"Aidan" wrote in message
m...
"Set Square" wrote in message
...

Surely , it would be safer for the secondary device to be
*non*-resettable?!

Hmmm, yes. I don't know the logic behind this. It might be that the
device could be tripped by a another fault, say the thermostat being
incorrectly wired or having been removed from the immersion heater
pocket. If the secondary thermostat has tripped it wouldn't
necessarily follow that the primary thermostat is defective, although
that would be the most likely cause.

When you say the older pattern can still be sold, does this have a
non-resettable secondary device, or doesn't it have one at all?

They don't have one at all, only the primary control thermostat. I
think the older type can still be bought and fitted. I believe the
legislation was rushed through and there are still stocks of the old
type in the distribution system.

Seen, installed and used electric hot water tanks here (Eastern Canada) for
almost 50 years. As far as I can recall, at least since 1960, of the eight
to ten of our own or other family members, never seen one that did not have
that 'overheat' bi-metal disc cut-out mounted, under cover, just above the
upper thermostat! They never seem to go faulty and I've always assumed they
are there as a heat cut-out of 'last resort'. "Better the tank don't work
than steam come out of the bath taps!"
Deteriorated tanks and corroded heater elements have been something of a
problem here; probably due to the frequent presence of iron in some of our
water supplies and/or ground-water or drilled wells. In fact, while
manufacturers will warranty a tank for five years elsewhere they will, now,
only do so for three years in this part of Canada! We are also starting to
hear of some 'pin-hole' leaks in copper pipes of older houses possibly due
to the same.
Over the years numerous tanks have been discarded, often someone, such as
myself take the thermostats as spares, the upper thermostat always seemed to
include the overheat device, no matter what manufacture or whether it was
Canadian or US made.
So I've always assumed the overheat device was 'standard'.
Previous postings interesting though; cos. British Standards in many
respects, especially electrical, seem to be stringent?
Also UK electrical devices are generally, it seems, substantial and well
made? Although I still don't like those little tiny brass screws and the two
individual base spring plunger contacts for bayonet type bulbs! Are they
still used? Or am I out of date?
Edison threads seems to be so much simpler. And yet cars over here use some
bayonet (and wire-ended) lamps!
Anyway safety of the best made equipment conforming with the best standards
can be invalidated by improper/stupid installation and practices.
This is great group for learning; enjoy the informative postings. Terry.