Copper Sulfide on Earth Wire

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A months back the a breaker on my consumer unit tripped. I flipped the breaker back and approx 20mins later I heard a pop sound from a socket. I removed the faceplate and could see the earth wire was blackened and a fair amount of blue dust around the earth wire at the point where the white insulation was cut. The red and black wires were perfect. The entire wire was blackened when I removed the earth sleeve.

I have about 8 inches of slack wire and when I stripped the earth back six inches its still blackened with small amounts of blue dust. I think this is copper sulfide.

I think the wire runs under screeded floor but in a cavity.

Can I clean the wire with sand paper or will it need replacing?

Any advice would be helpful
 
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It is common when a bare earth wire within a cable gets shorted with the live wire for it to be blackened further down the cable.

Clean the bare wire up with some wire wool - usually available at a DIY store either in the plumbing or decorating section.
 
Many Thanks Sparkwright.
Will clean with wire wool and reassemble.

Thanks again
 
I've cleaned up the earth wire and was wondering how the earth wire shorted with the live. Other than the blue stuff and blackened earth wire there are no other tell tell signs. I am sureish that the pop sound came from behind the socket but can't see any tell tale signs.
 
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Is there any trace of damp behind the socket, copper salts ( but not oxides ) are conductive and could have created a short circuit.

One of the most common causes of shorts behind sockets is when two wires are squashed against each other when the front plate is screwed into place. Or a Live or Neutral is squashed against the metal back box. Over time the squashed insulation deforms, moving away from the area of pressure and eventually the copper conductors touch
 
Thanks Bernard, Yep, about 3years ago I had a leak in the underground drains which was sorted. The area affected was 15 away under the screeded floor.

When I removed the front plate the metal box had small areas of rust covering about 30%.

The skirting board came off but was not rotted although the nails looked rusty. The old plaster/cement render behind the skirting dropped off pretty easily.

I am now wondering if some damp from the leak affected the socket?
 
Could be that the cable has shorted under the floor and you're looking at the other end of the damaged section. My mother's kitchen was installed with too much slack cable behind the sockets and the faceplates left loose so the live conductor rubbed against the screw and shorted. Result was localised damage and the live conductor burnt halfway through
 
Many thanks Dave & Bernard, ok, I'll change the metal back box for good measure and refit everything. Fingers crossed it won't trip the breaker again. If it does then I suppose I'll have to get the entire cable changed.

If there is anything else I can check or do then please let me know otherwise I'll keep you posted.

Many, many thanks again
 
All done now BUT to cut a long story very short I was almost ready to test the circuit when I noticed that when my multimeter was set on 200k the ring main in question was reading 35.2 rising to around 60.4. I have no idead what this meant but the other ring main reads 0.0. I knew instantly that it must be the same wire in question so isolated it from both sockets and sure enough that wire has the resistance. Once isolated the remainder of the ring main read 0.0.

Is this the reason the breaker was tripping after varying amounts of time?

Many thanks for any advice
 
I connected it up today and the breaker did not trip.

I am very worried about those high resistance readings especially as all the six wires were disconnected and yet there was a reading. I can only imagine that some corrosion is to blame.

Should I arrange to have this section of cable changed or have I missed something.

Sorry to go on but I want to be sure about this.
 
What two points are you measuring that resistance between? Did you have your fingers on the test probes?
 
Sorry for the delay...
I was sure my fingers were not touching any wires.

I measured between the red and black at both sockets. Weird thing is the wires are all disconnected. Red and black readings started at about 35.2 ohms and rising. Red and earth started at 55.0 rising and black and earth 55.0 ohms and rising. Its the same readings at either end of the wire.

I know this cant be right and reluctant to leave it this way without knowing whats going on

Thanks for replying
 
Red and black readings started at about 35.2 ohms and rising
Exactly what were you measuring ? Was one probe on the black wire and one on the red wire when you read 35.2 ohms ?

If yes and it was 35.2 Ω then something was still plugged in or otherwise connected between the red and black wires. From W = V² / R = (230 * 230) / 35.2 = 150 Watts. This could be a large lamp or a small heater maybe.

Or were you reading 3.5 kΩ or 3.5 MΩ Check the position of the Range Knob on the multimeter.
 

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