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I'm repairing a heater that someone threw in the trash (this model):

small fan heater

It has an internal thermostat next to the heating wires, plus a thermal fuse.

internal view

schematic

What is the reason for a fuse in addition to the thermostat? It seems to me that the thermostat alone is sufficient protection against overheating, since the fan does not produce heat.

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  • \$\begingroup\$ It is funny that 'fused' often means "welded together" but a 'fuse' is something that opens a circuit. What's up with that? \$\endgroup\$ – user56384 Jun 8 '17 at 18:06
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    \$\begingroup\$ a fan heater not much different from this one caused the Kaprun disaster. While investigators blamed most of it to modifications, it still is a very good idea to invest in something as simple as a fuse to prevent overheating. \$\endgroup\$ – dlatikay Jun 8 '17 at 19:24
  • \$\begingroup\$ @dlatikay Similarly, the Apollo 13 problem resulted from using an AC rated device with DC: contacts stuck closed. \$\endgroup\$ – user56384 Jun 8 '17 at 21:50
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    \$\begingroup\$ @nocomprende the Latin root at work means to melt or liquify. If two things fuse together it means they melt together into one whole (which is pretty much what welding is); a safety fuse opens the circuit by melting the fusible element. \$\endgroup\$ – hobbs Jun 8 '17 at 23:47
  • \$\begingroup\$ Among other things, it's fairly easy for some styles of resistance heating elements to short-circuit internally. \$\endgroup\$ – Hot Licks Jun 9 '17 at 2:46
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The fan doesn't produce heat, but if the fan never blows, the heating element might overheat and start a fire.

Thermostats fail. Safety regulations generally work on a "single fault" principle. Meaning no single fault in a product should lead to a safety hazard. In this case, the thermal fuse provides a backup to prevent a fire in case the thermostat fails (or, as @winny points out, in case the fan is mechanically blocked).

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    \$\begingroup\$ It may be different outside Europe, but an heating appliance is required to be safe beyond a single fault being thermostat failure. They are allowed to lock the fan from spinning too, so two faults. I suppose the reasoning is the inherent danger of fire being higher from a heating appliance than many other. \$\endgroup\$ – winny Jun 8 '17 at 14:41
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    \$\begingroup\$ Spot on. Fail-on is a failure mode for the thermostat, contacts welded shut. Upvoted. \$\endgroup\$ – TonyM Jun 8 '17 at 14:43
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    \$\begingroup\$ +1 Might be worth noting explicitly that if the thermal fuse is open then it must be replaced with a new fuse of the same type and rating, or the appliance may be rendered unsafe. \$\endgroup\$ – Spehro Pefhany Jun 8 '17 at 15:06
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    \$\begingroup\$ One more important safety advantage of the thermal fuse: if an unsafe condition leads to the fuse opening, it will stay open until manually replaced. A thermostat will recover. If a device is exceeding safe operation, you want it to be shut down permanently. \$\endgroup\$ – Evan Jun 8 '17 at 15:58
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    \$\begingroup\$ @ChrisH huh, the flames are supposed to come out the front! Definitely a fault. \$\endgroup\$ – user56384 Jun 8 '17 at 21:14
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In addition to the fused closed thermostat issue already mentioned, the other issue that can happen with heaters is the thermostat can be arranged such that it measures the ambient air not the temperature inside the heater itself.

This can mean the thing will call for heat but if the fan is not turning or is blocked by that carelessly discarded garment..... (imaginations run wild here)... the coil and internals will get really hot, really quickly. As such a second internal protection device that is more sensitive to the heater coil temperature is required.

Having said that, the proximity of the thermostat and the fuse in that particular heater does not look like it lends itself to that characteristic.

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  • \$\begingroup\$ Isn't "call for heat" the purpose of the temp control (presumably an adjustable bimetal switch/spring)? Then the simple thermostat would be an overheat cutout \$\endgroup\$ – Chris H Jun 8 '17 at 15:11
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    \$\begingroup\$ @ChrisH oh ya.. I didn't notice the extra thermostat... still the point remains. \$\endgroup\$ – Trevor_G Jun 8 '17 at 15:13
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Note that as drawn, if the thermostat opens the fan keeps running to cool the element/enclosure. This might happen if you partially block the airflow or return the hot air into the heater, even if the temperature control realises the room is cold. Then the thermostat will reset and everything is fine (your room just heats up more slowly than you expect). It also protects from failures/complete blockages of airflow.

For the thermal fuse to blow, on the other hand, something must be really wrong. This should be investigated rather than returning to an operating condition, so the fuse will open permanently. In practice most users would throw out the fan and buy a new one rather than investigating (this isn't something that will trouble the manufacturer).

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  • \$\begingroup\$ I want use it as a fan, to blow away fumes. I don't trust this heater. \$\endgroup\$ – user83628 Jun 8 '17 at 15:31
  • \$\begingroup\$ If nothing goes wrong, it will keep blowing (turn the temp control up to max) and the heater off. If something goes so badly wrong that the themral fuse blows, you want it to cut out, because the alternative is a high chance of fire. If the fumes are so bad that you're more worried about them than a slim chance of your fan shutting down, you need proper fume extraction. And I hope the fumes aren't flammable, because I for one wouldn't trust the motor not to spark. \$\endgroup\$ – Chris H Jun 8 '17 at 15:35
  • \$\begingroup\$ I will disconnect the heating wires and use it as a desktop fan while soldering things (that kind of fumes). \$\endgroup\$ – user83628 Jun 8 '17 at 15:42
  • \$\begingroup\$ And blow them out of the window I assume? If the fan shuts down you'll know and you can stop work. Solder fumes are almost all when the fresh solder touches the iron \$\endgroup\$ – Chris H Jun 8 '17 at 15:44
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A fan motor that is stuck (maybe from a worn bearing) or suffers from a winding short - maybe by plain material fatigue, production defect, or foreign object damage - can become very good at producing heat. And smoke for good measure.

After a fan failure, the heating element designed for forced convection heating will act as a radiation heater instead - likely heating (and trapping enough hot air) in the plastic casing to melt (or even light) it within minutes.

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  • \$\begingroup\$ Regardless of the type of catastrophic failure, the outcome is probably going to be either an open circuit (danger over) or a short - which will blow the safety fuse. Because heating appliances (toasters, coffeemakers etc) usually draw close to the limit for an outlet, a normal overlimit fuse will not work: it could not distinguish reliably between "enough current to do the job" and enough to start a fire, because it is the same amount. \$\endgroup\$ – user56384 Jun 9 '17 at 12:38
  • \$\begingroup\$ Winding shorts in transformers or motors can be just the right type of short to create plenty of trouble without tripping panel fuses... Same goes for a mechanically blocked electric motor. Also, you could probably take this kind of fan/heater and literally set it ablaze running and it would take quite some time until it actually opened or shorted... \$\endgroup\$ – rackandboneman Jun 9 '17 at 13:50
  • \$\begingroup\$ You know, they used to make home appliances out of metal. I have never seen steel catch fire. Too bad we aren't really concerned about safety. \$\endgroup\$ – user56384 Jun 9 '17 at 13:54
  • \$\begingroup\$ Oh, steel and iron can catch fire if the geometry is right :) Actually, very fine iron powders tend to catch fire all by themselves. But yeah, no way to set a sheet steel casing on fire. But then, people are afraid of steel cased electrical equipment due to a perceived electrocution risk. \$\endgroup\$ – rackandboneman Jun 9 '17 at 15:18
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The thermostat regulates the temperature within the normal working range. The fuse protects against exceeding that range. You have also already noticed that while the thermostat controls only the heating coil, the fuse powers down the entire appliance.

Their jobs are complementary, not mutually exclusive. They're like a regular brake and an emergency brake.

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This is common in anything with a heater, and also AC motors. If the fan was not working it would cycle on and off still, based on the location of the thermostat. It is there as a failsafe in case the thermostat gets stuck on, otherwise a stuck thermostat could cause catastrophic damage, perhaps even burn your house down. I'm not sure, but I would assume it would have to be there to get CSA approval. The fan motor most likely has one internal to it as well, in case the fan seizes.

source: apprentice electrician that likes to take things apart

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