Jeff Liebermann wrote:
On Thu, 22 Mar 2012 00:03:41 -0400, jeff_wisnia
wrote:


Over the years I've had a few thermal fuses in houshold appliances go
open for no apparent reason. When I've replaced them with ones with the
same temperature rating they stayed working fine for years more.

A couple of days ago our three year old Bunn coffee maker quit, because
a 141 degree celcius overtemperature thermal fuse on the water tank
opened. It might possibly have happened because the water tank
thermostat stuck closed, but after I repaired it the thermostat cycled
just as it should.


Yep. See UL CHAT (conductive heat aging).
http://www.intercontrol.de/00_img/pdf/article_TH100_en.pdf
UL tests for thermal cutoff problems in appliances. The logic is that
if the thermal fuse is going to fail, it should always fail in the
direction of safety. So, if the cutoff threshold drifts, it should
always drift toward opening early. The same logic applies to thermal
type electrical circuit breakers, which are designed to lower the
threshold value after being tripped a few times.


I was suprised to find that there were TWO identical thermal fuses
connected in series located right next to each other, the bodies were
actually touching. That seemed like a belt and suspenders approach,
unless there's a significant likelihood that a thermal fuse won't open
when it should?


The coffee distillery is double insulated, which means it doesn't need
a ground pin on the power plug. Since a fault can affect either side
of the power line in such a symmertical arrangement, two fuses are
required. Note that the thermal fuses also act as an internal short
protector. I doubt if the fuse died from CHAT. My guess is it had an
internal fault, probably from internal water accumulation during
washing, which blew the fuse.


Jeff


Yet another Jeff.



In response:

The Bunn coffee maker we have uses a 3 pin grounding cord and plug.

The two thermal fuses were definitely in series with each other in the
hot side of the line feeding the thermostat controlling the storage tank
water temperature, and the also feeding the switch which controls the
"keep warm" heater under the carafe.

I think we've about "saucered and blowed" this subjuct by now, thanks all.

Jeff

--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
The speed of light is 1.8*10e12 furlongs per fortnight.