Heating element earth trip

Alex1M6 · Oct 19, 2014

Hi,

Can a heating elements resistance to earth depend on its temperature? As I have one here that trips the RCD but when checking the resistance cold there is around 1.5M ohm-7M ohm to earth which shouldn't trip a 30mA RCD. Putting pressure on the element doesn't change it and there aren't any obvious burnt spots where arcing could have occurred.

Could the element still be faulty but only when it heats up? This is from a Kenwood bread maker. Between the element terminals it reads about 90 ohms so it appears to be intact, maybe the elements resistance wire only makes contact with the outside when it heats up.

Thanks.

telectrix · Oct 20, 2014

wids my lady up when she asks me to get the hoover and i ask "do you mean the kirby or the vax. we ain't got a hoover.?"

Alex1M6 · Oct 22, 2014

Thanks for the replies and advice everyone.

Well I don't have a megger so this is what I've done.

I removed everything else from the bread maker (yes PEG, it is my mothers lol) and then using a 48v DC power supply let it slowly heat up. After a minute I felt the heat rising so I disconnected power and put my ohm meter across one of the terminals and earth tab.

There was now less than 50kohm between them, when I leave it to cool down it slowly creeps back up. If its dropping this low with just under 25 watts of heat (48v/91 ohm=0.52 amp*48v=24.96 watts) then I guess it must be dropping low enough to trip the RCD when it reaches cooking temperatures.

telectrix · Oct 22, 2014

from that, i'm certain that a IR test would show it's faulting to E. your ohm meter puts about 3V across it. imagine what 500V would show. buy a new element. sorted.

Alex1M6 · Oct 22, 2014

Thanks, I'll go ahead and order a new one then. Luckily a replacement element is still cheaply available.

I left it running a bit longer to heat up more and saw less than 5kohm when I put the meter on it, more than enough to trip the RCD.

Just for giggles I placed an LED with a small 9v battery between the earth tab and used one of the terminals of the element as common, and sure enough it lights up nice and bright when the element is hot (the 48v supply is first turned off).