imersion heater thermostat fault finding

[Midwest]

result was 5000 Mega Ohm, which is 5 Giga Ohms (GOhm).

Thanks, i thought that would be the answer. I'd prefer to be sure the element is faulty first but so far no one has been able to suggest a method to test the thermostat.

A reading of 5Mohms, would be quite an acceptable reading IMO, but as tel says heating of the element can affect the insulation of the element.

A modern thermostat will have an overheat stat, but that's only to prevent the water overheating and circulating into the cold water tank, its not going to cause the fuse to go. As Westward said, and as I did before, you need to rectify these faults. Not suggesting domestic circuits are more complicated than aerospace circuitry, but its horses for courses; I still would recommend getting a competent sparks to investigate.

 

[Wilko]

Hi - just wondering what instrument you've got with 5,000 meg Ohms fsd ? (Mine only goes to 999).

 

[telectrix]

Hi - just wondering what instrument you've got with 5,000 meg Ohms fsd ? (Mine only goes to 999).

my megger Ir tester reads up to 999 Gig.so ner, ner.

 

[davesparks]

There seems to be a bit of a plumbers approach to fault finding going on here, just replacing parts until it starts working again without a real logical process of fault finding.

If the thermostat has failed closed and the circuit remains live for a couple of hours then someone would have noticed the water boiling or the overheat cutout (if fitted) will have operated.
If the thermostat has failed open a simple continuity test will identify it.

As far as I can see only the supply cableing has been IR tested and not the actual heater element so far?

The cable melting at the terminal in the CU suggests a possible poor connection there.

 

[Andrew B]

If the thermostat has failed closed and the circuit remains live for a couple of hours then someone would have noticed the water boiling or the overheat cutout (if fitted) will have operated.
If the thermostat has failed open a simple continuity test will identify it.

Continuity of thermostat has been checked and is closed as expected with a cold tank so i know it hasn't failed open.

Your might be right that someone would have noticed the water boiling if it had failed closed- although my parents-in-law are old and have medical issues so i wouldn't bet on it. (hence them currently selling the house to move into sheltered accommodation). This thermostat doesn't have a cutout

As far as I can see only the supply cableing has been IR tested and not the actual heater element so far?

I tested between heating element line connections and PE, this gave a high insulation resistance of 5 GOhms. I did actually state this in post #9

just replacing parts until it starts working again

I think that is a bit unfair.
On the first time round I identified a failed mechanical timer and no other faults: continuity, IR and element resistance all good. The mechanical timer is at least 30 years old. So i think it was fair to consider it a failed part and simply replace it.

The second time around a fuse had blown - again all tests came back OK, no obvious cause of the blown fuse. All tests, continuity, IR & element checked out. No visible damage other than the blown fuse. Again with nothing to go on I think it was fair to replace a fuse, consider it was perhaps due to a 30 year old fuse being disturbed when the circuit was first examined.

On the third time round, no blown fuse and no power to immersion heater. clearly continuity issues which on investigation was found to be broken wires due to thermal overload in the CU. Clearly there is an unidentified fault and that's how I've left it.

I'd like to hear what you would have done differently?

 

[telectrix]

suggest clamping feed to heater and waiting for it to heat up. see what happens.

 

[Midwest]

I think that is a bit unfair.

I'd like to hear what you would have done differently?

Don't mind davesparks, he's like that with everyone.

I think you need to go back to basics, back to the beginning.

If your CU has been damaged by this issue, that needs repairing or replacing.

Then get your immersion final radial circuit repaired, tested and inspected, so its functioning correctly, without any load attached to it.

After you have done all that, then you can move onto the immersion heater itself, and its ancillary controls.

Still think, you should admit defeat, and employ a competent electrician, which if your CU needs replacing, you'll have to do anyway.

 

[PEG]

Hi,i think you have done more than expected,to help the in-laws,it may be time for help.
As for if a "normal" spark could tackle the tank side...well,a normal one may have a go,but "special" ones,always have an 86mm spanner and some towels...

 

[DefyG]

What is happening to the fuse wire as that can tell you what sort of fault you are looking for.
Have you re-tested since replacing the timer as its possible that the element was good then but faulty now?
Is the flex between the timer and the element good, not squashed, burnt or damaged in any way?
Are you sure the time switch is connected correctly and you haven't created a dead short as soon as the timer operates, maybe contacts welded together causing permanent dead short?