DIY Doctor

Shed wiring... live wire with MCB off

Postby wandering_physio » Sat Mar 29, 2008 4:44 pm


I'm going through building regs (Part P) to renew the electrics in my shed. Building control have just been out to check that the wires were correctly placed (they were) so I'm getting all the hardware in (light/socket etc) ready for the electrician to check it.

Once it was all wired up I wanted to check it worked so turned all the switches off in the shed CU (bought as a 'shed pack' with 1x6A, 1x32A MCBs and RCD) then turned the MCB on in the house CU that feeds the shed. When I tried to turn the main switch on in the shed CU, it immediately tripped in the house (with a blue flash for effect!). The house fuse is 40A and is the one that fed the original shed wiring (that was very dodgy hence re-wire), its in an old CU that doesn't have an RCD.

I also noticed that even with the house CU MCB off, there was still power coming through the live wire to the shed... is this just some residual current or a fault with the MCB in the house?

Hoping someone can help so I can get as much sorted as possible before the electrician comes to check it. If not I'm hoping I might be able to pick his/her brains!


PS I don't think my last post went through properly so if it did and this is a duplicate I apologise
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Simply Build It

Postby ericmark » Sat Mar 29, 2008 9:28 pm

Something wrong!
1. Why blue flash? I would assume nothing turned on so lets look at easy mistakes. Neutral, live swap?
2. Still power! would this fall in with original fault possibilities. Yes
3. What next! First turn off all power at house then turn off power in shed and wait for electrician. If you do turn back on in house make sure no one visits shed.
4. Why! you may have put so much current through the switches/breakers that hey have welded and even if switched off you may get nasty shock.
5. Building controls, well if qualified? but often they are builders with very little knowledge about electrics.
Advice:- Leave well alone switch off all you can with rubber boots and gloves on and wait for electrician. Oh and remember most rubber boots have carbon or graphite in rubber to reduce static problems so even with rubber boots you can still get a belt. And of course tell the electrician don't let him find out hard way.

Postby wandering_physio » Sun Mar 30, 2008 9:57 am

Thanks for your reply Ericmark.

Just to confirm... all the wires were very carefully connected up with the colours in the right places and checked several times so. We also used brand new wire and have carefully routed into the shed right next to the house. All the power is off in there now and we're steering well clear!

I understand that the guy who came first was literally just checking the wires were horizontal and vertical so appreciate it didn't mean it was 'safe'.

As we are doing it ourselves and going through Building Control then we'll have a visit from an electrician. I'm assuming that he/she may be able to point us in the right direction of the problem so we can either fix it or pay a professional if it's complex?

Once again thanks for your reply... it did just confirm what I thought but I wanted to check we hadn't made a simple error that could be sorted before the electrician came.
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