UK Find a Fault on a Ring Final Circuit

[john999uk]

I am not a qualified electrician, I was employed as an electrician after my MoD electronics apprenticeship but that was 30 odd years ago. So I have basic skills and an insulation tester with a low resistance capability.
This is an old installation with a new cu installed in 2014 (EICR etc done). Some cable is the stranded red and black t&e, some solid red and black t&e, some blue and brown.

I recently replaced a 2 gang socket with a usb version and moved the old 2 gang to replace a faulty one. 2 days later the rcd and elcb tripped.

I inspected the 2 sockets and cables but couldn't see any issues.
I disconnected the usb sockets and connected the cables with choc blocks.
r1 = 0.5 ohms, r2 = 0.87 ohms rn = 0.50 ohms.
I have no idea of the route of the cables for the final ring circuit so made it into 1 radial and tested low ohms at each socket and that allowed me to roughly work out the order of sockets on the final ring.

I disconnected the ends of the final ring and get a low insulation reading between line and cpc (less than 1M Ohms). I split the ring and now get low insulation readings on both legs! I checked my insulation tester on another circuit and that shows good readings, so I believe tester is OK.

So any ideas how I can find where the issue/s are?
Thanks

 

[catdog1121]

Try connecting live and neutral together and testing to cpc?

 

[GBDamo]

Are you absolutely sure there are no loads on the circuit? Not always practicle to remove them all.

If its an RCD tripping you have an L-N imbalance so quick LN-E IR @ 250V will give you a good idea as to the state of the cable without damaging connected loads.

A low reading here indicates a L-E or N-E

Then you need to be identifying all accessories on the circuit and inspecting them for loose wires, including the fuse board.

If that draws a blank, from the above you will have an idea of the routing, then spit in half at the nearest accessory and find the faulty leg, keep splitting until you have just one run of cable left, which must be your fault.

My most frustrating was doing the above and getting a low reading, splitting the ring and finding both legs gave a very high reading, spent 20 mins scratching my head and checking my MFT, doubting the operator before looking at the FCU I'd removed and the neon tail had been pinched by the patress screw.

I'd actually split the ring at the location of the fault and removed it without noticing.

 

[john999uk]

Try connecting live and neutral together and testing to cpc?

Insulation value 0.06Mohms so still very bad. Cheers.

 

[Mikegh]

That's very low

Like a reading you'd get with a faulty water heater connected

Could be anything I guess

 

[catdog1121]

And you get about 0.06 even after separating and then splitting the ring? On both sides? Are you sure you split the ring and there was not a spur off a spur that you split, or there is a ring within a ring?

When you have split the ring connect line and neutral together then go check the sockets with continuity tester between line and neutral to make sure there are actually some sockets no longer on one side of the split ring... If that makes sense.

 

[john999uk]

Cheers for the suggestions.

I have now double checked where I split the ring and retested and one half of ring is good. No idea how both were testing bad. So that's progress. Part of the problem is that there is only 1 ring covering downstairs and the 'room in roof' plus 2 radials. The ring goes up and down due to alterations that replaced the kitchen and bathroom floors with concrete. Plus at some point some sockets that I assume used to be mounted on skirting boards were raised without extending the ring so they are spurs. I am guessing it's one of these spurs that's causing the issue but loathed to rip up the floor till I can identify the location of the fault.
Whole lot needs rewiring really....

 

[Mikegh]

Can be a nightmare in a random installation with points and branches anywhere

 

[john999uk]

Have a little more progress. I did have a couple of neons in the ring - cheers!!! I have traced the fault down to the final 2 double gang sockets in the ring. One of which was replaced on Saturday!!!! So looks like floor up and find the issue then probably replace cable, but that's tomorrows job.

Cheers for everyone's help.

 

[john999uk]

Well after a fair bit of head scratching I found the issue!!!!! Looks like whoever did the original install about 60 years ago nicked the line when stripping the outer insulation. Over time moisture has got to the copper and corroded it to push out of the nick and make enough of a path to the cpc to trip!!! Obviously I found the fault after chasing out the cable to the socket I replaced last week!!! Morale to the story test each cable as you go to find the issue, don't go hacking walls!!!

Thanks everyone.

Attached is an image where you can see the copper corrosion!.