Dryer Thermal Fuse?

[AcHmed99]

How would a dryer thermal fuse blow open when the thermostat seems to be functioning properly?

My dryer is a Samsung model 5 years old and the thermal fuse (non-resetable) blew today. The heating element has a thermostat that is in series with the fuse. I tested the thermostat on a hot pan and measured the temp with a thermocouple, the thermostat opens (audible) at 250F which is what it should do and re-closes at about 190F. Is it typical to have such a wide hystersis band, about 60F in this case? Do these thermal fuses suffer from fatique like typical fuses? I can't get any data on the temperature the thermal fuse blows but I'm assuming it would be at least 300F. So why would the fuse blow if the thermostat seems to be working correctly?

I'm asking because it's recommended to replace both when one fails.

Thermostat
https://www.reliableparts.ca/product/inv_DC47-00018A_thermostat

THERMAL FUSE
https://www.amresupply.com/part/9279538

[joseph nicholas]

The thermal fuse is there to protect the electronic portion of the dryer.  I you have a short on the circuit board it will blow.  The thermostat monitors the temperature of the inside of the dryer, like inside your coffee pot.  If the inside of the dryer gets too hot it will temporalily disconnet the power.  I think you are comparing apples and oranges here.

[SaabFAN]

Check for clogs in the pipes of the dryer.
The one we had regularly blew the fuse because fibers from the clothes that managed to get through the filters clogged up some area near the fuse. In the end we used at least 3 or 4 fuses every year because of this.

[MosherIV]

I had a similar issue with a rice cooker.

I just replaced the thermal fuse years ago and it is still working today.

My guess is that during normal operation a (sustained) voltage spike causes the heating element to overheat and pop the thermal fuse.
I say 'sustained' because a short spike in voltage will not cause a problem. The over volt either has to be there for 100s ms or the over volt is very high in V, either way causes the heater to over heat.

[Gyro]

I tested the thermostat under load in the dryer with a thermocouple directly beside it and the thermostat failed to open at temperatures of 295F. The thermostat should open at 250F.

Just as well you checked, well spotted - be very grateful to your thermal fuse.

House fires caused by defective tumble dryer models have reached pretty much scandal proportions in the UK....

http://www.bbc.co.uk/news/business-37572532

[Seekonk]

In your case it was the thermostat.  They suggest replacing  both because the thermostat continually opens and closes to maintain temperature. It failed and the fuse likely acted in a protective way The thermal fuse is chemical and they are known to open for no reason.  I've replaced many when nothing else was wrong.

[Gyro]

At a guess, something like welded contacts or melted contact actuator. Something that still allows the bimetal disc to click but no longer actuates the contacts properly. It would be interesting to know for future reference.

P.S. I wasn't suggesting that Samsung were involved in the problem in the UK, just be grateful to the thermal fuse for doing what it was designed to do, when it needed to.

[Seekonk]

Interesting paper.  Some circuit breakers have have coils in them to blow the arc away and make the path longer.  I have to use a compressor with a wand to clean my dryer blower out at least once a year.  Amazing the buildup.  Dryer sheets and laundry additives serve to bind the lint.

[P90]

I've replaced the thermal fuse on a few dryers, and it's surprising just how much lint gets packed in the blower housing and exhaust path. It's recommended to replace both the thermal fuse and cycling  thermostat together.