A while ago, a Mr. Coffee maker I have started leaking. I don't know how old it was, maybe 10+ years at least and well-used to be sure. So I opened it up to see where it was leaking. I quickly saw that one of two hoses had developed a crack and I proceeded to apply a small amount of silicone adhesive/sealant (see pic below).



After thinking about it for a bit, I decided against resuming use. Partially because I was not crazy about drinking regularly from a glued tube, but also because the Teflon warming plate was rather rotted. So, I just bought another one - less than $25 and pretty much identical.

Naturally, I started cannibalizing the old unit and I came across this piece (see below).


Of interest to me were the two fuses.


These are maked:
SEFUSE / SF214E /216 o C / 10A / C0921 So, thermal fuses with those ratings. They are probably the same as these.

But, there are two connected in serial. Presumably a fail safe? I don't know when I have seen this arrangement. Is it common? Are thermal fuses inherently less reliable? I mean, I don't see duplicate fuses in the house fuse box?

 

I would assume the thermal fuses were in different positions on the heater assembly to look for a localized hot spot on the heater coil.

 

I would assume the thermal fuses were in different positions on the heater assembly to look for a localized hot spot on the heater coil.

I don't know and I am not sure exactly what you mean. I do not fully understand how it works....as far as I can tell, there are two heat generators a "boiler" and a warmer. The boiler is that KSD303 and the warmer is a resistive element that traverses the aluminum tube, upon which the warming plate sits (see below). But, I don't know what turns the boiler off?

In any event, you can see that the two fuses are attached inline with both heating sources.



Edited to add: Here is a blow up parts diagram - maybe not the exact model, but similar at least.

 

Those are thermal SEFUSE fuses set to open and stay open at a specific temperature (216C) with a load up to 10A (interrupt current). The other inline device is a common thermal cutout switch normally attached a heated surface that can reset after an overtemp event.

Heater elements often crack as a failure mode due to mechanical thermal stress. This crack reduces the resistance at a point in the element making the voltage drop across the crack higher and hence the power greater at that point on the entire heater so heating won't be uniform. I'm just guessing on why.

 

Those are thermal SEFUSE fuses set to open and stay open at a specific temperature (216C) with a load up to 10A. The other inline device is a common thermal cutout switch normally attached a heated plate that can reset after an overtemp event.

Heater elements often crack as a failure mode due to mechanical thermal stress. This crack reduces the resistance at a point in the element making the voltage drop across the crack higher and hence the power greater at that point on the entire heater so heating won't be uniform. I'm just guessing on why.

The two "fuses" are absolutely identically marked - to each symbol - I checked and rechecked (and I just checked again before typing this)...they are the same part. What do you mean by the "other inline device"...it's "wrecking my head"

Edited: Ohhhh I see what you are saying (I think)...the KSD part is a reset-able switch not a heat source. OK, that makes some sense. But the two inline fuses remain. You are saying that it is a fail safe as I originally thought?

Nice design that the one element can boil the water, and run the warmer with the reset-able switch staying off.

 

I wouldn't call it a fail-safe, just extra protection from localized overheating. The two 'fuses' (they are not electrical current fuses) are electrically connected in series to open the heater circuit if there is a overtemp event in either of their physical sensing zones.

 

Ok, I think I understand now - thanks. I don't remember exactly where the fuses were initially, but I think the first picture is pretty much how it was set up. One of the fuses may have been under the cross bar that connects to two spots on the heater.