Then to prove that the immersion is faulty, rewire it from a plug and
see if that blows the fuse too. The thermostat will not cause fuse
blowing in my experience, while the element will.
I have changed lots of elements over many years and only once has the
cylinder started to twist and buckle because of tightness, although
the element still undid ok and a new one went in ok. Always start the
undoing process with the cylinder full, even if the element is at the
bottom, the weight of water keeps everything stable, also use a socket
with crossbar if possible.
The element could have shorted to the casing, causing it to draw
more current. This will make the fuse carrier very hot (and the
cable quite likely warm too).
If it's an intermittent short such as a cable fault, then the
fuse carrier won't have time to heat up (although it might be
slightly warm just from normal operation of the immersion heater).
This is a handy technique to diagnose an overload current blowing
a fuse verses a fault current (short circuit) blowing a fuse,
but you have to get there immediately after it happens.
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