Normal questions in design and use

Thermal cutoff is a precise, stable safety component. The TCO would fuse abnormally due to improper design, installment and work environment. In order to use TCO properly, here comes some info for your reference.

Q: Technical Terms— What are the meanings of rated functioning temperature (Tf)Holding temperatureTh)、Maximum temperature limit(Tm)?

A: Pls refer to the explanation of technical terms. (Explanation of nouns)

Q:  Can thermal cutoff be used as current fuse?

A:  The answer is NO. The TCO is only sensitive to temperature, Pls use the CTO strictly under the current and voltage which required.

Q:  Why there is no conduct for TCO sometime after soldering? Why TCO can be open in test sometime after soldering and fixing? How to avoid these kinds of problems?

A: This is maybe caused by the temperature of welding. During the process of welding, the ambient temperature of TCO has reached the functioning point. As a result, the TCO could fuse or serious be damaged. The welding point is too close to the CTO, the welding time is too long, the temperature of welding is too high are some of the wrong behavior.

To reduce and avoid the damage made to the CTO, we suggest customers pay attention to the following tips:

1. The distance between the welding point and the body of the CTO should be more than 10mm. ( As showed in Figure4.)

2. The welding time should be less than 3s.

3. Welding fixture should be applied when the welding is manual. Such as cliping an aluminum block (or other metals which are good at dissipating the heat) with the lead wires to help dissipate the heat.

4.  Immerse the lead wires into the cooling liquid (like alcohol) after the welding.


Q The TCO is connected by rivet. During the whole process, there is no heat source. How could the TCO fuse when the appliances run the test for whole equipment?

A:  Maybe this is caused by the high resistance of the rivet part. When loading the current, the temperature of the rivet part would become higher and pass the heat to the TCO. When the temperature is up to the rated functioning temperature of the TCO, it will function and cut off the circuit.

Q: Can TCO bear tension and pressure? What is the value?

A:  The caseepoxy and lead wires could not be pulled or pushed no matter when it is installed under the normal temperature or when it is used in the environmental temperature. These actions could damage the internal sensing temperature part of the TCO.

Q:  Why do the TCOs function in bulk when they are applied in the appliance devices and put in the oven?

1The buffer temperature created by the heating process of the oven may be high enough to make the TCO function.

 2The temperature beside the oven is not uniform. Some positions are high, the others are low.


3The air inlet and air outlet are blocked because of the amount is too much. Once holes of oven are blocked, the ventilation will be poor and temperature will get higher partly, finally make the TCO fuse off.

Q:  Can TOC be put inside of the liquid or gaseous environments?

A:  That’s forbidden. Due to the craftwork of TCO, it is not allowed to be used under the environment of liquid, just as water or organic solutions, as well as the dewfall condition. It cannot be applied in the environment of corrosive gas (such as sulfur dioxidehydroxide) either.

Q: Why does the TCO function when the heat shrinkable tube is shrunk? How to avoid this kind of situation?

A: During the process of shrinking the heat shrinkable tube, the lead wires should feel the heat and transfer the heat to the thermal alloy. The TCO functions because the heat it gets from the lead wires is high enough. Pls pay attention to this: High temperature or long time of shrinking maybe damage the TCO or make the TCO function.

In order to avoid the damage from the heat shrinking process, we propose that customers should check their craftwork of shrinking. Pls refer to the question “Why do the TCOs function in bulk when they are applied in the appliance devices and put in the oven?”. If you are using air heater during the process, pls guarantee the distance and time is reasonable to ensure the heat transferred to the TCO would not exceed its functioning temperature.

Q:  How to test the functioning temperature when make the incoming inspection?

A: If you have a silicon oil bath used for testing the TCO’s functioning temperature. Blow tips are important.

1. The current passing the TCO is less than 10mA.

2. Put the TCO into the silicon oil bath at a point of Tf-12.(For example, TCO of A4-F series should be put into the silicon oil bath when the temperature achieve 118). You’d better keep the temperature constant for about 30 minutes.

3. Put the temperature probe into the silicon oil bath together with the TCO. Then rise the temperature of silicon oil bath at a rate of 0.5-1 per minute till the TCO fuse-off.

4.The probe should be put beside the TCO as close as possible.

You could use a constant temperature oven with the blowing device to do the test if you don’t have a silicon oil bath. When using a constant temperature oven pls pay attention to:

a. Put the TCO into the silicon oil bath at a point of Tf-12. You’d better keep the temperature constant for about 30 minutes.

b. To ensure the accuracy of temperature inside of the oven, you’d better put the TCO into the oven together with a probe.

c.Then rise the temperature of silicon oil bath at a rate of 0.5-1 per minute till the TCO fuse-off.

The rate of heating up can not exceed 1 per minute after putting in the TCO

Q:   How to install the TCO to let it achieve the optimal performance?

A: There are two ways for CTO to sense the temperature.

1.   The body of TCO to be put close to the heat source. The TCO feel the temperature through the body. As the body of TCO (alloy type) has no direct contact with the thermal alloy, it will take more time when the heat from the body transfer to the thermal alloy. The heat transfer rate is slow in the kind of installation. The fusing movement has some delay.

2.                The lead wires of TCO to be put close to the heat source or to be connected with the heat source directly. The TCO feel the temperature through the lead wires. As the lead wires are connected to the thermal alloy directly, the thermal alloy could feel the heat source once the lead wires feel it. Because the lead wires are no insulated, we suggest customers to put good heat transfer and good withstand voltage bushing outside of the them.

ps.  TCO is not allowed to be suspended inside of the equipment.