I've been having some fun with a cooker, and wondered if others have experience of odd behaviour of the mineral insulation in the elements. After an RCD trip,I found what I considered unacceptably low resistance in an element, (about 2 megohm) and purchased a replacement (at a price that hurt !). On checking the replacement I found it wasn't that much better, at about 5 meg. Now this is a good brand spare, supplied in a sealed polythene bag. I then tested it hot, and to my horror found it had dropped to under 1 megohm. When it cooled the resistance increased again, reaching a higher level than original. Cycling it several more times, over a couple of days, I now have a hot reading around 5 meg, cold greater than 20. Now, I'm GUESSING that what is happening is that there is water in the mineral insulation, and that when the element heats up this is forced to the cool bits at the end, where the increased moisture presence causes the low resistance. Some is driven right out of the end "seals", so there is a gradual improvement. However, has anyone else had similar experience ? Is this the expected behaviour of these elements ? And worst, have I wasted my money on the replacement? Should I have simply baked it for a while on a non-rcd circuit.
Hi Geoff, I have had similar experiences, cooker elements can a major PITA when protected by 30mA RCD's. The UK trade associations recommend that where the supply is TNS or TNC-S (low loop impedance values), cooker control switches that DO NOT incorperate a socket are fitted, and the circuit supplied from a non-RCD protected MCB. This reduces the problem of tripping due to moisture in the elements. Personally, I wouldn't regard 2 Megs as an unacceptably low reading for a heating element, unless it went really low (below 0.5meg) after heating. You may well have wasted your money here. I would have tried the cooker on a non-RCD circuit, as you suggested, then retested when hot. Is it possible that there could be a fault with the cooker's internal wiring, perhaps damaged insulation which shorts to earthed metal when hot? Just a thought.
#138086 - 08/13/0307:57 PMRe: Cooker element insulation
I wouldn't consider 2 meg to be unacceptably low either, although most new elements certainly seem to megger far above that value.
Even at 1 meg insulation though, it still shouldn't trip the RCD, as the leakage will be only 240uA. What's more likely happening with the faulty element is that it's shorting straight out to the casing at some point when it gets hot.
Did you meg the cooker's internal wiring by the way? I've found grease deposits can cause problems sometimes.
#138087 - 08/18/0302:36 PMRe: Cooker element insulation
Thanks for the advice. I do feel better now about putting the cooker back into service with what I considered still rather poor insulation resistance even after repair. I checked the cooker wiring with the elements disconnected and that was fine. There was no external evidence of a hard fault on the element I replaced, but it tripped several times during warm up, not at switch on, and worked OK on a non-RCD circuit. Anyway I've discarded it now. I'm still surprised at the way the insulation resistance drops so dramatically as the element warms up, even with new ones.
#138088 - 08/19/0308:35 AMRe: Cooker element insulation