockets. also none of those readings will trip a RCD.

 

[MalcyB]

Update:

The cables going to the FCU, came out of some boxing in, we dismantled this and first found the cables wrapped around the central heating pipes! First thought was melted insulation. Cables found to be ok. Then we found rodent damage. so we cut cable back as far as possible and tested, found there was still a problem with this leg, but unable to get access. The lady did not want here decor damaged, so we decided to get rid of this leg all together and make the circuit into two radials on 15A fuse.

We also found a fault on another leg direct from the CU, we tried to trace this leg of cable to try to find the area of the fault but that proved impossible so we replaced the whole leg.

Everything tested and works fine now.

 

[Zeespark]

15A fuse?, are you using a 3036 board or got mixed up Malcy? if it is a 2.5 ring I would have been tempted to put them on 20/25A radials rather than 16, but thats just Me.

AndyGuinness