Unconnected wires causing fuse blow

[Crescent]

Hi all,

I have been in the situation described here but I didn't want to resurrect an old thread.

//www.diynot.com/diy/threads/pull-cord-switch-with-3-live-3-earth-wires.367354/

Refitting a pull cord switch with three red wires (+ earth) I had to break off half-way through.
leaving each wire unconnected and isolated, (not good practice but wires were safely out of reach )

However on switching the power temporarily back on from the fuse box while away from the fitting, the downstairs light circuit fused. There was no likelihood of any of the wires touching.

I'm guessing the reason is pretty basic electrical theory but I'd be grateful for an explanation why this happens.
Thanks

 

[DetlefSchmitz]

The scenario you describe is extremely unlikely to have the effect you describe. Most likely one or more of your wires is touching something earthed.

 

[bernardgreen]

the downstairs light circuit fused.

What exactly "fused" ? was it an MCB or an RCD ?

As Detlefschmitz said most unlikely, in fact 99.9% impossible for disconnected wires to cause an MCB to trip. But there is way that a disconnected wire might cause an RCD or RCBO to trip.

MCB like a fuse trips on excessive current
RCB trips if current is leaking to ground and it has a test button
RCBO is a combined MCB and RCD trips on leaks t ground and excessive current

 

[Crescent]

Thanks for the responses.

No, just an old fashioned consumer unit with 5 Amp wire fuse. replaced fuse and got the same result indicating the fusing didn't happen prior to switching mains back on.

I'm beginning to think that any short circuit may be out of sight; above the ceiling hole.

 

[deadshort]

Yes, i would agree with you. It's perfectly possible that the fixing screws for the switch have damaged the cable when originally installed and when you have unscrewed the old switch it has mashed the cable within the ceiling void.

I have had cables with screws through them that have worked for years ! Usually N to E shorts which go undetected until the board is changed and a RCD is fitted !

Regards,

DS

 

[Crescent]

Thanks, DS
Sounds a likely solution, ceiling is wooden cladding so a bit of excavation required, I think.

Bit of further info:
Non-contact testing with voltage pen gives;
2x red wires live
1x red wire not live
earth not live

There are two lamps on the same switch both in good condition.

 

[ban-all-sheds]

Non-contact testing with voltage pen

Ah - the magic wand.

2x red wires live
1x red wire not live
earth not live

That might be correct.

It might be incorrect.