Electrical Fault - Correct Diagnosis?

[Christopher Key]

Hello,

Just noticed that electric boiler in out flat didn't appear to be heating any water. The fault was quickly traced to the DP switch supplying power to the heating element, which would no longer physically switch from the on position, and which no longer had the neon light illuminated.

Examination of the back of the switch showed significant charring at the top of the dry-lining box, on the back of the switch where it had been pressed up against the neutral wire, and about 1 inch of the insulation from the neutral wire had burnt away, leaving significantly discoloured copper underneath. The other wires looked largerly unaffected. Finally, the neutral wire was loose in its terminal on the switch.

My conclusion would be the original installer either hadn't tightened the neutral terminal, or had tightened it onto the neutral wire's insulation, leading to very poor contact, and the consequent arcing which overheated the neutral wire and damaged the switch.

Firstly, does this sound like the correct diagnosis?

Secondly, how cross ought I to be with the builders / developer. Would you consider such wiring as grossly incompetent and as having come close to starting a fire, or just as slightly sloppy and of no real danger? Does it seem unreasonable to demand a full wiring inspection to ensure that there aren't any similar issues lurking elsewhere (New build flat, completed just over a year ago)?

Many Thanks,

[newshound]

Diagnosis sounds OK (I'd bet on slack screw), just had a similar failure myself in a metal box after about five years. I guess drywall boxes are pvc or some other fire resistant plastic, so not a large fire risk. If arcing or overheating gets beyond a certain point usually an ELCB trips and the "fire" goes out. My gut feeling is that 95% of such faults are fail safe (perhaps someone in the trade has some statistics?). Of course this isn't to say that electrical faults don't sometimes cause serious fires, but I suspect the problems arise when there are wood shavings or a mouse nest or similar in the wrong place, or a non-compliant bundle of wires. This is another advantage of all-metal boxes and metal conduit as used in schools / hospitals / factories.

[Andrew Gabriel]

It's quite likely if it's a recent installation. If it's not recent, then I would say slightly more likely, particularly if it's a cheap make of switch, is that the switch contacts have gone high resistance, generated excess heat which is conducted back to the screw terminal. This might result in the screw terminal failing too when it overheats, or it could just be suffering directly from conducted heat.

Replace the switch with a good make such as MK, and not the cheapest on the shelf. Check tightness of the other terminals on the circuit.

For a one-off, I would forget it and move on.

[Dave Plowman (News)]

Yes. Inadequate tightening is pretty common. Dunno why. Perhaps just using a screwdriver that's too small. It's near impossible to overtighten them without employing a gorilla.

The whole idea of a backing box is that it will contain any fire.

I'd first remove a couple of random sockets and check if the screws are loose.

[The Medway Handyman]

You would be surprised how often I find this. Light switches, sockets, DP shower switches, ceiling roses. Very common fault.

My Dad always taught me to 'tighten it up till it squeaks' which I still do today.

[rrh]

Tighten, push accessory fully into place on back box, gently pull forwards and tighten again. Cables often move slightly as the accessory is moved into place, however tight you did the screws up first time.

And always try and pull the cables out to test if tight.

[Dave Plowman (News)]

I've never done this and never had a problem with loose cables.

[ARWadworth]

Instead of a few random sockets, you could test all the sockets at once by doing a continuity check on the final ring.

says that whichever of the live/neutral/earth readings is high will only be found by removing and tightening up the terminals on the last remaining socket you have to check and this socket will also be in the most inconvenient place.

A simple work around to the above is to start with the most inconvenient socket and this time the problem socket will be the easiest one to access. It will still be the last one on the ring circuit that you have to check though.

Adam

[rrh]

You obviously have stronger wrists than me!

[Harry Bloomfield]

Tighten, waggle the wire to settle it in, then tighten again - finally give the wire a tug just to make absolutely sure.

[Dave Plowman (News)]

Just a question of using the correct screwdriver. Many seem to think a 'mains tester' ideal for such work - they're not. You need a decent one with a shaft the same diameter as the screw head (as a rough guide).