8.5kw shower on 6mm cable melting into switch points

[RazorD]

Hey guys,

We've got a mira sprint electric shower, its rated 8.5kw at 240v. (something like 7.8kw at 230v), and the voltage at the shower is about 243v (i assume this fluctuates a little, but that was last night)

Towards the end of 2008, the shower switched stopped working, as either the switch, or the cable had melted at various points - this could have been an incorrectly rated shower switch though.

We're covered by british gas homecare, so they came out and replaced the switch with one i know was rated for 45A.

On saturday, the shower stopped working again. 243v was between live and earth, but only about 40v between live and neutral. So i opened the switch again to find the shower-side neutral cable had melted once again to the switch.

This doesn't make sense to me, as i'd of expected the live to melt if anything, and more so, if 6mm cable should be able to handle 40A ish, the circuit break for the shower circuit is a B32, so why isn't that tripping out if the shower is drawing over 32A.

I believe i'm correct in saying its the amps across the cable which would cause it to melt?

Anyway. British gas came back out this morning, but refused to change the switch again, saying we need 10mm cable and they'd quote me for it.

Does anyone have any advice on if it is the 6mm cable causing problems, or if theres something else i'm missing?

Thanks.

-Liam

 

[rebuke]

I suspect someone terminated an already slightly damaged bit of copper in to the new switch, at which point it's capacity is reduced, so it just melts again - you need to make sure that you strip it back to nice clean copper, and terminate that in to the switch.

Also, make sure the terminals are nice and tight (the switches with two screws are good for this), as if not then there will be small amounts of arcing etc...

 

[RazorD]

This would make sense. there is electrical tape all around it. Problem is, to strip back to clean would not give me enough room to wire it in, and looking online, you can't buy 45A termination blocks. I'm guessing i couldn't just "make do" with a 30A termination block in this instance?

I'd happily just replace the whole cable from the switch to the shower, but it goes through the roof of our bathroom - which is a flat roof, and therefore a complete nightmare!

 

[RazorD]

I uploaded a photo of the switch so you can clearly see exactly whats happened:

http://www.flickr.com/photos/lgladdy/4284156447/sizes/l/

Does that change the diagnosis at all, or match what you'd expect?

 

[ban-all-sheds]

This doesn't make sense to me, as i'd of expected the live to melt if anything

Why?

The current is the same in both conductors.

the circuit break for the shower circuit is a B32, so why isn't that tripping out if the shower is drawing over 32A.

They don't trip on small overloads. A B32 will pass over 40A indefinitely.

//www.diynot.com/forums/viewtopic.php?p=76467#76467

This would make sense. there is electrical tape all around it. Problem is, to strip back to clean would not give me enough room to wire it in, and looking online, you can't buy 45A termination blocks. I'm guessing i couldn't just "make do" with a 30A termination block in this instance?

No.

You can get 60A ones.

Or you could use crimps, but you'd need a proper tool:

not one of these:

I'd happily just replace the whole cable from the switch to the shower, but it goes through the roof of our bathroom - which is a flat roof, and therefore a complete nightmare!

If the cable is damaged then it must be wholly or partially replaced.

The nightmare of doing that is considerably less of a one than the house burning down.

 

[ban-all-sheds]

I uploaded a photo of the switch so you can clearly see exactly whats happened:

The overheating damage is quite clear - it has got to be cut back.

BGas had no business bodging that repair - they could have killed you.

 

[mikeyd]

Why is the earth wire in that photo not connected to anything?

 

[RazorD]

Why is the earth wire in that photo not connected to anything?

Thats a british gas special When they fitted that switch a year ago, i questioned it to the installer (who spent 30 minutes on the phone to try and find out if 6mm cable was okay for an 8.5kw) - he said it was fine.

 

[timtheenchanter]

BG special or not, whatever is at the other end of the earth, be it the shower or CU means you have no earth protection at your shower, someone needs a slap!

they should also have not left the cable like that, the damaged copper is most likely reason for the second failure, get them to come back and fix it properly, its an ongoing fault, NOT a new call, if it had been repaired properly in the first place, it probably would not have failed again so soon (I say probably because there are occasionally faulty switches.

 

[bernardgreen]

A qualified technician apparently approved by BGas does work like that ?

Dangerous on at least two counts.

Doesn't say much for the ongoing assessment of these "approved" technicians ( or tool twiddlers ) by "the system" that is supposed to keep installations safe.

Would they allow a damaged end of a gas pipe be re-used in a new fitting ?