Went to visit a friend today. He has an oil filled electric towel-rail in the bathroom, 700W @ 230v. It's practically brand new and worked fine when [ professionally ] installed, but he's been away in England for several weeks. The whole house has been professionally rewired right back to new consumer unit / panels. When plugged in it runs for 30 seconds, then trips the 30ma RCD unit. I put a meter on the plug [ cord-cap ] terminals and got a reading of about 9000 ohms hot to ground, ie a leakage of about 20ma with the unit cold. I told him to get the electrician back in to fix the problem as he has a serious fault, but why does it trip out after 30 seconds?

Alan

[This message has been edited by Alan Belson (edited 12-15-2006).]

Thermal expansion of some kind in the element inside the oil chamber possibly Alan?.

Could there possibly be enough inductive reactance to reduce the AC current to borderline? I really question the mfg. tolerances on GFCIs. I faulted hot to ground through my Fluke and a variable resistor on a GFCI at work. It didn't trip until about 9mA, which was more than I expected. I had tested one this way in response to a thread here where someone had suggested using a Wiggy to test a GFCI. I seem to remember the Wiggy flowing a current several times that considered lethal.
Joe

Alan

Have a quick look at this link see if it helps

IEE-Understanding RCDs

Kenny

Heating elements can often develop a fault in which they short out to the grounded casing as they heat up.

I'd disconnect the element entirely from the thermostat/timer/switching and megger it, although if you've already read 9 kohms on a low-voltage DC test that probably confirms it anyway.