In article , fred writes
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.

It is now law that that immersions without such a device can not be
supplied.
On 1 Apr 2004 BS3456 section 2-21 was withdrawn
BS EN 60335-2-73:1997 was introduced.
The thermal trip must not be automatically resettable (like those in
kettles etc).
Wholesalers and retailers will only have the new standard ones in stock.
There was a shortage of immersion stats in the sales outlets when the
new standard came in but all old standard stocks were removed from the
market.

The introduction of non resetting trips was because of serious scalding
injuries and in some cases fatalities ( an elderly lady in Aberdeenshire
died of a scalding incident just days before the introduction of the new
standard)

The really old stats were much more reliable than cheap recent ones
which I was forever replacing.
--
Z
Remove all Zeds in e-mail address to reply.