Burnt earth terminal.

[whistler]

A couple of months ago as we were undergoing a kitchen refurb, I replaced three double sockets. No wiring changes, just replacement sockets.

All was ok until the early hours of this morning when we heard a bang and some power went off. Checked the consumer unit and both the mcb and the rcd had tripped. I reset them and around 30 mins later it happened again. There wasn't much plugged in, but I unplugged everything and tried again. The power tripped again sometimes instantly and sometimes up to 20 minutes later.

I decided to leave the power off and went to bed with a view to checking things in the morning. Today I opened up the first socket, and the earth terminal was partly burnt out. However all wires were terminated correctly and no obvious signs of a short circuit.

The switch was a relatively low cost one but from a reputable shop. The live terminal is quite close to the earth terminal, but I checked with a meter and all seemed OK.

I replaced the socket with a better quality one and up to now it has been ok.

I'm confused as to what the fault may have been. If there was a s/c then I would have expected burning on both the live and earth but the live wires and terminal are clean.

Any ideas on this ?

 

[bernardgreen]

Pictures would help. Back of the suspect socket and the box in the wall.

 

[JohnD]

was anything plugged into the socket?

I once had a bad flashbang in a kitchen, when a water leak had flooded a fridge plug.

 

[securespark]

That earth terminal looks as if it has shorted against some wiring in the back box - a "pressure fault" - when it was screwed back. These kinds of faults can take a good while to manifest themselves.

Turn off the power, pull out the replacement socket and let's have a photo of the wiring behind.

 

[bernardgreen]

It looks as if the plastic has melted. Which suggests the material the socket is made from is sub standard. It should not melt or deform when heated.

Next thing is why was there any current flowing onthe Earth wire. If there was nothing plugged in then either there is a fault in the socket. Or there isa fualt else where in the house that is causing current to flow in the Earth wire from another socket. If the Earth terminal in the damaged socket was not tightened then the fault current, flowing from wire to wire through a poor contact would cause heating.

 

[John D v2.0]

Perhaps the back box became/was live for some reason and the cpc wire touched it?

 

[securespark]

We need pictures!

 

[ban-all-sheds]

Which suggests the material the socket is made from is sub standard.

Or it suggests that the material was subjected to heat far in excess of what the standard requires it to survive.

 

[whistler]

On closer inspection, there is a tiny cut in the live wire. The socket has a metal strip with quite sharp edges which runs across the the back between two earth terminals. There is a tendency for the wires to press against this strip. It may or may not be relevant, but last night was a very warm night. Maybe be a bit of expansion?

I'm off to the shops to buy better quality sockets!

 

[securespark]

On closer inspection, there is a tiny cut in the live wire.

That earth terminal looks as if it has shorted against some wiring in the back box

 

[ban-all-sheds]

No wiring changes, just replacement sockets.

Were they flat plate ones?

I'm off to the shops to buy better quality sockets!

Possibly of a different style, and/or deeper back boxes?

 

[securespark]

From the picture they look like white standard plate sockets but they are the type (there are 100's on the market) that have a sharp edged earth bar running across them and it is not recessed like on some sockets.

They are a short waiting to happen. I have come across loads of these in my career.

 

[rsgaz]

From the picture they look like white standard plate sockets

Agree. The two crosses on the left of the photo are where those little screw cover thingys come on a new socket.

 

[crystal ball]

I just installed four Schneider metalclad FCUs for roller doors, the box earth terminals have very pointy corners on them, so I dressed the cores away from them, unfortunately the roller door fitters did not, got a call one morning, "we can't get into the workshop", some hand cranking then redressing

 

[DaveHerns]

Check the wire hasn't been melted where it was shorting out.