In article ,
skilso writes:

Recently a plumber changed our immersion heater and after being called
back twice to rectify afailed thermostat he embarked on other work
nearer home and I ended up changing the thermostat myself.I have now
changed it three times and each time the same problem arose.even though
the thermostat is set to a low temperature the water overheats and the
power wire to the thermostat melts and eventually disconnects.I have
tried a heat resisting wire but this burns too as do the plastic top of
the thermostats.The fuse is 15amp and has not blown atall during these
traumas.The cylinder when connected gets very hot at the top and is
often quite cold below.I wonder if anyone can suggest something before
I purchase another thermostat .Thanking you ...Skilso

I wonder if you have a long thermostat and a short element?
Not that I would expect this to cause a wire to melt, but it
could cause the water to be too hot at the top.

--
Andrew Gabriel