Why TWO Thermal Fuses?

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
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Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
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Could be a positioning strategy...
http://forum.allaboutcircuits.com/showthread.php?t@819
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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
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On 11/2/2013 9:36 PM, bob haller wrote:

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

under 25 cents

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Christopher A. Young
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On Sun, 03 Nov 2013 02:58:59 -0500, Stormin Mormon

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wrote:

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.
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'Jeff Wisnia[_8_ Wrote: > ;3143616']

>

>

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.
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.
--
nestork


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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.

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On Sun, 3 Nov 2013 01:40:24 -0700 (PDT), snipped-for-privacy@my-deja.com wrote:

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 ASCAP's.

A disk drive is cheaper than its contents, or a lawyer.
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On Sat, 02 Nov 2013 20:19:26 -0400, Jeff Wisnia

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.
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wrote:

Soldering is no problem. Be quick, with a hot iron.
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snipped-for-privacy@attt.bizz wrote:

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

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

Thanks.

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.

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On Saturday, November 2, 2013 5:19:26 PM UTC-7, Jeff Wisnia wrote:

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/
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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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wrote:

Thanks!
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