Truma Electric Heater tripping RCD

[pashelley]

Hi, we have a truma gas / electric blown air heater in a compass omega 524. Problem is it has just started tripping the RCD which I understand is when there is earth leakage.
Problem is that the fire works fine on 500w which I assume is both elements in series. When I turn heater up to 1000w or 2000w the rcd trips, but not immediately, sometimes after a few seconds, others after 15-20 minutes.
I have tested the elements and both are showing resistance and appear ok.
It does not trip the mcb at any time.
Im thinking that one of the elements is breaking down under load? ie when 240v is applied. Could this be and if so how do I test to find which element it is?

Regards

Paul

 

[VicMallows]

It does sound like a classic case of earth leakage in the element. The reason why it does not trip on 500W could be that the elements (when in series) do not get hot enough to create the problem; or it could be that the fault location is close to the centre tap and therefore at half the voltage, resulting in a leakage current less than the trip threshold.

You could establish which element is the culprit with an insulation tester ('megger') ; or by trial-and-error disconnecting each in turn.........but if you want to maintain full functionality why bother, as the element is only available as a complete unit anyway! They are about £70. Access involves quite a bit of dismantling.

 

[Dustydog]

Paul
The heater contains two small TODs. Thermal Overload devices. One trips at circa 120 degrees then re trips as it cools. The other hits 180 degrees and fails for ever. They are approx a fiver each and situate at the bottom rear of the heater.
May be worth changing them first.

 

[VicMallows]

Dusty,
Why do you suspect the thermal trips of causing an RCD trip? - seems very unlikely to me. Have you experienced such a problem caused by them?

 

[Dustydog]

Dusty,
Why do you suspect the thermal trips of causing an RCD trip? - seems very unlikely to me. Have you experienced such a problem caused by them?

Hi Vic
I had RCD trips. It transpired both TODs were very brittle and breaking down.
I am not a spark but in my case it fixed the problem. A cheaper first option before buying a new element. That said if I had to change the element I'd still change the TODS to be on the safe side

 

[VicMallows]

It transpired both TODs were very brittle and breaking down.

Interesting. Reminiscent of old distributor caps! .. especially in the hot, dusty , situation they are in (and mounted on a metal plate.). The fact that in OPs case it does not trip on 500W may be accounted for by the (weird) switching circuitry placing the over-temp devices in a different part of the circuit when on 500W.

 

[Damian]

Thermal overload breakdown is very common in the Ultraheat.
Main element faults are very rare indeed.

I guess I have had to replace somewhere in the region of 50 over a 10 year period as opposed to no main elements replacements.

 

[pashelley]

Thank you, done the megger test, element gone.

 

[VicMallows]

Thanks for reporting back. Always nice to hear the eventual outcome.
Just hope you don't get a similar failure on the Utrastore element (if you have one)! Had to replace mine a few months ago ....and it's another £70-£80

 

[Hutch]

New circuit boards for a 2013 Thetford fridge, (now come as a repair kit) £177.