Installing Shower - Final Stages - Help Appreciated!

mrchook · 10 Aug 2005

I am in the final stages of installing a new 10.5kW electric shower in our top bathrooom. I have installed the shower, cable goes into a 50A ceiling-mounted isolator switch, then down two floors, into new 30mA RCD, then into spare 40A MCB on existing consumer unit. Two questions/issues:

1. When I finally got it all going last night, turning on the isolator switch caused the switch to blow ("bang!") before tripping the MCB. Any ideas what would cause this? I'm sure I wired the switch correctly.

2. I know I probably need a 45A or 50A MCB rather than the 40A I am trying to use, but can't seem to find one of these for my CU (100A Wylex), and am I right in thinking this wouldn't be the cause of the problem above anyway?

Help really appreciated.

Thanks,
Steve Hadaway

Qedelec · 10 Aug 2005

Steve
A 10.5Kw shower will draw 46Amps so no wonder it blew a 40Amp MCB but it sounds like you have a short as drawing more the the MCB can handle is unlikely to cause a 'bang'

Try checking for a short between phase and neutral on the shower side of the switch.

Are you aware of part P and any circuit should be successfully tested before energising, this is for your safety.

pdcelec · 10 Aug 2005

What readings did you get for insulation resistance and earth continuity before you energised the circuit?

Qedelec · 10 Aug 2005

Testing????
Surely the cicuit was tested before energising!!!!!!!!!!!!!!!

Would anyone like to place any bets.

mrchook · 10 Aug 2005

Thanks for your help, and no (as an amateur) I didn't carry out the level of testing you indicated (have no tools to measure insulation resistance or earth continuity), although I did as much research as I could, followed detailed instructions to the letter and checked and rechecked everything I did!!

That all said, clearly something is wrong so will check connections on shower side of the switch as suggested.

On the MCB point, will a 45A current definitely trip my 40A MCB (i.e. any chance I can get away with using that?) Have searched for a 45A/50A MCB for my CU, but can't seem to find one (e.g. screwfix sell this 40A one but nothing more:

http://www.screwfix.com/app/sfd/cat/pro.jsp?id=77992&ts=87803

Does anyone know if MCBs >40A of this type are available?

Thanks again for help
SH

Spark123 · 10 Aug 2005

A 45A current will eventually trip a 40A MCB but not instantly with a bang as you describe. A current of 1.45 x 40A = 58A must cause a trip within 1 hour. The bang you describe indicates there is a short circuit.
It is difficult to fault find without specialist equipment such as an insulation resistance tester. You are probably better calling in a sparky friend to sort this out.

You can buy a 45A (green base) cartridge fuse. This fuse / breaker must go next to the switch as it is keyed. http://www.tlc-direct.co.uk/Manufacturers/Wylex/Wylex_Standard_Carriers/index.html

Screwi · 10 Aug 2005

Youve wired the double pole switch up wrong - youve wired the switch so it shorts live and neautral instead of live to live etc etc.

Spark123 · 10 Aug 2005

Screwi said:
Youve wired the double pole switch up wrong - youve wired the switch so it shorts live and neautral instead of live to live etc etc.

Maybe, but also could have damaged cable, insecure connections, faulty shower unit......

BJS_Spark · 10 Aug 2005

mrchook said:
I'm sure I wired the switch correctly.

How did you wire the switch?

Supply cable L&N to Supply and shower cable L&N to Load??

mrchook · 10 Aug 2005

Will check but 99% certain that's how I did it (except switch was marked feed rather than supply - I'm sure semantics).

Thanks for everyone's help

mrchook · 10 Aug 2005

Oh: one other question - as I said, switch made a pretty big flash and bang before flipping the MCB. Can I assume I should fit a new switch rather than reuse that one....?

Cheers.

pdcelec · 10 Aug 2005

mrchook said:
Oh: one other question - as I said, switch made a pretty big flash and bang before flipping the MCB. Can I assume I should fit a new switch rather than reuse that one....?

Cheers.

By asking that question, I am sure you already know the answer.

Spark123 · 10 Aug 2005

Don't refit a switch which is too small again, fit a 45A fuse instead.
Better fix the problem first though, replacing the fuse and switching it on again isn't going to fix anything.

ban-all-sheds · 10 Aug 2005

Qedelec said:
Steve
A 10.5Kw shower will draw 46Amps so no wonder it blew a 40Amp MCB

Probably not - the headline rating quoted by manufacturers is nearly always at 240V, i.e. 43.75A. Since our supply usually is 240V in practice, that's a likely actual current - if we ever do end up on 230V, it'll be 40.2A.

But anyway - 43.75A is 1.09 x In - the MCB will pass that for hours, possibly for ever.

Are you aware of part P.

A question still unanswered by our petitioner.

mrchook said:
switch made a pretty big flash and bang.

Strongly indicative of a supply/load rotation, or a downstream cable fault.

If so will almost certainly have knackered the switch.

mrchook · 10 Aug 2005

Thanks - have ordered a new switch and will check my wiring and cabling again.

Wasn't aware of Part P., but have read about it now.....

Thanks - SH.

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Installing Shower - Final Stages - Help Appreciated!

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