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fred
 
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In article , Set Square
writes
In an earlier contribution to this discussion,
Ken Knott wrote:

i have just fitted a resettable immersion heater thermostat. It has a
dial to select the temperature and a reset button. Since you can
select the temperature using the dial i don't really understand what
resettable means in this context. can anyone explain? thanks.


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.
--
fred