SWMBO went to make a pot of coffee with our Bunn coffeemaker and
reported to me that the water which flowed through the ground coffee was
I checked for the obvious and then drained the unit and took it down to
my basement shop for a look see.
I quickly found the problem was an open 141 degree C thermal fuse
mounted on top of the heater tank. That fuse was the first thing in
series with the hot side of the power cord and fed everything else in
But, I was surprised to find that there wasn't just one thermal fuse
there but two identical ones in series, clamped to the top of the tank
within a millimeter of each other.
I resisted the temptation to bypass the open thermal fuse and was
pleased to find that the Radio Shack store a quarter mile from our home
stocked them. Fifteen minutes later I was back home with a new $1.99
thermal fuse, installed it and the Bunn was back in business.
But why does Bunn use TWO identical thermal fuses in series? Are those
little suckers so unreliable that they felt the need to use two in case
one of them failed to open when the tank temperature soared because of a
My curious mind wants to know.
In order for Bunn to sell that Coffee Maker in the USA, it has to make
it past UL, or Underwriter's Laboratories. In Canada, it has to be
approved by CSA, or the Canada Standards Association.
If Bunn comes to them with a coffee maker that has a single thermal fuse
and says "We want to sell a million of these things in the USA", UL is
going to think: "If that thermal fuse doesn't open to shut off power to
the heating element if the thermostat sticks, then we have a potential
fire. Even if only one in 100,000 thermal fuses fails to blow, we're
still talking about 10 potential fires. For the few cents that a second
thermal fuse costs, it buys a lot more security that power will be shut
off to this thing if it gets too hot." After all, the chances of one
thermal fuse not blowing are low, but the chances both won't blow are
miniscule, and therefore worth the few cents that a second thermal fuse
is gonna cost.
So, most probably UL told Bunn they wanted a second thermal fuse in that
coffee maker just to be on the super safe side or Bunn designed it with
two thermal fuses just to streamline the approval process.
On Saturday, November 2, 2013 7:47:49 PM UTC-7, nestork wrote:
In the US there's no requirement for products to be safety approved, but
I had a $4 no-name Walmart drip coffee maker that was ETL approved that
had only one thermal fuse in series with the bimetal thermostat.
Here are some UL approved and non approved power supplies for external
hard disk enclosures. The one on the bottom is really bad and has a
low voltage circuit board (swung to the left) that normally sits right
above the high voltage board, separated only by a thin but tough piece of
plastic. The dealer claimed it was UL approved, but when I pointed out
it wasn't, they corrected their website in 30 minutes. Another customer's
hard disk blew out in such an enclosure, and the dealer not only refunded
his money but also paid for his hard disk, which had been bought elsewhere.
The AC adapter in the middle pictures, included with Bytecc and some other
brand USB hard disk enclosures, is also not safety approved, and despite
the fact it takes a 3-wire AC cord, the ground connection is fake. The
photos at the top are for a UL approved AC adapter that replaced it.
On Sun, 3 Nov 2013 01:40:24 -0700 (PDT), firstname.lastname@example.org
Correct. There is no requirement at all to have products safety
approved but it does leave one wide open to lawsuits. The first
products a former employer designed weren't approved at all. We then
went to UL for a contract, and ETL for everything else.
..or UL's lawyers would have been all over them. They're as nasty as
A disk drive is cheaper than its contents, or a lawyer.
Yes, if one in one-hundred thousand thermal fuses fails to break circuit (o
n an average), and just one is in series to the circuit, and they sell a mi
llion units, They can expect to burn down ten homes by calculated chance, c
ould be more or less. Or P(fire)= 0.00001x1,000,000
But with two TF... P(F)=.00001x.00001x1,000,000=.0001
Or, having sold a million, there would be a 1:10,000 chance of any fires at
all. They'd have the probability that they could build ten-billion units b
efore burning one house. Life safety vs $0.99...worth it.
Thanks. I have a little set of 5 heat sinks, spring clips in two
sizes, one with the jaws bent 90^, and a fifth one with a metal clad
magnet connected to it. I also have forceps that I got at a
hamfest. But it sounds like I can do without all that if I'm quick,
and I'm quick if I try to be.
soldering thermal fuses leads to premature failure. it stresses the fuses.
I service roll laminators for a living, one manufacturer had mass thermal fuse failures, traced back to soldering them.
there are crimp connectors made just for thermal fuses
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