Why TWO Thermal Fuses?

Could be a positioning strategy...

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

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.

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.

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:

belts and suspender approach to make certain they

never cause a failure.

under 25 cents

Cheaper than fire liability claims?

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.

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.

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.

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

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

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.

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.

Thanks!

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

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=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 b efore burning one house. Life safety vs $0.99...worth it.

That's assuming statistical independence. If they use two thermal fuses fro m 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 t wo different manufacturers. But, of course, they don't do that.

To a lawyer, two identical thermal fuses from the same batch probably sound s like the manufacturer was being extra careful. To an engineer, two identi cal 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 hav e 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 the y weren't identical.

Note that if either or both thermal fuses fail "closed" then the coffeemake r 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 identic al drives, with the same date of manufacture, from the same factory, and su bject 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 loo se their data from their "redundant" RAID array.

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.