Switching & protectionRebuild shelfTest and identify

Thermal fuse

Also called thermal cutoff, TCO, one-shot fuse.

A one-shot link that opens at a stamped temperature and stays open. It is why a lot of dryers, kettles, and hair dryers are in the trash. The element is often still good. The fuse opened because something else was wrong.

If you are new to this category

A switch opens or closes a circuit. Some switches are for your finger. Some open when they get too hot and stay open. Those do not reset. A relay is a switch that a small current turns on so it can control a larger current. Use the rating printed on the part.

Click it and listen. Then use a meter on continuity, which beeps when the contacts touch. Do not guess that a small switch can run a motor.

A heater is a chain

Wall power goes through a switch, a one-time thermal fuse, and often a resettable thermostat, then the element. If you remove the fuse because it is open, the element can stay on after something else has already failed. That is how the donor caught fire the first time.

Heaters without their cutout. Nichrome and tubular elements are happy to start a fire if the thermal fuse or tip-over switch is left out of the new circuit.
Mains voltage. Line voltage, and the DC bus just after the rectifier, both kill. A disconnected cord is not a discharged supply.

Specification. The right-hand column is what is typical. The label on the part in your hand wins if they disagree.
What to look for	Usual figure
Marking	A temperature in °C, sometimes a current (10 A, 15 A)
Look	Small axial can, two leads, no polarity
Reset	Never. If it beeps, it has not opened. If it is open, it is done.

Volts are electrical pressure, amps are how much current is flowing, and ohms are how hard the part resists that flow. A higher voltage than the part was built for is how salvage parts die on the bench. If a cell in the table says “read the stamp”, that stamp is the whole specification.

What people actually use it for

Series protection on a new heater, at the same temperature or lower
Understanding why the donor died

Leave this one when

You need a thermostat. This part will not reclose. Also skip any plan that replaces an open fuse with a wire.

Buying a new one is the better idea when

A new thermal fuse of the same temperature and current. They are cheap, and an open one cannot be reused.

How to tell if this one still works

Continuity. Then decide whether you trust it in a new design. Once opened, there is nothing to test.

Step-by-step on the bench: Tell a thermal fuse from a thermostat

Tools you will want: Meter.

Words on this page

Short meanings, so the rest of the entry is readable if you are not already doing this every weekend.

polarity: Which side is plus and which is minus. Reversed polarity ruins a lot of electronics. The symbol on a power brick is the map.
continuity: A meter mode that beeps when two points are connected. It is how you map a switch: beep, then click, then see if the beep changes.

Where people actually find it

6 notes, most useful first. “Leave it” means the part is in that machine and you should not take it: it is dangerous, worn out, or only useful if you already know that exact repair.

Harvest at a glance
From	Call	How many	How often	Where it sits
Microwave oven	Worth it	1	usually	On the magnetron housing or the cavity.
Clothes dryer	Worth it	1–2	always	Blower housing and heater.
Electric kettle	Worth it	1	usually	In series, often against the element.
Toaster and toaster oven	Worth it	1	usually	In the wiring.
Hair dryer	Worth it	1	always	In series with the element.
Space heater	Worth it	1	always	In series.

Worth pullingMicrowave oven1 · usually
In this particular machine
One-shot, temperature stamped.
Where to look
On the magnetron housing or the cavity.
How to get it out
It is a protection part, not a thing to bypass.
The condition to expect
Open means the oven overheated.
Worth pullingClothes dryer1–2 · always
In this particular machine
One-shot. A good one still reads shorted.
Where to look
Blower housing and heater.
How to get it out
Do not replace an open fuse with wire.
The condition to expect
Open is common, and it is evidence.
Worth pullingElectric kettle1 · usually
In this particular machine
One-shot, stamped.
Where to look
In series, often against the element.
How to get it out
Open means it already protected someone.
The condition to expect
Often open.
Worth pullingToaster and toaster oven1 · usually
In this particular machine
One-shot.
Where to look
In the wiring.
How to get it out
Test continuity.
The condition to expect
Often the actual fault.
Worth pullingHair dryer1 · always
In this particular machine
Stamped temperature.
Where to look
In series with the element.
How to get it out
Continuity.
The condition to expect
Often open.
Worth pullingSpace heater1 · always
In this particular machine
One-shot.
Where to look
In series.
How to get it out
Continuity.
The condition to expect
Often open.