Why TWO Thermal Fuses?

[Jeff Wisnia]

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
cold.

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
the coffeemaker.

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
stuck thermostat?

My curious mind wants to know.

Thanks guys.

Jeff

[DerbyDad03]

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 cold.
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 the coffeemaker.
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 stuck thermostat?
My curious mind wants to know.
Thanks guys.
Jeff

Could be a positioning strategy...

http://forum.allaboutcircuits.com/showthread.php?t=40819

[bob haller]

the manufacturer of the coffe maaker is using a beltsand suspender approach to make certain they never cause a failure.

purchased in bulk the extra fuse likely cost under 25 cents

[Stormin Mormon]

Post by bob haller
the manufacturer of the coffe maaker is using a

belts and suspender approach to make certain they

never cause a failure.

Post by bob haller
purchased in bulk the extra fuse likely cost

under 25 cents
Cheaper than fire liability claims?

[c***@snyder.on.ca]

On Sun, 03 Nov 2013 02:58:59 -0500, Stormin Mormon

Post by Stormin Mormon

Post by bob haller
the manufacturer of the coffe maaker is using a

belts and suspender approach to make certain they
never cause a failure.

Post by bob haller
purchased in bulk the extra fuse likely cost

under 25 cents
Cheaper than fire liability claims?

At the factory door in China, closer to $0.025.

[k***@attt.bizz]

Post by bob haller
the manufacturer of the coffe maaker is using a beltsand suspender approach to make certain they never cause a failure.
purchased in bulk the extra fuse likely cost under 25 cents

Probably more (I've been trying to something that cheap).

They're probably in series because they aren't very reliably and
lawyers are expensive.

[nestork]

;3143616']
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 cas
one of them failed to open when the tank temperature soared because of
stuck thermostat?

Most probably.

In order for Bunn to sell that Coffee Maker in the USA, it has to mak
it past UL, or Underwriter's Laboratories. In Canada, it has to b
approved by CSA, or the Canada Standards Association.

If Bunn comes to them with a coffee maker that has a single thermal fus
and says "We want to sell a million of these things in the USA", UL i
going to think: "If that thermal fuse doesn't open to shut off power t
the heating element if the thermostat sticks, then we have a potentia
fire. Even if only one in 100,000 thermal fuses fails to blow, we'r
still talking about 10 potential fires. For the few cents that a secon
thermal fuse costs, it buys a lot more security that power will be shu
off to this thing if it gets too hot." After all, the chances of on
thermal fuse not blowing are low, but the chances both won't blow ar
miniscule, and therefore worth the few cents that a second thermal fus
is gonna cost.

So, most probably UL told Bunn they wanted a second thermal fuse in tha
coffee maker just to be on the super safe side or Bunn designed it wit
two thermal fuses just to streamline the approval process


--
nestork

[l***@my-deja.com]

Post by nestork

Post by Jeff Wisnia
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
stuck thermostat?

Most probably.
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.

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.

Loading Image...

[k***@attt.bizz]

Post by l***@my-deja.com

Post by nestork

Post by Jeff Wisnia
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
stuck thermostat?

Most probably.
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.

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.

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.

Post by l***@my-deja.com
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.

..or UL's lawyers would have been all over them. They're as nasty as
ASCAP's.

Post by l***@my-deja.com
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.
http://www1.picturepush.com/photo/a/11504419/1024/Anonymous/USB-enclosures-power-supplies.jpg

A disk drive is cheaper than its contents, or a lawyer.

[e***@gmail.com]

Yes, if one in one-hundred thousand thermal fuses fails to break circuit (on an average), and just one is in series to the circuit, and they sell a million units, They can expect to burn down ten homes by calculated chance, could be more or less. Or P(fire)= 0.00001x1,000,000=10

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 before burning one house. Life safety vs $0.99...worth it.

[n***@gmail.com]

...But with two TF... P(F)=.00001x.00001x1,000,000=.0001

That's assuming statistical independence. If they use two thermal fuses from the same batch, from the same factory, statistical independence is a very bad assumption.

If the coffeemaker manufacturers were smart they'd use thermal fuses from two different manufacturers. But, of course, they don't do that.

To a lawyer, two identical thermal fuses from the same batch probably sounds like the manufacturer was being extra careful. To an engineer, two identical thermal fuses from the same batch is probably no better than just one.

I have a Mr. Coffee sitting here with two identical thermal fuses which have BOTH failed "open," at the SAME time, after years of use. That surprised me, because for them to BOTH fail "open" in this Mr. Coffee, they must have failed at EXACTLY the same time. But it would surprise me even more if they weren't identical.

Note that if either or both thermal fuses fail "closed" then the coffeemaker will continue to work. So even if they don't fail "closed" at exactly the same time you have the same problem of two identical fuses being no better than one.

The thing that saves these manufacturers in spite of themselves is that is very rare for the thermal fuse to ever NEED to work.

People make the same mistake with RAIDed hard disk drives. They use identical drives, with the same date of manufacture, from the same factory, and subject them to identical usage patterns, and then they're surprised at how "unlucky" they are when two of the drives fail on the same day, and they loose their data from their "redundant" RAID array.

[J. Jones]

Post by n***@gmail.com
I have a Mr. Coffee sitting here with two identical thermal fuses which have

BOTH failed "open," at the SAME time, after years of use. That surprised me,
because for them to BOTH fail "open" in this Mr. Coffee, they must have failed
at EXACTLY the same time

Perhaps they both failed at the same time because they're both in proximity to
a heat source that exceeded their rated temperature -- say, when the
thermostat failed closed and didn't shut off the warmer? I'd guess both
failing open has less to do with some mystery defect in the batch of fuses and
more to do with there being a legitimate failure event that tripped them both.
Thermal fuses don't blow instantly (it takes a few seconds for the fused bit
to melt) so it's quite possible one failed shortly after the other within a
few seconds of hitting melting temperature; cutting power doesn't instantly
reduce the temperature below that point, so after the first popped and killed
the element, the second proceeded to melt anyways and boom, two open thermal
fuses.

[micky]

On Sat, 02 Nov 2013 20:19:26 -0400, Jeff Wisnia

Post by Jeff Wisnia
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.

How did you attach the thermal fuse? I'm afraid to solder them, and I
seem to have nothing good with which to crimp them.

[k***@attt.bizz]

Post by micky
On Sat, 02 Nov 2013 20:19:26 -0400, Jeff Wisnia

Post by Jeff Wisnia
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.

How did you attach the thermal fuse? I'm afraid to solder them, and I
seem to have nothing good with which to crimp them.

Soldering is no problem. Be quick, with a hot iron.

[Jeff Wisnia]

Post by k***@attt.bizz

Post by micky
On Sat, 02 Nov 2013 20:19:26 -0400, Jeff Wisnia

Post by Jeff Wisnia
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.

How did you attach the thermal fuse? I'm afraid to solder them, and I
seem to have nothing good with which to crimp them.

Soldering is no problem. Be quick, with a hot iron.

Yep, that's just what I did.

I thought about using a pair of long nosed pliers with a rubber band
around the handles as a heat sink between the solder point and the fuse
body, but didn't bother.

Jeff

[k***@attt.bizz]

On Sun, 03 Nov 2013 12:00:42 -0500, Jeff Wisnia

Post by Jeff Wisnia

Post by k***@attt.bizz

Post by micky
On Sat, 02 Nov 2013 20:19:26 -0400, Jeff Wisnia

Post by Jeff Wisnia
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.

How did you attach the thermal fuse? I'm afraid to solder them, and I
seem to have nothing good with which to crimp them.

Soldering is no problem. Be quick, with a hot iron.

Yep, that's just what I did.
I thought about using a pair of long nosed pliers with a rubber band
around the handles as a heat sink between the solder point and the fuse
body, but didn't bother.

If you've got the space to do that, they work fine. Forceps are the
perfect tool for this. For $5 at Harborfrieght, can't go wrong.
Harborfrieght is good for some things.

[micky]

On Sun, 03 Nov 2013 12:00:42 -0500, Jeff Wisnia

Post by Jeff Wisnia

Post by k***@attt.bizz

Post by micky
On Sat, 02 Nov 2013 20:19:26 -0400, Jeff Wisnia

Post by Jeff Wisnia
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.

How did you attach the thermal fuse? I'm afraid to solder them, and I
seem to have nothing good with which to crimp them.

Soldering is no problem. Be quick, with a hot iron.

Thanks.

Post by Jeff Wisnia
Yep, that's just what I did.
I thought about using a pair of long nosed pliers with a rubber band
around the handles as a heat sink between the solder point and the fuse
body, but didn't bother.

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.

Post by Jeff Wisnia
Jeff

[l***@my-deja.com]

Post by Jeff Wisnia
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
cold.
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
the coffeemaker.
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.

They probably used 2 thermal fuses in series to avoid the problem GE
had with many of its drip coffee makers produced from 1976-1984
causing house fires when their single thermal fuse failed to melt:

http://www.cpsc.gov/en/Recalls/1991/General-Electric-Voluntarily-Recalls-Certain-Drip-Coffeemakers-That-May-Pose-A-Fire-Hazard/

[bob haller]

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

[micky]

Post by bob haller
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

Thanks!

[Davej]

Post by Jeff Wisnia
[...]
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
stuck thermostat?
My curious mind wants to know.

I suspect they are so cheap that it makes sense to use several
since they each detect heat only in one very small location. Plastic
appliances are quite a fire risk and can turn into balls of flame.