I decided to make this simple write up after some of my devices; especially the rice cooker and fan constantly blowing up due to the thermal fuse getting faulty yet the current fuse remained intact. As a biomedical engineer, I hadn’t much interacted with thermal fuses among medical devices but I thought it would be good for me do some simple write up about these small components that protect my devices from catching fire.
What is a thermal Fuse?
A thermal cut-off/ thermal fuse is an electrical safety device that interrupts electric current when heated to a specific temperature. Unlike a thermal switch/ thermal start which may automatically reset itself when the temperature drops, the thermal fuse is more like an electrical fuse: a single-use device that cannot be reset and must be replaced when it is triggered.
While these small yet crucial devices may be familiar to biomedical/electrical/electronic engineers and designers, the end user is largely ignorant of the important role thermal cut-off fuses play in keeping them safe on a daily basis. I also didn’t know not until recently. When a thermal fuse functions correctly, nothing may be immediately obvious to the user, not until the device suddenly stops working like what happened to my fan.
But while most people have at least a surface level knowledge of their fuse boxes, most people have no idea how a thermal fuse protects their home, device or medical appliance in hospitals and how it differs from a current line fuse. Some also have inefficient electrical knowledge needed to safely install or replace thermal cut-offs, which is why the repair should always be left to a professional or the manufacturer of the device.
What really happens?
When the temperature in an electrical component exceeds the designed thermal cut-off temperature of the thermal fuse, the thermal element in the fuse melts, interrupting the electrical flow. The thermal fuse functions as an extra line of protection, preventing overheating. But once the component is triggered, it’s permanently destroyed. That leaves some end users wondering if it’s possible to safely replace these devices on their own. So is it possible, and more importantly, is it advisable? Unlike electrical fuses, thermal fuses only react to excessive temperature and not to excessive current (unless the excessive current is sufficient to cause the thermal fuse itself to heat up to the trigger temperature).
The precise answer is NO; leave it to the professionals. Trying to work with an electronic device is more likely to do harm than good. It’s crucial that end users understand that only trained professionals should try and repair electronic equipment that use thermal fuses. In many cases it is not possible to replace a thermal fuse since it may be enclosed and sealed in the device.
Operation principle thermal fuses for most high current application devices
Its based on a small meltable pellet that holds down a spring. When the pellet melts, the spring is released, separating the contacts and breaking the circuit. Different series/ types may use alloy pellets that contain copper, beryllium, and silver to melt at a precise temperature.
Some key terms definitions;
- Functioning temperature; the temperature at which the thermal cut off opens (fuses) while passing 0.1 amp or less in a controlled oil bath at a rise of 1’C per minute. The functioning temperature is not the rated functioning Temperature.
- Rated Functioning Temperature (Tf); rating of the functioning temperature on safety standards. Tolerance of rated functioning temperature is +0,-10’C for IEC 60691.
- Holding Temperature (Th or Tc); the highest temperature of the thermal cut off at which it will not open the circuitry for 168 hours while passing rated current.
- Maximum Temperature Limit (Tm); the highest temperature at which continuity does not occur for 10 minutes after the function of the thermal cut off.
- Rated Current (Ir); the allowable maximum current which the thermal cut off is able to carry.
- Rated voltage (Ur or Vr); the allowable maximum voltage which the thermal cut off/ fuse is able to be applied.
How to fix a blown or faulty Thermal Fuse
- Examine well the device you want to open.
- Choose the right tools that you may need to open the device e.g. long nose pliers, screw drivers, metal wire cutters, digital multi meter, etc.
- As usual unplug the device from the power outlet.
- Unscrew the device to expose the necessary circuitry where the thermal fuse may be located; is it’s present!
- Use a multi meter to test for the continuity across the thermal fuse. (On assumption the reader knows how to use the multi meter specifically for this purpose). In brief, if there is no beep from the multimeter, then there is no continuity across the fuse and thus the fuse is blow. Otherwise if there’s a beep, then it’s not the thermal fuse that is faulty; you’re then advised to check well other electrical components.
- Remove the thermal fuse. Properly uncramp or unscrew the two terminals of this component.
- Replace a new one with the same ratings. Do not use/replace a thermal fuse of a different rating and if you must, then take one with a slightly higher temperature and current.
- Make the connections very tight. Avoid soldering because solder can easily melt depending on the temperatures that the device normally works with.
- Check the new replaced thermal fuse with continuity test using the multimeter. It should be positive with a beep sound.
- Replace the cover and all the screws that were there.
- Plug the device in the power source and it should be working well.
- Take care of your device well. Avoid repairing the device if you’re not a known well conversant with electronics. Please consult a technician to do it for you.
Lastly don’t play with your electrical device because of curiosity.
compiled By BRIAN MATOVU, Biomedical Engineering Makerere University
- IEC 60691:2015 Thermal-links – Requirements and application guide