Dual Element Immersion Tripping Fuse

[Rob_Hammered]

I have a dual Element 2Kw/3Kw immersion heater installed on the top of my hot water tank. This is wired through a bath/sink switch & has been working fine for last 2 years. However it trips the consumer unit fuse if switch on either setting. I checked the wiring in the switch & all is fine here. I removed the top to reveal the thermostat.
Would it be worth looking at the thermostat first? or will I replace the entire immersion. If I do this, do I need to drain the tank?
The immersion is on the top of the tank. The tank is gravity fed from the attic .

Any help appreciated as my folks are arriving next week & at the moment only the oil fired central heating heats the water..

 

[JohnD]

Does your consumer unit really have a fuse in it? Or is it an MCB? Or an RCD? What is written on the thing that trips (all the letters and all the numbers, please)

 

[Rob_Hammered]

It has both RCD & MCB. It is the MCB it is tripping as instant the immersion is switched.

 

[JohnD]

Sounds like an internal short in the element, in which case you will have to run off the hot water and fit a new one. Disconnect the wires from the heater first to check that the problem is not caused by a damaged cable.You do not have to drain the entire cylinder, but a pint or two of water will spill out when you unscrew the old heater (provided you have turned off the water supply and opened the hot taps until they run dry).
Or you can get a plumber in to do it for you.

 

[Rob_Hammered]

Drained the tank & removed the old one.. The copper element had corroded so much that the heating wire was visible, broken & touching the other element. Fitted new dual element immersion €27 & added an immersion timer plus double lagged the tank. First bath was a bit rusty coloured but it worked & was a simple enough job to do.