Immersion Heater switch failure advice

[Rannsachair]

Hi folks, I am no electrician, though I have done the basics most of my life - covered electrical in my engineering studies many years ago. My house is 15 years old and wiring as done by the builder. So would appreciate some thoughts on:

So the other morning the wife advised there was no hot water, I had a quick look in the boiler cupboard and could smell something. There are two switches in the boiler cupboard: 1 for the overnight electric immersion and one for the booster:

I found I could not switch the power off, so isolated the power and opened it up:

Looked worse when off the wall:

compared with the unmelted booster switch:

So the immersion heaters are 3 kW, so 3000/240 = 12.5 Amps, the fuse is 13 Amps then onto a 16 Amp circuit breaker.

So how has this happened - I am assuming that the switch is not rated for the current spikes when the immersion is switched on?

Why did the fuse not pop first?

Why is there a fuse here when it goes through a 16 amp circuit breaker?

I have just cut the slightly melted wiring back and fitted 20 Amp switches, replacing the fused 13 Amp switches - should that suffice?

 

[brightspark]

no problem changing them to 20 amp double pole switches as there fed from 16 amp breakers . its not good to have them on a 13 amp switchfuse. loose wires wont help neither 3kw on a 13 amp fuse ran constantly overheats them and the contacts get weaker then burn out ced are cheapo gear

 

[Kayos]

I’ve seen a few melt

Swapped one recently that was on an odd control system, they had used a 6a double as it was the only double pole they could get, made a right pop apparently....

 

[brightspark]

I bet that stunk of fish .I wear gloves so it don't go all-over me

 

[Rannsachair]

I bet that stunk of fish .I wear gloves so it don't go all-over me

The wife complained there was a funny smell in the cupboard, I was not sure so she just put an air freshener in.

New ones fitted:

 

[Rannsachair]

My stepson has told me that the cause is the cheap CED switches have poor contact on the fuse bars causing hot spots. He does not rate the new CED ones I fitted and says they should be fused, so new MK switches now on order.

 

[Barlidge]

Fuses are nowhere near an exact science.

https://www.pat-testing-training.net/articles/fuse-operation-characteristics.php

 

[ronan]

That happened here a few years ago, i found the replacements not much cop to be honest. I don't mind swapping out the switch, but its the worry of a fire starting.

 

[Spark plug]

Items like these are becoming more and more of a mine field to separate the good from the bad, as said stick to known brands.

I’m defiantly seeing more and more burnt out fixtures and fittings, faulty MCB’s etc than I did say 5-10 year ago.

 

[northwest]

I don't know what to buy anymore. I recently had an MK switch break in me and now I have a GEC light fitting giving intermittent trouble.

 

[ronan]

Its the race to the bottom, to ever make things cheaper and squeeze a bit more profit from things.

 

[dobbslc]

I had the same trouble as the OP but I had fitted a "quality " MK spur with a 3kw immersion heater.
The spur was hot when I checked it! Too hot! I swapped it for a proper immersion heater timer and its been ok since....
I think that the 13a spurs are just not capable of taking 12 Amps continuously on a high load. I've seen a lot of them melted in site drying rooms.....

 

[WorkshopChris]

I think that the 13a spurs are just not capable of taking 12 Amps continuously on a high load. I've seen a lot of them melted in site drying rooms.....

I have a feeling its a design flaw (comprise) with the 13a fuse, we have probably the safest and effective plug and socket system anywhere in the world.
But the 13a fuse will happily run a well over 13a for a good while to allow and compensate for inrush situations. The fuse and contacts will get warm over time with a heavy load, when fitted in a good quality plug mated with a quality socket in good condition the amount of metal in the system generally dissipates any excessive heat. In a FCU the reduced metal content combined with modern designed down to the last penny situation leads to excessive heating of internal parts.

 

[jimbo84]

My stepson has told me that the cause is the cheap CED switches have poor contact on the fuse bars causing hot spots. He does not rate the new CED ones I fitted and says they should be fused, so new MK switches now on order.

If your cable between the switch and immersion is 2.5mm flex, there's no need for the switch fused spurs, as brighspark says the 20a double pole switches are spot on. Double back the ends of the flex and cables too, it'll help the screw clamp down onto the cable and maintain a good connection.

If they're on a 16a MCB you'll most likely find that in a fault situation the MCB would trip before the fuse anyway.

 

[Barlidge]

Double back the ends of the flex and cables too, it'll help the screw clamp down onto the cable and maintain a good connection.

I was thinking this also, on the original plate the Load Live (and possibly others) looks to be down the side of the terminal screw so wont be making a good surface area of contact and could be a contributory source of heat build up.

 

[Matrixx]

Looks like a bad/loose connection on the centre live cable causing a slow build-up of heat melting the switch.

 

[Matrixx]

Fuses are nowhere near an exact science.

https://www.pat-testing-training.net/articles/fuse-operation-characteristics.php

They are actually very accurate , designed to blow at very specific times and loads. as per the article.

 

[Seadog]

Loose terminal screw

.