"tony sayer" wrote in message
...
In article , Set Square
writes
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.]
FWIW I've just changed one in my daughters flat, had a small BI-metallic
switch type thingy on in series with the thermostat, over temp cut out I
suppose.....
--
Tony Sayer
I agree:
Sounds like a resettable 'over temperature safety switch'! The OP will
probably never have to look at it again? Similar 'overheat' device are
scattered through electrical appliances.
As far as I have seen all electrically operated hot water tanks/cylinders
here have this 'overheat' device; typically it's small black plastic block
which mounts just above the upper thermostat wired in series with one leg of
our 230 volts supply, (I guess in the UK that would be the 'hot/live' 230
volt input lead). If the tank overheats for any reason which might include
low water and therefore maybe steam, in top of the tank, a thermostat goes
short circuit etc. a bi-metallic disc inside the device 'clicks' open a
single contact disabling the input voltage. The bi-metallic (when
everything has cooled down!) can be reset by a red button. The upper
thermostats and this gadget are revealed by removing the cover on the side
of the tank where the wires are connected.
In over 30 years I don't think I've seen one bi-metal that has operated and
they never seem to go faulty, but since they seem to come as part of a new
upper thermostat replacement, I must have several in the junk box! Unlike
thermostats and heater elements.
A typical tank/cylinder, often referred to as "A hot water heater" or "Hot
water tank", contains about 35 Imp. Gals and has upper and lower 3000 watt
230 volt heating elements each with its own upper/lower thermostat which are
adjustable. Instructions regarding setting the water temperature vary.
Depts. of Health demand and will test for a minimum of 160F. But Safety
Organisations will recommend a max. of 120F "To avoid children scalding
themselves"!
Conservationist recommend low temperatures, such that you then need to use
more 'hot' water mixed with cold for a reasonable shower!
As shipped the upper and lower heaters are wired 'Flip-Flop', in other words
the upper heater heats top of tank first. When top reaches the set
temperature the upper thermostat flips over and the lower heater under
control of the lower thermostat then heats the lower section water to its
set temperature. etc.
It is possible by moving one wire to have both upper and lower heaters work
independently, each under control of its own thermostat, for faster
recovery, using a maximum of 6000 watts, which may require heavier wiring!
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