Donate to Remove ads

Got a credit card? use our Credit Card & Finance Calculators

Thanks to Bhoddhisatva,scotia,Anonymous,Cornytiv34,Anonymous, for Donating to support the site

Which switch failed first?

Does what it says on the tin
bungeejumper
Lemon Half
Posts: 8021
Joined: November 8th, 2016, 2:30 pm
Has thanked: 2831 times
Been thanked: 3921 times

Which switch failed first?

#493374

Postby bungeejumper » April 10th, 2022, 4:43 pm

A mid-priced wall-mounted heater in a rental property, running off its own dedicated cable, via a decent quality Crabtree 20 amp isolator switch, and thence back to its own dedicated RCD at the consumer unit. Everything was professionally installed about eight years ago.

So the rocker switch on the heater (dual pole, single throw) stopped rocking recently, and upon closer inspection it became obvious that things had been getting pretty hot in there. I thought I'd better take a look at the isolator switch, and whoops, it turned out to be a candidate for the black museum. All four line and neutral cables had been blackened and thoroughly fried for the last inch or so, and even the thin little wires that fed the neon light had been roasted, although it was still operational.

I've junked the wall heater, because it's against my religion to try and repair leccy devices when they go wrong in a tenant's home. :? As for the isolator switch, I've also replaced it - fortunately the fitter had left generous lengths of cable in the switch box, so I was able to cut the ends back to healthy cable.

Problem solved, then. And the sparky will be along soon to certificate the building. But I'm still left wondering how (and whether) a failed switch on an appliance can comprehensively fry the isolator switch that feeds it? Or did the isolator switch go down first, and take the heater switch with it?

And another thing. Why didn't the consumer unit trip if things were getting that warm?

BJ

Mike4
Lemon Half
Posts: 7039
Joined: November 24th, 2016, 3:29 am
Has thanked: 1632 times
Been thanked: 3761 times

Re: Which switch failed first?

#493383

Postby Mike4 » April 10th, 2022, 5:49 pm

Wires get hot where there is an electrical resistance. I'd suggest the wall plate failing and the appliance switch failing are not related. Neither could cause the other AFAICS. In fact I'd suggest one switch or the other was still capable of passing current up to the point the other failed, despite the scorching.

The appliance switch probably got hot simply from resistance arising from use and age. The wall plate is more of a mystery. The immediate cause of a resistance which gets hot is loose screws holding the wires in. I regularly encounter the cable fixing screws in wall plates loose. so often that the loosening can't possibly be caused simply by sloppy installation. They must loosen after installation. But why?

Its been discussed on here before and my preferred explanation is that copper 'creeps' once under prolonged sustained force, leading to the screws becoming loose. Possibly the screws are not tight enough, or possibly too tight. I've heard of electricians using torque wrenches to tighten wire fixing screws to an exact tightness but I've heard nothing about how effective this is.

I think this problem is the reason for the steady shift towards spring-loaded clip terminals instead of clamping screws.

Might be an idea to check all the other switch and socket plates in the house for loose terminal screws.

XFool
The full Lemon
Posts: 12590
Joined: November 8th, 2016, 7:21 pm
Been thanked: 2600 times

Re: Which switch failed first?

#493388

Postby XFool » April 10th, 2022, 5:58 pm

Mike4 wrote:Wires get hot where there is an electrical resistance. I'd suggest the wall plate failing and the appliance switch failing are not related. Neither could cause the other AFAICS. In fact I'd suggest one switch or the other was still capable of passing current up to the point the other failed, despite the scorching.

The appliance switch probably got hot simply from resistance arising from use and age. The wall plate is more of a mystery. The immediate cause of a resistance which gets hot is loose screws holding the wires in. I regularly encounter the cable fixing screws in wall plates loose. so often that the loosening can't possibly be caused simply by sloppy installation. They must loosen after installation. But why?

Heating and cooling cycles? Just a quick thought.

bungeejumper
Lemon Half
Posts: 8021
Joined: November 8th, 2016, 2:30 pm
Has thanked: 2831 times
Been thanked: 3921 times

Re: Which switch failed first?

#493392

Postby bungeejumper » April 10th, 2022, 6:05 pm

Mike4 wrote:Might be an idea to check all the other switch and socket plates in the house for loose terminal screws.

Thanks Mike, I'm already on that one. I'll also check the rating of the RCD. You never know. :D

BJ

9873210
Lemon Slice
Posts: 974
Joined: December 9th, 2016, 6:44 am
Has thanked: 224 times
Been thanked: 293 times

Re: Which switch failed first?

#493397

Postby 9873210 » April 10th, 2022, 6:13 pm

bungeejumper wrote:
And another thing. Why didn't the consumer unit trip if things were getting that warm?

BJ

This one's easy. It's not the type of fault an over current device is designed to detect.

If you put a 0.1Ω resistor in series with a 240V 10A heater there will be little effect on the performance of the heater or the current and so it will not trip the consumer unit. However the 0.1Ω resistor will dissipate 10W. 10W in a small, poorly cooled enclosure is enough to do damage.

You can get 0.1Ω of resistance from a poorly done connection. The protection against this (other than using good technique) is that connections should not be near anything combustible. Ideally in a fireproof box that is large enough that the outside will not reach dangerous temperatures dissipating a few watts of heat.

My guess at this point would be the two switches failed independently (apart from them both being on the same heavy load)

Itsallaguess
Lemon Half
Posts: 9129
Joined: November 4th, 2016, 1:16 pm
Has thanked: 4140 times
Been thanked: 10020 times

Re: Which switch failed first?

#493413

Postby Itsallaguess » April 10th, 2022, 7:23 pm

bungeejumper wrote:
Or did the isolator switch go down first, and take the heater switch with it?


I strongly suspect it's two completely different types of high-resistance, heat-related issues.

The one in the appliance switch is likely to have occurred at the contacts of the appliance switch itself, over time. This is often characterised by a rocker-switch operation being difficult to manually change position, which you seem to have experienced here.

The one in the wall-switch is likely to have occurred at the rear connections of the wall-switch, and is often a consequence of having to connect up a high-load switch or socket when it's in an 'away from the wall' position, and then the electrician trying to push the switch into the back-box, with little care as to what happens to the tight wiring as they do so.

Pushing back face-plate devices such as wall-switches and sockets like this often shifts the previously-tightened electrical wiring at the rear of the items, and I would always recommend wiring and tightening as normal, then moving the item back into it's back-box position, taking care to blend any wiring into a suitable position in the back-box behind it, pushing up to a point where the item is flush to the wall, but without actually screwing the face-plate to the back-box at that point...

Then, carefully lift the face-place of the item back away from the back-box just enough to be able to get a screwdriver back in to all the electrical connections at the rear of the item, including earth connections, and only then, following that second-tightening operation, should the face-plate be pushed back into the back-box and the face-plate screws inserted and tightened.

The above two-stage process takes care to ensure that the bulk of any bent-wiring positioning that may take place on first push-back, and which might subsequently shift solid-core electrical wiring in their initially-tight connections, are then re-tightened for a final time with most of that wiring positioning having taken place, and which then gives the best chance of maintaining the tightest connection when the face-plates are finally reintroduced to the face-plates...

Cheers,

Itsallaguess

Mike4
Lemon Half
Posts: 7039
Joined: November 24th, 2016, 3:29 am
Has thanked: 1632 times
Been thanked: 3761 times

Re: Which switch failed first?

#493465

Postby Mike4 » April 11th, 2022, 12:24 am

9873210 wrote:
bungeejumper wrote:
And another thing. Why didn't the consumer unit trip if things were getting that warm?

BJ

This one's easy. It's not the type of fault an over current device is designed to detect.


Point of Order...

The RCD is not an over current protection device. That's the function of each of the MCBs. The RCD is looking for tiny current differences between the live and neutral conductors, thereby indicating a leak to earth somewhere in which case it disconnects. Overheating wiring will trip neither type of protection device.

Edit to add: For that, we rely on smoke alarms!

Or the occupier's sense of smell. I occasionally get called out to investigate "a fishy smell" around the area of an electric boiler. I nearly always find some loose terminal screws and some scorched wiring.

csearle
Lemon Quarter
Posts: 4748
Joined: November 4th, 2016, 2:24 pm
Has thanked: 4785 times
Been thanked: 2076 times

Re: Which switch failed first?

#494925

Postby csearle » April 18th, 2022, 9:24 am

FWIW I agree with all that and would only note that, as has pretty much also been said, copper is ductile so it deforms plastically when squeezed in terminals. It will do this quite a bit initially then settle down so it is always a good idea to re-tension the terminals a short while after doing them up.

The torque screwdrivers always seem to leave the terminals slacker that I would instinctively leave them. I have a set and have tried colleague's ones too in case mine is not set correctly.

Another thing is that the nature of the current flowing through the terminals is that there will be a minuscule alternating force exerted as the magnetic field, caused by the current flowing, interacts with any prevailing magnetic field. This over years can, I understand, help loosen the terminals. They certainly can, and do, loosen whatever the reason.

Almost the best thing that a homeowner can do to help prevent fires caused by electrics is to check (or have checked) the tightness of all screw terminals. Electricians for some time now have not been allowed to include any screw terminals in the electrical installation where they are not accessible (That is why for example these round junction boxes are no longer installed.)

If accessories associated with high current consumers (like heaters) start to smell of fish then it might be worth having a look too.

Chris

bungeejumper
Lemon Half
Posts: 8021
Joined: November 8th, 2016, 2:30 pm
Has thanked: 2831 times
Been thanked: 3921 times

Re: Which switch failed first?

#494936

Postby bungeejumper » April 18th, 2022, 10:07 am

Excellent answers. Thanks, all!

I have replaced the switch and the heater, and all is working well. Will get the sparky to check all the screw terminals throughout the building at the next inspection.

BJ

quelquod
Lemon Quarter
Posts: 1009
Joined: November 5th, 2016, 12:26 pm
Has thanked: 193 times
Been thanked: 187 times

Re: Which switch failed first?

#494972

Postby quelquod » April 18th, 2022, 12:39 pm

csearle wrote:Another thing is that the nature of the current flowing through the terminals is that there will be a minuscule alternating force exerted as the magnetic field, caused by the current flowing, interacts with any prevailing magnetic field. This over years can, I understand, help loosen the terminals. They certainly can, and do, loosen whatever the reason.

Almost the best thing that a homeowner can do to help prevent fires caused by electrics is to check (or have checked) the tightness of all screw terminals. Electricians for some time now have not been allowed to include any screw terminals in the electrical installation where they are not accessible (That is why for example these round junction boxes are no longer installed.)

Yet another reason for loosening is electromigration caused by the warmth and electric field across the deformed copper in the screw connection, worst in high current situations.

Over the years like Chris I’ve replaced hundreds of sockets ruined by the effects of poor connections to either the wiring or the inserted plugs, commonly hidden away behind a washing machine or tumble drier, and in inspections it’s entirely commonplace to be able to lift a socket away from the wiring after unscrewing the faceplate. It even used to be common to find the old fused consumer units similarly affected. It’s vanishingly uncommon for homeowners to go round occasionally checking mains wiring connections after all.


Return to “Building and DIY”

Who is online

Users browsing this forum: No registered users and 5 guests