In an earlier contribution to this discussion,
fred wrote:

In article , Set Square
writes

Sounds like a secondary safety device which trips if the main stat
fails to operate for some reason.

I'll go with this one, it's all to do with health & safety (the good
kind) and the single fault principle. The idea is that a single fault
(say the main thermostat contacts welding closed with age) should not
cause harm to
the user, say by the water boiling in the cylinder & consequent
risks. In this case, the second safety device will cut out, removing
the power and protecting the user. It's such a fundamental principle
in the certification of all potentially hazardous electrical &
electronic equipment that I'm surprised it's taken so long for these
to appear in immersion heaters - kettles & coffee makers all have
them as standard.

I'm sure you're right. It seems a *bit* odd for it to be re-settable though
because if the main stat has welded contacts you shouldn't be able to turn
it on again! [An electric kettle is different because some fool might turn
it on with no water in. When it's re-set and re-filled, it's fine.]
--
Cheers,
Set Square
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