Immersion heater problem

2 Nov 2010
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United Kingdom
I recently had a problem with the wiring in my house to the immersion heater. It looked as if it had melted at one point in one of the switches on the way to the immersion heater. Yet this had not blown the fuse (old fashioned fuse box)

So using a work around I connect the immersion heat to a normal plug
and used an extension lead to a plug outside the bathroom to
provide temporary hot water while I fix the wiring.

Now the strange thing. I have unwound the extension lead to stop the cable overheating but after an amount of time the safety button on the extension lead pops. Thinking there was a fault with this extension lead I swapped to another. Exactly the same problem the power goes to the immersion heater for an amount of time then the extension lead trips.

Is this a fault with the immersion heater? A problem with the extension lead not being able to supply the 13A for the immersion heater for a prolonged period?
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I have no idea in your case!

But I would be measuring the current taken by the heater!

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With a 3KW immersion heater you're be right at the top limit of your 13A cable. Chances are the cable won't be rated for 13A continuous use, ergo the heat and the thermal trip on the extension is then popping after a while.
Could it be when the thermostat is switching?
Should make absolutely no difference if 10 min or 10 hours.

Unfortunately there is no control on the quality of imported electrical equipment and much is seriously dangerous.

Any cable running near it's max capacity will warm up and could reach the thermal threshold of the cut out and then trip. 1.5mm flex cable will top out at 16A so running at >12A for a length of time will heat the cable. This is normal behaviour.

Wouldn't say it's necessarily to do with the quality of your extensions, though I must agree that there is a lot of dodgy stuff out there that uses thinner cable more prone to overloads. Any CE/BS tested cable, even if made in the UK to the highest standards and tests will heat up when it's running near maximum.
But this is we told an extension cable which has been unrolled from the drum so cable heat should not be affecting it.

Dodgy stuff also has the appropriate CE and BS markings.

In South Africa I saw some very poor quality Chinese 4 socket extension leads. One had a plug which said it contained a "puse".

They used wire hardly 0.5 mm and one had no fuse in the plug but instead one in the socket box so that the cable was totally unprotected and that with a seriously undersized cable too.

A correctly sized and rated extension lead will carry the rated current of 13A without overheating. But only very few are made to this standard.

Generally I would say don't use more that 50% of the rated current even on reasonably well made stuff.

In this case I even wonder if there is not a fault on the element causing it to take excessive current.

The consumer unit is clearly seriously out dated and I am surprised they are still classed as legal!

immersion heaters are one of very few domestic electrical appliances that use their full rated current for extended periods. Maybe about two hours to heat a fully cold cylinder. So warming of cables is bound to happen.
Its not helped that most are fitted with wrong cable.

A proper job would use heat resistant 2.5 mm cable.

Most use 1.5 mm standard cable.

The 1.5 mm gets gently warm when looped across a space. Much hotter if lying on a surface and dangerously hot if lying on an insulating surface like a towel.

Indeed, I would be surprised that an unwound cable would get to it's thermal limit unless it's sitting on a pile carpet or something insulating but as mentioned quality is not a given and it's maybe not the cable rather the plug unit, where the cutouts are usually located, that may be the problem, causing the thermal switch to trip.

Something must be getting too warm, with the extended high current draw of an immersion element, to trip the switch.
Blar Blar blar...

A extension lead isn't designed to take a continuous load of 13 amps.

Another old thread resurrected.

In this case I even wonder if there is not a fault on the element causing it to take excessive current.
That's not really possible.

The consumer unit is clearly seriously out dated and I am surprised they are still classed as legal!
There is nothing wrong with fuses.

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