Immersion heater

24 Dec 2009
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United Kingdom
Can anyone please help on this Christmas Eve. At this moment in time I have no hot water. I have a problem with a blocked pipe in the central heating system (a whole different problem) which requires me to now use the immersion heater rather than the oil-fired boiler.

However when I switch on the immersion heater, it trips the RCD between 0 and 5 seconds. I have connected the immersion switch directly to the immersion heater and it still trips between approx 0 and 5 seconds (therefore eliminating the thermostat). When I disconnected the immersion heater and left the wires in mid-air, the RCD does not trip. I therefore suspected the immersion heater. I then measured the resistance across the immersion heater (live and neutral) and it is 18 ohms (as expected). I connected the immersion to a 40 Amp RCD (rather than the 20 Amp circuit it is usually on) and it still tripped.

I therefore think I need a new immersion heater, but don't understand why I get the correct resistance across the immersion heater. Any other ideas please?
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RCD checks for current leaking to earth. although your element is reading the correct resistance, the element is leaking current to earth.
You can prove this by using a Mega and reading between element casing and one of the terminals i.e. live. You should get a minimum reading greater than 2M ohms anything less and RCD will trip. However, the fact that your RCD is tripping already proves this so new Immersion element required.


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