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What Is a Thermal Fuse and How Does It Work?

Author: Apogeeweb
Date: 6 Mar 2021
How can Thermal Fuses be classified


Ⅰ What is a Thermal Fuse?

Ⅱ What is the structure of Fuse?

Ⅲ How can Thermal Fuses be classified?

Ⅳ What are the characteristics of the Thermal Fuse?

Ⅴ What are the types of Thermal Fuse?

Ⅵ How does a Thermal Fuse work?

Ⅶ Precautions for Thermal Fuse

Ⅷ Some Frequently Asked Questions about Thermal Fuse


Ⅰ What is a Thermal Fuse?

A thermal fuse is a new type of electrical overheating protection element. This kind of element is usually installed in heat-prone electrical appliances. Once the electrical appliance fails and generates heat, when the temperature exceeds the abnormal temperature, the thermal fuse will automatically fuse to cut off the power supply to prevent the electrical appliance from causing a fire.


The thermal fuse is the same as the fuse we are familiar with. It usually only serves as a powerful path in the circuit. If it does not exceed its rated value during use, it will not fuse and will not have any effect on the circuit. It will fuse and cut off the power circuit only when the electrical appliance fails to produce abnormal temperatures. This is different from a fused fuse, which is blown by the heat generated when the current exceeds the rated current in the circuit.

Thermal Fuse

Ⅱ What is the structure of Fuse?

Generally, a fuse is composed of three parts: one is the melted part, which is the core of the fuse, which cuts off the current when it is blown. The melt of the same type and specification of the fuse must have the same material, the same geometric size, and the resistance value. It should be as small as possible and consistent. The most important thing is to have the same fusing characteristics. Household fuses are usually made of lead-antimony alloys.


The second is the electrode part, usually two. It is an important part of the connection between the melt and the circuit. It must have good electrical conductivity, should not produce obvious installation contact resistance; third is the bracket part, the melt of the fuse is generally slender and soft, the function of the bracket is to fix the melt and make the three parts a rigid whole for easy installation and use, It must have good mechanical strength, insulation, heat resistance, and flame resistance, and should not be broken, deformed, burned, or short-circuited during use.

Structure of Fuse

Ⅲ How can Thermal Fuses be classified?

The thermal fuse can be divided into:

According to the material: it can be divided into the metal shell, plastic shell, oxide film shell

According to temperature: it can be divided into 73 degrees 99 degrees 77 degrees 94 degrees 113 degrees 121 degrees 133 degrees 142 degrees 157 degrees 172 degrees 192 degrees...


• Commonly used fuse specifications

Commonly used fuse specifications


Ⅳ What are the characteristics of the Thermal Fuse?

Thermal fuse has the characteristics of accurate melting temperature, high withstand voltage, small size and low cost. The thermal fuse shell is marked with the rated temperature value and the rated current value, it is not difficult to identify, and it is very convenient to use. It can be widely used in electrical equipment, electric heating equipment and practical electrical appliances for overheating protection. Thermal fuse mainly has the following parameters:


①Rated temperature: Sometimes called the operating temperature or fusing temperature, it refers to the temperature at which the temperature rises to the fusing temperature at a rate of 1°C per minute under no-load conditions.


②Fusing accuracy: refers to the difference between the actual fusing temperature of the thermal fuse and the rated temperature.


③Rated current and rated voltage: Generally, the nominal current and voltage of thermal fuse have a certain margin, usually 5A and 250V.


Thermal fuse is a one-time-use protection element. Its use affects not only depends on the performance of the element itself but more importantly, on how to select and install the thermal fuse correctly. The thermal fuse is generally connected in series in the circuit when it is used. Therefore, when choosing a thermal fuse, its rated current must be greater than the current used in the circuit. Never allow the current through the thermal fuse to exceed the specified rated current. Before selecting the rated temperature of the thermal fuse, you must understand and measure the temperature difference between the temperature to be protected and the location where the planting fuse is installed.


In addition, the length of the fusing time and the availability of ventilation are also closely related to the selection of the rated temperature of the thermal fuse.


Ⅴ What are the types of Thermal Fuse?

There are many ways to form a thermal fuse. The following are three common ones:

The first type: Organic Thermal Fuse

Organic Thermal Fuse

It is composed of a movable contact (sliding contact), a spring (spring), and a fusible body (electrically nonconductive thermal pellet). Before the thermal fuse is activated, the current flows from the left lead to the sliding contact and flows through the metal shell to the right lead. When the external temperature reaches a predetermined temperature, the organic melt melts and the compression spring becomes loose. That is, the spring expands, and the sliding contact is separated from the left lead. The circuit is opened, and the current between the sliding contact and the left lead is cut off. 


• The second type: Porcelain Tube Type Thermal Fuse

Porcelain Tube Type Thermal Fuse

It is composed of an axisymmetric lead, a fusible alloy that can be melted at a specified temperature, a special compound to prevent its melting and oxidation, and a ceramic insulator. When the ambient temperature rises, the specific resin mixture begins to liquefy. When it reaches the melting point, with the help of the resin mixture (increasing the surface tension of the melted alloy), the molten alloy quickly shrinks into a shape centered on the leads at both ends under the action of the surface tension. Ball shape, thereby permanently cutting off the circuit.


• The third type: Square Shell-type Thermal Fuse

A piece of fusible alloy wire is connected between the two pins of the thermal fuse. The fusible alloy wire is covered with a special resin. Current can flow from one pin to the other. When the temperature around the thermal fuse rises to its operating temperature, The fusible alloy melts and shrinks into a spherical shape and attaches to the ends of the two pins under the action of surface tension and the help of special resin. In this way, the circuit is permanently cut off.


Ⅵ How does a Thermal Fuse work?

When the current flows through the conductor, the conductor will generate heat because of the resistance of the conductor. And the calorific value follows this formula: Q=0.24I2RT; where Q is the calorific value, 0.24 is a constant, I is the current flowing through the conductor, R is the resistance of the conductor, and T is the time for the current to flow through the conductor.


According to this formula, it is not difficult to see the simple working principle of the fuse. When the material and shape of the fuse are determined, its resistance R is relatively determined (if the temperature coefficient of resistance is not considered). When current flows through it, it will generate heat, and its calorific value will increase with the increase of time.


The current and resistance determine the speed of heat generation. The structure of the fuse and its installation status determines the speed of heat dissipation. If the rate of heat generation is less than the rate of heat dissipation, the fuse will not blow. If the rate of heat generation is equal to the rate of heat dissipation, it will not fuse for a long time. If the rate of heat generation is greater than the rate of heat dissipation, then more and more heat will be generated.

And because it has a certain specific heat and quality, the increase in heat is manifested in the increase in temperature. When the temperature rises above the melting point of the fuse, the fuse blows. This is how the fuse works. We should know from this principle that you must carefully study the physical properties of the materials you choose when designing and manufacturing fuses, and ensure that they have consistent geometric dimensions. Because these factors play a crucial role in the normal operation of the fuse. Similarly, when you use it, you must install it correctly.


Ⅶ Precautions for Thermal Fuse

The following items must be observed to ensure the normal operation of the fuse:

1 Each thermal fuse has rated current and voltage, melting temperature (Tf), operating temperature (Th), and maximum temperature (Tm), which must be used under specified parameters.


2 When selecting the fuse installation location, be careful not to shift the stress to the fuse due to the vibration in the finished product and the displacement of other accessories.


3 It must be installed in a place where the temperature will not rise above the maximum operating temperature after the thermal fuse is blown.


4 Can not be used in liquids or in machines where the humidity is maintained above 95%.


5 The thermal fuse should be installed in a place that can only sense the heat source of the thermal fuse. When it is unavoidable in the structure, a thermal barrier should be installed. For example, when installing on a heater, be careful not to connect directly to prevent the hot wire from heating to the thermal fuse


6 To increase the current flow of the thermal fuse, if it is connected in parallel or continues to pass overcurrent and overvoltage, the internal contact of the thermal fuse will be damaged, which will affect its normal operation. Therefore, it cannot be used under the above conditions.


Although the thermal fuse has high reliability in design, the abnormal situation that a single thermal fuse can deal with is limited after all. Coupled with man-made or unpredictable force majeure, the thermal fuse is damaged and cannot function normally, and the circuit cannot be cut off in time when the machine is abnormal. Therefore, when the machine is overheated, when the wrong action directly affects the human body, when there is no circuit cut-off device other than the fuse, and when a high degree of safety is required, two or more thermal fuses with different fusing temperatures should be used.


Ⅷ Some Frequently Asked Questions about Thermal Fuse

1. How is a thermal fuse different from an electric fuse?

An electric fuse is a common name of a thermal fuse. The thermal fuse is of two types.

The one which melts at a certain high temperature

The one which disconnects due to sub-zero temperature as required.

Hypo thermal fuse is made of Biometal but a simple electric thermal fuse can be of any metal or alloy.

There is another fuse that does not blow but disconnects the electric circuit. This is called a magnetic fuse. This used in circuit breaker.


2. Are thermal fuses universal?

If by “universal” you mean “one size fits all”, then no. Thermal fuses come in a range of temperatures. The only ones I’ve bought are to replace failed ones in coffee makers, and I picked ones rated at around 110*C with an appropriate current capacity. Did not search for anything else, but higher current capacity units must exist.

For those who have not run into these devices, they operate like any other fuse in that they are installed in series with the power source, but are designed to be relatively insensitive to current and to open when their temperature exceeds the design point. A valuable safety device in heated appliances.


3. How do I test a dryer thermal fuse?

First of all, understand that once a certain amount of current goes through any kind of fuse, the fuse blows and can only be replaced, not repaired. So then, the only test you really want to do is to see if the fuse can still conduct electricity. Unplug the power cable and disconnect either end of the thermal fuse. Connect any cheap ohm meter to the loose end and the other end. If you get a reading, you may consider the fuse to be good. Don’t have a meter? In that case, you can use an old flashlight bulb (not LED), along with a battery and a piece of wire to test the fuse. Press the base of the bulb against one node of the battery while pressing the opposite end of the battery to one of the 2 fuse connections. At the same time, hold a test wire between the side of the bulb and the other fuse wire. If the fuse is good, the light will turn on.


4. How do I know if my thermal fuse is blown?

Using a digital or analog multimeter, or other resistance-measuring instruments, check the resistance across the thermal fuse (preferably when it’s out of the circuit, which can affect the reading), If you read continuity (in the range of several ohms or less, depending on its rating), the fuse is still functional. If you read an open circuit, the fuse is blown, and has to be replaced.


5. How do you test a fuse using a multimeter?

Testing connectivity is the best way of testing a fuse. A fuse works as long as its two terminals are connected by wire i.e. the two terminals of the fuse are shorted.


If the connectivity test fails then it is sure that the fuse isn’t working. However, there might arise a case if the fuse isn’t using proper material. There might not be any connectivity, however, testing the resistance between the two terminals would give a small non-zero value. Even in such cases, we say that the fuse is working. However, such cases rarely exist and if they do we don’t consider as a good fuse (at least for the small power applications like a household)


6. What is the function of the thermal fuse of an electric fan?

When the oil in the cheap sleeve bearings in the cheap shaded pole motor gets gummy, the motor will start drawing more current and run hotter. If the motor is not re-lubricated in a timely manner, eventually the sleeve bearings will get stuck and the rotor will fail to turn. This results in a locked-rotor condition and the windings draw more current and produce more heat than they can dissipate with no airflow over them to provide cooling. Eventually, the enamel insulation degrades and gets hot enough to smoke, possibly producing shorted turns that draw even more current.


The thermal fuse is a safety device to prevent the cheap motor from actually catching on fire. Sometimes the fuse can be replaced and the bearings can be relubricated to get another year or two of service if the windings haven’t discolored from overheating, but you can be sure that the end is near. You are better off getting a fan motor with sealed ball bearings. They cost more but last much longer, and usually give you a warning by making a rattling noise when the bearings start to wear out rather than seizing silently.


7. What material is used for making electrical fuses and why?

Electrical fuses are generally made from materials having low melting. It acts as a low resistance path when the current flowing through it exceeds its rating by even a small amount. This is done to protect an electrical device from getting damaged. Thus, it acts as an overcurrent protection device. During faults, especially short circuit faults, when heavy currents suddenly flow, the fuse wire gets heated up and melts down, thereby preventing damage and fires from occurring. The fuse wires in general are made of nichrome, etc.


8. What is a fuse?

It’s a safety device used to provide overcurrent protection of a circuit. Its main component is a metal wire/strip that melts when there’s too much current flowing through it and thus interrupts the current. This element can be made of zinc, copper, silver, aluminum, or some alloys. Fuse body is made of ceramic, glass, fiberglass, molded mica laminates or molded compressed fiber.


9. What is the difference between fuse and circuit breaker?

Fuse-it is such a type of device which breaks the circuit one time when overcurrent in the circuit. you cannot break the circuit or open-close according to your choice.

Breaker-it is such type of electrical equipment which breaks when overcurrent, other faulty conditions in the circuit. you can easily control the breaker for opening and closing the circuit ut is such a type of automatic switch. Mainly the big breakers are mainly run with the help of a relay.


10. What causes fuses to blow?

A fuse is a safety device that should protect the rest of the circuit from (more) damage when there is a FAULT in the circuit. This can be caused by:

  • component failure
  • wiring failure
  • placing a load in the circuit that exceeds the Circuits safe level.

Note that some circuits (example: motors) can have a very large starting current and special (slow blow) fuses are designed for this type of load.

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