Yet Another Immersion Heater Tripping Question

[Housey99]

Sorry - another one! We don't use our immersion heater much, but we had a problem with the usual boiler heating method so turned on the immersion. It worked ok the first time ... but a day later, tried it again, all the lights went out.

The immersion is on its own circuit (actually shared with a socket for the shower pump) protected by an MCB. The RCD covers the whole house and it is that which is tripping every time I try to turn on the immersion.

What I've tried so far:
- I've tested the thermostat and it seems to be working correctly. Resistance between terminals is either 0 or infinite (using multimeter). No detectable connection from either terminal to the probe sheath. In any case, I also tried removing the thermostat completely and connecting the element directly to the supply - that tripped when turned on, so its not the thermostat.
- I've tested the resistance of the heating element - 19ohms, which I think is correct. No detectable connection from either terminal to the tank itself, though I'm only using a multimeter and not an insulation tester.
- the cable leading to the immersion heater looked a bit iffy - some heat damaged insulation - so I replaced it with a new rubber sheathed "immersion heater" cable. No difference.
- in case there is something weird going on elsewhere, I plugged a 2kW heater into the shower pump's socket (same circuit as the immersion) and ran it - no problem, from which I infer that the general cabling is probably ok.

What's left?

 

[AdrianUK]

- I've tested the resistance of the heating element - 19ohms, which I think is correct. No detectable connection from either terminal to the tank itself, though I'm only using a multimeter and not an insulation tester.

I think you've answered your own question A multimeter is unlikely to find an insulation fault between the element & the sheath because the test voltage is not high enough. You've eliminated everything else so its time to get the immersion heater out of the tank...when you do, I suspect that you'll find the sheath has split.

 

[Housey99]

I'm afraid you're right. I was just looking for a nice straw I could clutch at...

 

[Murdochcat]

As the immersion element gets older it can lead to leakage from the neutral to the earth and hence tripping the RCD. Unless you have access to an insulation resistance tester it’s unlikely you will be able to confirm this

 

[Taylortwocities]

 

[ericmark]

There has to be some guess work, when you are using extra low voltage to measure MΩ, normally we use low voltage (500 volts DC) to test with. Meter looks something like this but we can if the RCD does not trip, measure how close we are to the limit with a clamp-on meter

we are permitted 30% of the RCD rating so a 30 mA is allowed 9 mA and a 100 mA allowed 30 mA, which TT (used and earth rod) we would often use 100 mA RCDs for the whole house, but it was considered these were too large to protect personal, so we moved to using a pair of 30 mA RCDs with TT, and half the circuits protected with a TN install using a 30 mA RCD, in both cases, since there is a method of moving devices to be powered from a non-protected or different RCD, one did have some way to work out likely cause.

My old clamp-on multi-meter had increments of 10 mA, so with a 100 mA RCD one did have a method to test, but as we moved to 30 mA we needed a clamp-on with 1 mA increments, the ones designed for the job went down to 0.1 mA increments.

With the meter we have an idea how close to the limit we are, it can be we have an earth - neutral leakage, and any load can cause the RCD to trip, as it moves the neutral voltage away from the earth voltage, so current can flow.

The first meter shown uses DC, so may not show all faults, and the clamp-on does use AC of course, but can't test a tripped circuit.

Today we use RCBOs which are MCBs and RCDs combined, so making it a lot easier to test. But they cost more, so basic thing is by saving money you make it necessary to have someone with meters, to work out what has failed.

 

[HERTS P&D]

Just replace the immersion.

Andy

 

[ericmark]

I seem to remember paying around £60 to have mine replaced, the tester cost me £35, so likely replace is best option.

 

[Housey99]

Plumber booked. Thanks for the inputs.