What would cause this melted switch? (pics)

[coolhands]

This switch is outside the bathroom on the wall and switches on / off the electric shower (9.5kw from memory). Thing is it's been in service for about 9 or 10 years! So why now would it suddenly melt? It has never got wet or anything.

The shower was replaced about a year ago and is working fine do I don't think it's excessive load that's caused the problem. It's not that far from RCD to switch to shower either.

I've replaced with a crabtree switch which I quite like as it has twin screws to hold each cable. But obviously the melted original concerns me (fortunately there was enough slack so I could cut all the bad wire right out before connecting the new switch). It was on the side going to the shower.

pics (bit blurry cos on my phone unfortunately).

the switch I used:

http://www.screwfix.com/p/crabtree-45a-2-gang-switch-neon/46279

 

[Iggifer]

looks like something has worked its way loose, although is that bar that goes to the screw hole at the top burnt away?

Was there a big bang or was it a gradual thing you discovered because of the smell?

 

[JohnD]

the cables appear to be extremely old, in which case they were probably supplied for an older, lower-power shower, so are probably overloaded. It would be interesting to know what teh size of cable is, what the power of the shower is, what the MCB rating is now, and what the fuse rating used to be when it was installed. The cable has the appearance of rubber insulation.

The cable cores are stranded, which can lead to them compressing in the terminal and becoming loose.

The overheating is probably because the cable was not tight enough in the switch, and was not retightened once a week for the first month or so.

You will have to cut away all the cable that has been discoloured by heat, because it will have softened and will not make a good connection in future. Subject to the other points, you may well need to run a new cable in a size appropriate to the load of the shower. What did that cable supply before you changed it?

I am especially alarmed that the earth terminal appears to have overheated. there should never be any load on the earth wire.

 

[Taylortwocities]

Oooh look. It's the live terminal/conductor that has failed. Usually its the neutral.

Agree with the above. Could be the shower is too big for the cable.

Coolhands - Need to know the answers to johnD's questions. TYVM!

 

[coolhands]

you can click on these two pics to make bigger

here's the RCD



and the fusebox

 

[Iggifer]

what size mcb is the shower on in the fusebox? I can't quite make it out

 

[coolhands]

the cables appear to be extremely old, in which case they were probably supplied for an older, lower-power shower, so are probably overloaded. It would be interesting to know what teh size of cable is, what the power of the shower is, what the MCB rating is now, and what the fuse rating used to be when it was installed. The cable has the appearance of rubber insulation.

It's not rubber. From memory it was 10mm squared. Is there any way I can check now it's 'in situ'?

You will have to cut away all the cable that has been discoloured by heat, because it will have softened and will not make a good connection in future.

Hi have done so it's now back to completly good copper.

Subject to the other points, you may well need to run a new cable in a size appropriate to the load of the shower. What did that cable supply before you changed it?

It was supplying a shower before I bought the flat.

I am especially alarmed that the earth terminal appears to have overheated. there should never be any load on the earth wire.

It has just been blackened (and the earth bar) it is not melted (or the earth bar)

 

[JohnW2]

what size mcb is the shower on in the fusebox? I can't quite make it out

Difficult to read, I agree, but it looks like B40 to me. However, we don't seem to know what cable we are dealing with, either in terms of CSA or 'type' (PVC, VIR etc.).

Kind Regards, John

 

[coolhands]

what size mcb is the shower on in the fusebox? I can't quite make it out

I think it says 40A 30mA (I am not there right now).

 

[coolhands]

ISTR the cable was 10mmsquared

 

[coolhands]

here's a pic of the cable back in 2004 (you can see the larger cable). I didn't fit the cables!! (click on the pic twice to make it full size)

 

[JohnW2]

what size mcb is the shower on in the fusebox? I can't quite make it out

I think it says 40A 30mA (I am not there right now).

We can read that one. It's the shower MCB (next to the main switch) in the main consumer unit that we're not sure about - as I indicated, I think it probably says 'B40', which would make sense.

Kind Regards, John

 

[riveralt]

what size mcb is the shower on in the fusebox? I can't quite make it out

I think it says 40A 30mA (I am not there right now).

That is the RCD - the size of the MCB is required.
For that size shower a 10mm ² T&E and B40A MCB would probably be appropriate.
As for why it went now (and on the line side) it doesn't really matter - they just seem to go whenever they feel like.
A couple of weekends ago I got called out to two showers that had gone. Showers had been in for just over 2 years (love how many go pop just after the guarantee runs out).
Both neutrals on the load side had gone - cable and terminals were tight - correct sized cable for the showers - only thing that was noticeable was that both (pull switches) were of an unknown brand.
One new 10mm ² T&E cable required (no slack) for one shower - new MK 50A fitted to both (although even they don't seem to be as well made as I remember).

 

[JohnW2]

here's a pic of the cable back in 2004 (you can see the larger cable). I didn't fit the cables!!

It does look rather like 10mm². However, it's proximity to that red-taped (presumably hot) water pipe, particularly in the joist notches, doesn't bode too well!

Kind Regards, John

 

[Iggifer]

nor the red pvc tape underneath the nut on that compression elbow!

I prefer to see soldered joints under floorboards personally.

If that green bag was cable clips, then it likely is 10mm