RCD Tripping - IR readings of Electric Fan Steam Oven.

[eveares]

Hi all,

Want to know what sort of IR readings I should be expecting between the case and the various elements (with the elements disconnected of course) of my Electrolux Steam fan oven as in recent times it has been tripping the RCD.

I have had it switched off at the local isolator for the last few months as I have a second Electrolux fan oven above it, what is the main oven for daily use. The faulty oven in question was installed in 2013.

IR readings at 500V taken with my Megger MFT1730 are as follows. Don't know if there is a separate steam element for the steam tray/pit, but I could only see three elements when I took the back cover off.

500V IR Readings:

Grill element: 0.22 MΩ
Fan element: 1.4 MΩ
Bottom element: 0.35 MΩ

So my question, is it a combination of the grill and bottom element's low IR reading what is causing the nuisance RCD trips, or is it leaking current to earth through other means as well?


Regards: Elliott.

 

[ericmark]

Although the readings are low, that is only around 2 mA to earth, however I know when I have come to use an appliance with a mineral insulated element the unit will often switch on, but then trip as warming up, the moisture clearly moving in the elements, and once hot it will often drive out the moisture and with daily use no problem until again left to stand.

I will hold up my hands and say I have never tested an installation for earth leakage with a clamp on ammeter, I know we should, as impedance is not the same as resistance and my insulation tester uses DC so measures resistance not impedance. I am told 9 mA is the maximum when using a 30 mA RCD however simply have no way to measure. My clamp on meter only goes down to 100 mA so rather useless for that job.

As far as I am concerned nuisance RCD trips are when there is no reason for it to trip, since 1MΩ is considered as the normal pass point, to trip with 0.22MΩ is not nuisance RCD trips, but I would not expect that to cause a trip when not using the oven, the neutral - earth current is dependent on the voltage differential between neutral and earth, so for those values to cause a trip while not being used I would be looking else where, but because I know as element heats up the moisture moves if it only trips is use, then yes likely the elements causing the problem if it trips the RCD when used.

We tend to think of our own installation, and I am all RCBO's, so yes would worry me if my cooker tripped, but with MCB's and a RCD covering many circuits it would not.

 

[eveares]

Thanks ericmark, not sure what you mean by you would not classify it as a nuisance trip, but I simply meant nuisance trip as it is a nuisance when your using the oven and it causes the RCD to trip and following using the oven and it causes the RCD to constantly trip time after time until the oven has fully cooled down.

I did apply ohms law to the IR readings and thus calculated the leakage current, and as what as you said, the calculated leakage current was very low and well below the required threshold for a RCD to trip. Of course there is going to be some other leakage current from other circuits that the RCD also feeds. I suspect if re-did the IR tests following heating the oven up, I would get very different results.

I have a leakage current clamp meter that measures in 0.1ma increments, so will also use that in my further testing.

Regards: Elliott.

 

[ericmark]

A nuisance trip is when it trips without good reason, but there is a good reason for it tripping so it is doing it's job, the mineral insulated heating element has always been a problem where the end seals allow moisture to enter, the mineral used is a hygroscopic substance that is one that readily attracts water from its surroundings, if you can get the element hot it will often drive out the moisture but clearly it travels while doing that and the resistance to earth will change as it is being driven out, which means it can fall below the threshold where the RCD trips.

Where an oven has been in storage it is some time worth driving out the moisture as once in regular use it never has chance to absorb it again, but where it is in normal use, what ever normal is for that household, then it is only a temporary fix, as clearly the end seal has gone and it will simply absorb the moisture again, so only option is new elements.

Having seen you so many times on this forum I am sure you can work out how to drive the water out, but it is a dangerous process, so don't post how it is done, others may copy and not take the safety measures required. I consider if some one has the intelligence to work out how, they also have the intelligence to work out how to do it safely, but in your case I would say not worth the effort or risk, as it will just sit there again unused and your back to square one. Only real option is new elements.

 

[eveares]

but in your case I would say not worth the effort or risk, as it will just sit there again unused and your back to square one.

Yea, the steam fan oven only gets used when we have lots of people around like at Christmas time.