Preventing Another Disaster

[novaf4]

I have a flat on the first floor of a large Victorian house. I have a single garage as part of a block of garages about 100 feet away from the house. None of the garages have power, but as I needed power, about 6 years ago I ran a cable from the garage under the ground, up the wall and into my bedroom though the house wall.

It's all properly insulated and protected. In the garage there is a 4 way gang plugged into that end of this cable. In the bedroom it was plugged into a normal double wall socket.

I used the power for all sorts of usual DIY jobs one would do in a garage and everything was fine for many years. It powered my dehumidifyer successfully too which went on and off on it's own accord depending on the level of dampness it needed to tackle. All good.

Then one day, fortunately when I was inside the flat, I smelled burning. It was coming from the double socket in the bedroom. Immediately I shut of the power to the socket ring main and then removed the socket cover. The insulation on both black neutral wires had melted and were very close to the twin red live cables which still had insulation on. It was fortunate that it wasn't worse.

I went to the garage and saw that the dehymidifyer had been the only thing that was on. I brought it into the flat and tried it at another plug. It failed to work. I took it to the supplier who confirmed that it had a faulty relay or something. So, even though it had a 13amp fuse at the garage end and my plug was fused at the bedroom end it still f**cked up and ruined my socket wiring.

The supplier replaced the dehumidifyer and I've been using it since - in the garage - via another socket in the bedroom. It has been fine.

Recently I finally got round to fixing the wiring on the offending socket which has been a right ball ache because it meant removing carpet, underlay and floorboards. In the end I made the job even worse by enlaring the hole in the wall and installing a deeper back box for a more attractive flush cover plate. I am now at the point where I'd like to re-use this refreshed socket for the garage power cable. But I'm worried, after all this work, that the same thing could potentially happen again.

So... I'm wondering if there's anything I can do about this to prevent this happening again. I don't know anything about this kind of thing but how about wiring some kind of surge protector into the actual dehumidifyer cable that is in the garage plugged into the 4 way gang? Something whereby if the dehumidifyer goes wrong again it will protect my wiring?

Thanks. Sorry it was so long and boring.

 

[ericmark]

I would guess some thing loose causing the socket to be damaged rather than the dehumidifier. The extra load may have been the last straw, when the dehumidifier went faulty or the dehumidifier went faulty due to the volt drop caused by the faulty socket.

Any refrigeration device unless inverter controlled takes a lot higher current on start, and the motor needs to get up to speed before the pressure builds up. If there is a volt drop the motor can stall, and the over load will trip, if this happens repeatedly then the over load will fail, this was a regular problem with air conditioning units when I worked in Algeria, we were told, if for any reason we needed to switch off the supply, we should wait 5 minutes before putting the power back on so as not to stress the AC units.

 

[terryplumb]

The melted insulation on the neutrals was more likely caused by overheating due to loose connections ,and nothing to do with your appliance.

 

[AndyPRK]

Ditto.

Probably a loose terminal behind socket.

You could fit a 13a fused switch next to the socket and connect the garage wire is properly.

This then connects into the back of the socket.

 

[novaf4]

Very interesting. Thanks for your explainations of the cause.

 

[garyo]

I'd assumed that this is why electricians have expensive test meters. So rather than just measuring continuity, they also push some amps through and see what happens to resistance at load?

 

[Risteard]

I'd assumed that this is why electricians have expensive test meters. So rather than just measuring continuity, they also push some amps through and see what happens to resistance at load?

Continuity testing is typically only at 200mA with an open circuit voltage between 4-24V d.c.

Loop testing, however, could be up to 25A.

 

[garyo]

Continuity testing is typically only at 200mA with an open circuit voltage between 4-24V d.c.

Loop testing, however, could be up to 25A.

Is that kind of test usual (or mandatory?) when an electrician fills out his test sheet?

 

[SimonH2]

It could well be that with the length of cable (I assume something like 1.5mm² flex), the resistance of the whole circuit is too high for a 13A fuse to blow.
This is bad for a number of reasons ...
If there is a fault, then a large current may flow for an indefinite time with bad consequences.
Also, whike such a fault current is flowing, the "earth" at the garage end could potentially be at (or slightly above) half the mains voltage - so at 120V or more.
As has already been suggested, overload protection in appliances might not work. Hence a faulty appliance could draw a significant fault current indefinitely.
And as a related issue, the supply impedance may affect startup of some loads.

It needs someone with the right knowledge and measuring equipment to work out what combination of cable and protection is acceptable.