Copper Sulfide on Earth Wire

Thanks bernardgreen.
I was initially checking earth continuity before turning the power back on after cleaning up all the wires. When by accident more than any particular reason I noticed that when I probed the live and neutral the multimeter showed 35.2 ohms when set on 200K
I thought it best to disconnect all the wires from the wall socket and the next socket inline and check again.

It appears that 35.2 ohms reading is only on that section of wiring as the remainder of the ring main reads 0.0.

I can't understand when the wires ( between that section ) are all disconnected and I probe live and neutral at either end only it should read 0.0 as nothing is connected at the sockets but bare wires.

I've taken a spare lenght of 2.5mm twin and earth and probed both ends........ to be clear..... probed black to black and red to red at the left end only reading 0.0, same for the right end. Should that not be the same for the wire in the wall between sockets when disconnected?

Currently leaving the ring main off until I feel happy about whats going on...... so desperate.

This is giving me a massive headache.

Thanks
 
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No, it shouldn't read 0, it should read infinity between L and N. Also, are you sure you're reading 35.2 ohms with your meter set to 200k? More likely to be 35.2 kilohms. The rising reading suggests that there's something capacitive across your live and neutral.
 
I am so grateful for the replies.
I think I may be over thinking this so have decided today to remove the floor units and ALL sockets. I'll expose all the wires and check eack one at a time. I will then have proper and clear information to post here otherwise I'm affraid of wasting everyones time.

Gonna start tonight and continue tomorrow and hopefully post some updates Saturday evening.

I am most grateful to Stillp and BernardGreen.
 
As has already been said, have you got anything plugged in or any switched fused spurs switched on? If you have the readings you have got are normal.

You should also been checking the ring final continuity, with all the sockets re-connected. You should have two cables at each socket, measure between both line conductors, neutral conductors and cpc/earth conductors
 
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Thanks TomH93 too for helping me.
Everything is unplugged and as of yesterday all sockets have been removed too.

Here is what I've done and found.

I removed both live wires from the breaker and both neutral and earth wires. Then removed all the sockets from the walls and removed the sockets from the wires. I used my mutimeter (set on continuity) to trace the route from the consumer unit to each socket. Using a spare length of wire I confirmed continuity between live neutral and earth at both ends. I also checked for any ohms readings.

After mapping out the route and confiming all the above I arrived at the original socket and wire which had copper sulfide which I earlier had subsequently cleaned and replaced the metal back box. This wire shows continuity between live and neutral BUT NO earth continuity. It also shows a reading of 95.2 ohms rising to 195.6ish ohms before settling. Multimeter set on 200k.

All other sections of wiring show continuity and NO ohms readings (or infinity).

The wire in question runs 6 feet within the wall and then disappears under a concrete tiled floor for another 6 feet. I think it runs next to the wall in a small cavity but not within the floors DPM so exposed to moisture. This may account for the rust and copper sulfide especially after the underground leak a few years back. Also there is some slight powerding of the wall paint behind some kitchen units which rise about 8 inches but only small areas.

None of the wires in the floor and within the walls are in any tubing.

I don't know why there are ohms readings on this wire but I do know I was right not to trust it. There is usually a scar when there is a bang!

I think the bang happened not at the socket but under the floor and the sound and flash travelled along the cavity.

So I have a dodgy length of inacessible wire with no earth continuity and a unexplained ohm reading.

Any advice would be really helpful
 
If I understand you correctly then it does sound like a damaged cable.

At the socket where the problem is how many wires are there?
 
Thanks TomH93,

The socket has 2 twin and earth cables 6 wires in total. The other end of the cable the socket has again 2 twin and earth cables 6 wires in total.

The damage is probably somewhere inbetween under the floor.

Let me know if I can add anything further.
 
Since you've worked out the route of the socket circuit, a different way of fixing this without actually replacing any cables could be done by converting the socket ring circuit into two radials. You would need to isolate the damaged cable at both ends (at each socket) and then at the consumer unit separate the two twin and earths into two 20a breakers

However this would only be sufficient if the damaged socket is roughly in the middle of the ring (which you will know now you've traced the circuit) and if it's possible to source breakers in your consumer unit (take a picture and I'll be able to tell you)

How much of the house does this circuit cover?


Otherwise you will need to replace the damaged leg of the ring
 
It also shows a reading of 95.2 ohms rising to 195.6ish ohms before settling. Multimeter set on 200k.
Are you sure about that? As I think has previously been said, if you get readings of "95.2" and "195.6" with a meter set on its 200kΩ range, that would normally mean 95.2 and 195.6 kilohms, not ohms.

Kind Regards, John
 
Since you've worked out the route of the socket circuit, a different way of fixing this without actually replacing any cables could be done by converting the socket ring circuit into two radials.
Apart from anything else, a practical issue about that approach would be that it would result in the creation of a 'new circuit' (hence notifiable), which could make it a very expensive exercise for the OP.

Kind Regards, John
 
The damaged section is all but one from the consumer unit so no way a equal divide. The ring covers the kitchen and dining so about 50% of the ground floor.

There is another cable which I can see alongside the faulty one which routes to the kitchen's oppsoite wall socket. It too is also partly underground and may fail eventually.

I am concerned about the integrity of the remaining cables on that side of the kitchen as the other side is stud wall and has a clever inspection panel.

I am now thinking if runing new cable in trunking and surface mounted boxes on that side of the kitchen would be safer option.

Thanks again
 
I have decided (unless there is a better option) to run a length of MMT2 to incorporate 2.5mm twin and earth six inches below the underside of the worktop and position 2 double sockets for the washing machine and dryer and the electric oven and hob ignition. I was hoping to save the tiles above the worktop and drill from underneath a channel six inches long and insert a tubing to the 2 above counter sockets.

In total 4 double sockets which would be 1 less than before.

The only issue I think I have is making a continuous join from one socket on the wall to the other wall which is stud. The cable runs from under the kitchen floor ( in cavity ) into the dining room under the joist. But I will need to pull up the remainder of the laminate floor to reach the area where I believe it enters the stud wall.

Can I cut and join under the dining room floor or will I need to make the length from one socket to the other continuous?

I think this will be the safer option than reparing the damaged cable but welcome some advice.

Thanks as always
 
If the your resistance reading is rising it suggests something copulative on the circuit. i.e. something still plunged in or a fix appliance Fused spear? If you bypass this cable you will leave whatever it is disconnected.

My main point:
If the cabling is damaged by damp or ingress causing water corrosion. Then there is a high likelihood other cables are in a poor state.

A multimeter may not show the damage only an isolation resistance tester will (MEGA)

In my experience corrosion to conductors will effectively reduce the CSA, increase the resistance and therefore the Zs. meaning the disconnection for the protective device may not be achieved. also if the damage is bad the cable may just melt at a weak point under fault conditions.

You may not like the idea but I would suggest calling an reputable electrician to do a full inspection and test.
 

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