Do thermal fuses fail from old age?

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On Sun, 25 Mar 2012 07:53:23 -0400, "Stormin Mormon"

A 15A thermal fuse will not blow on a 20A inrush. It needs some time to melt the wax inside the fuse and blow. I'm not sure how much is required to blow it up reliably, but a short circuit across the line should do the trick.
The problem with unequal current distribution is that the fusing current is not predictable. However, you're partially correct. If one fuse blows at some current, the full current load will transfer to the other parallel fuse, which will then have enough current to blow. The problem is that if it takes one minute for the first fuse to blow, it will probably take more than an additional minute to blow the 2nd fuse because of the higher series resistance which is what causes the unequal current distribution in the first place. In effect, such a derangement extends the time it takes for the power to be cut, and a fire prevented.

That's a big if. Presumably, if the heating element was one contiguous device, with no possibility of a partial short to the metal water tank, there would be no thermal gradient across the water tank. However, if the heater was a series of heating elements, distributed in some artistic pattern across the bottom of the water tank, and only a partial short occurs, the resulting thermal gradient will cause one thermal fuse to be much hotter than the other, especially if the water tank is empty.

From: UL 60691 Pg 17 NOTE 2 For reasons of safety, it should be made clear in the documentation that a THERMAL-LINK is a non-repairable item and that, in case of replacement, an equivalent THERMAL-LINK from the same manufacturer and having the same catalogue reference should be used, mounted in exactly the same way. This might explain why every pot warmer I could find has the plastic base solvent welded shut, or metal base riveted together, to prevent (or at least discourage) thermal fuse replacement.
However, in the case of the coffee machine, UL contradicts itself: From: UL 1082 Pg 26 18.1 If an appliance is provided with a thermal cutoff, it shall be secured in place and shall be so located that it will be accessible for replacement without damaging other connections or internal wiring. See 50.6.
So, one spec says it should be un repairable, while another says that it should be accessible for replacement. Toss a coin.
Incidentally, if the thermal fuse blows, one should consider asking why it blew. It isn't always old age or crappy quality. There may be an intermittent or obscure fault causing it to blow.

Look harder and you'll see. What you're suggesting is not in itself unsafe. It is possible to run parallel thermal fuses and still have it perform its intended function. However, it's much safer to put them in series.
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a 15 amp thermal fuse on a high inrush load causes premature failure of the fuse, not from device malfunction but the repeated high current rush.
this can and does cause cause the thermal fuse to fail while the device is fine.
with gasoline so expensive i want to minimize unnecessary calls. they annoy the customer and waste my time.//
I am ordering some high current thermal fuses in the AM
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wrote:

What is the device? I ask because it might be easier to install a thermistor inrush current limiter on the input power leads, than a bigger thermal fuse.
Inrush current limiting devices: <http://www.ametherm.com/inrush-current/ <http://www.rtie.com/category-s/48.htm According to the data sheets they have devices rated from 1 to 36 amps.
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On 3/25/2012 11:08 PM, Jeff Liebermann wrote:

Back about 4 decades when I tried my hand at TV repair, me and the guys I worked with had to replace a lot of what were called "glowbars/globars", they were actually PTC devices hooked to the degaussing coil around the CRT of a TV set to power the degaussing coil for a short period every time a TV set was turned on. When you first turn on a TV or CRT monitor equipped with one you may hear a "GRONK" sound and that's the degaussing circuit. The PTC thermistor allows a big rush of current to the coil until it heats up which causes its resistance to increase stopping the flow of current until it cools off. There could be a way to use a globar and one of those mil spec power resistors in that finned aluminum case that has mounting ears and one flat surface. The resister could be epoxied to the metal in the appliance and hooked in series with the thermistor across the incoming power to temper any inrush current before it hit the heating element circuit. I see several diagrams forming in my mind's eye including one with a low value power resister in series with the heating element connected to the thermistor circuit and another with an NTC device in series with the heating element and the power. The only problem would be coming up with a thermistor that could handle the current of the heating element when it's connected in series with it. Oh well, it's my mind, I'll have fun playing in there. ^_^
TDD
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On Mon, 26 Mar 2012 00:53:26 -0500, The Daring Dufas

That type of thermistor works backwards from the flavor I'm proposing. For the degaussing coil, the thermistor has a low resistance when cold. When it gets warm, the resistance increases dramatically. When installed in series with the degaussing coil, it allows an initial blast of current through the coil, followed by effectively turning itself off.
In the case of protecting the thermal fuse, it's the other way around. When cold, the thermistor has a fairly high resistance, thus limiting the peak inrush current. As the current heats up the thermistor, the resistance decreases, allowing the device to operate normally.

It's not current handling but rather energy handling capacity. If the inrush current surge lasts too long, the thermistor will get rather hot. If this were a design exercise, I would need the steady state current, the peak inrush current, and the approximate time duration in order to calculate the energy dissipated (in joules or watt-seconds) and eventually the maximum thermistor resistance.
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On 3/26/2012 2:13 AM, Jeff Liebermann wrote:

That's why I thought an indirect approach might work and last longer. ^_^
TDD
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That's a PTC (positive temperature coefficient) thermistor. Its resistance goes up with temperature.

Nope. A PTC protection device (a.k.a. "polyfuse" or "polyswitch") is also a PTC device. It has very low resistance when cold. When excessive current flows through it, it heats and becomes open, more or less. It's only reset when the voltage is taken off and it's allowed to cool. It wouldn't work if it had a high resistance when it was cold.
Both devices are PTC thermistors, albeit with different characteristics. NTC thermistors are normally used as temperature sensors.

Any sane person would just buy the one designed for the application.
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the machines are roll laminators that put plastic on paper, think of menus, often they are laminated.
the high inrush current leads to all sorts of troubles, with burned out switches, fried connectors, burned out boards etc..
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That may be true, but what does it have to do with the price of eggs in China?
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wrote:

One of these? <http://www.ledcolaminator.com I do computah work for a print shop that has several of these. I don't recall hearing about any thermal fuse failures.

Do you have any specs or measurements for one of these machines? 1. Operating voltage: 2. Peak inrush current: 3. Steady state current: 4. Duration of inrush surge: If you want, I can then grind the numbers and offer a recommended thermistor. I have that horrible feeling that the power consumption may be high enough that a suitable inrush thermistor might not work (or exist).
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Not only thermal fuses, as you've found. I've also seen compressor thermal overloads hanging loose in the air. With the compressor cooking hot.
One other one I saw, was a dehumidifier (commercial model, mounted in a cellar) that would regularly freeze over. I had a look, and find the freeze sensor hanging in the air, rather than clipped to the (frozen) suction line.
Christopher A. Young Learn more about Jesus www.lds.org .
I used to subcontract electronic repair for a fire restoration company. Most of their jobs were to repair the buildings so the heirs could sell them. They wanted nothing to do with the place their parent or parents died. More than one death was caused by an incompetent repair. A thermal overload failed? No big deal: "he terminals on the wires will push together, so all I need is some tape and I'll charge the suckers a couple hundred dollars." :(
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On Thu, 22 Mar 2012 10:12:07 -0400, Phil Hobbs

Maybe. Such guesswork would be greatly reduced if the original poster would kindly provide the Braun model number. <https://www.google.com/search?&q=braun+coffee+maker&tbm=isch
If the coffee maker is of the drip type automatic variety, then there are two heaters. The upper heater, that heats the water before it goes through the coffee filter, and the warmer at the base, that keeps the pot of coffee warm. Such a derrangement requires two thermal fuses, one for each heater.
Incidentally, I had a really old Mr Coffee maker overheat and melt the plastic case. The thermal fuse never blew. The consensus was that water somehow invaded the Microtemp thermal fuse, and rusted everything in place.
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Jeff Liebermann wrote:

Our Bunn is a Model GRX.
The two thermal series connected fuses are both located side by side on top of the heated water tank.
There was no accessable thermal fuse I could see located on the warmer heater, but maybe one was buried inside it.
Jeff
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On Sat, 24 Mar 2012 13:11:45 -0400, jeff_wisnia
Thanks. Is it GRX-B, GRX-W, GRX-Basic??? <http://www.walmart.com/ip/Bunn-O-Matic-10-cup-Black-Professional-Coffee-Brewer-GRX-B/4891346 <http://www.bunnathome.com/products/velocity-brew/velocity-brew-gr <http://www.bunnathome.com/sites/bunnathome.com/files/GR_use&carebooklet_english.pdf That style would require two thermal fuses. One on top and one on the warmer. I couldn't find anyone selling internal repair parts that might also have an exploded view showing the fuses.
Looks like it has a 3 year warranty. Find your receipt. Support 1-800-352-2866. Call and ask.

Well, that's the most likely to overheat, but it certainly would not require two fuses to do the job. Something is wrong here.

Maybe, but more likely the factory forgot to install one on the warmer. So they put it where it would fit easily, which is next to the upper heater fuse. QA? Whazzat?
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Jeffry Wisnia (W1BSV + Brass Rat '57 EE) The speed of light is 1.8*10e12 furlongs per fortnight.
Jeff Liebermann wrote:

<http://www.walmart.com/ip/Bunn-O-Matic-10-cup-Black-Professional-Coffee-Brewer-GRX-B/4891346
<http://www.bunnathome.com/sites/bunnathome.com/files/GR_use&carebooklet_english.pdf
I'm finding it somewhat hard to believe that the warmer heater would overheat, since there's no thermostat in or on it which could fail in the closed mode and cause such overheating.
I'll await comments from others regarding the need and/or usage of a thermal fuse on the warmer.
Jeff
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On Sat, 24 Mar 2012 18:55:44 -0400, jeff_wisnia

UL coffee maker standards: <http://ulstandardsinfonet.ul.com/scopes/1082.html <http://bbs.dianyuan.com/bbs/u/33/1127541096.pdf Section 18.x for thermal cutoff requirements. Section 23.x for whether a thermal cutoff is required. Table 33.1 shows the maximum temperature rise allowed. As I read between the lines, any part that might rise above the stated temperatures, requires a thermal fuse.
UL 60691 for testing of the thermal fuse: <http://bbs.dianyuan.com/bbs/u/32/1122972217.pdf Section 10.8 discusses its use as a short circuit protection fuse.
You might find this interesting: <http://www.fixya.com/search/p89511-bunn_nhb_coffee_maker/thermal_fuse It's a different model Bunn coffee maker, but the problems are probably similar.

Ah, truth by consensus and acclamation. Much as I like the concept, I've seen it fail far too often.
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the
because
That is because they dont trigger on current(mainly). They trigger when their surrondings get to hot. Imagine a current carrying spring, soldered to the other side with solder of a particural melting temperature. Of course you can also heat them by massive overload in current, but that is not the way they ought to work.
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wrote:

I had one of the hot water dispenser tank settups that goes under the kitchen sink. A few years in, it failed due to the thermal fuse failing for no reason. The original was no longer available and had been replaced by a different type of design, attachment method, etc. Looks to me like they had a problem with them and changed the design.
I would suspect that part of the problem today is a lot of the thermal fuses are being made in places like China with poor quality control.
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Jules Richardson wrote:

Not only that but the vendors at your end don't want them to last anyway. Except for consumer electronics, people don't get rid of old items and buy new ones because they are obsolete with enough frequency to keep them in business.
They really do make it in volume, and not on single sales.
So if your coffee maker did not die after a year or two you would probably be using it for the next 20 years.
The latest innovation in coffee makers is the cartridge ones, where the profit is in the single use cartridges that you must buy.
I am packing up for a crosstown move, after 16 years in this apartment, and I a finding kitchen items that I had when I moved in, some of which are still in use everyday. Some I have had that are still in use, but not daily, since the 1970's, but they are not electric.
Geoff.
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Geoffrey S. Mendelson wrote:

It gives me great joy to beat the manufacturers at that game Goeff.
Instead of spending about an hour driving to and from a Walmart to buy a new Bunn Coffeemaker for $99.00 (plus 6.25% sales tax) I spent less than a dollar on a thermal fuse and maybe half an hour in my workshop fixing it while SWMBO cooked up a batch of eggplant melanzano (yummy) for dinner. (She loves to cook and I love to fix busted things others have to toss out.)
'Course the current miniscule size of todays electronics and my aging eyesight makes it a no-win game for me to try and do much fixing of that kind of stuff those days. 'Twas much easier in the vacuum tube era of my youth 65 years or so ago when I occasionally even ecountered those old fashioned resistors which were just round sticks of carbon composition with right angle solid wire leads wrapped around each end, then painted and color coded with paint dots.
Jeff
PS: Do you find folks screw around with the second "e" in your first name by leaving it out or moving it ahead of the "r"?
My first name has only one "e" in it, but unless it goes from my keyboard to an address or salutation without ever encountering a human the odds are many to 1 that some jerk will assume I don't know how to spell my own name and stick an extra "e" in it for me.
The spelling of my name isn't that unusual:
Google Hits on:
Jeffry    9.3 million
Jeffery 48.8 million
Jeffrey 264.0 million
But yours is the winner...
Geoffrey 86.2 million
Geoffry 0.812 million
Geoffery 1.8 million
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