Thermal Fuses – Working Principles, Types, and Safety Certifications

Published on:

Jul 24,2026

A thermal fuse (also known as thermal cutoff, thermal link – as defined in international standards such as IEC 60691, UL 60691, and GB/T 9816.1) is a temperature-sensing circuit-interrupting device. It detects overheating caused by abnormal operation in electrical and electronic products and interrupts the circuit to prevent fire hazards. Once activated, a thermal fuse cannot be reset and operates only once at its designated melting temperature.

I. Key Terminology

Chinese	English	Definition
额定温度	Rated temperature	The maximum temperature at which the thermal fuse melts. Example: F00240C → Rated temperature 240°C.
额定动作温度（Tf）	Rated tripping temperature (Tf)	According to standards IEC 60691 and GB/T 9816.1, the fuse must trip within the range of rated tripping temperature +0/-10°C. (ARLIN corporate standard: rated tripping temperature +0/-5°C).
保持温度（Th）	Holding temperature (Th)	According to standards, the maximum temperature at which the fuse can maintain its non-conductive state for 168 hours (UL standard) when rated current is applied.
极限温度（Tm）	Ultimate temperature (Tm)	According to standards, after the fuse has tripped, the maximum temperature at which the fuse can maintain its mechanical and electrical integrity for a specified time at twice the rated voltage.
额定电流（Ir）	Rated current (Ir)	The maximum current the thermal fuse can carry.
额定电压（Ur）	Rated voltage (Ur)	The maximum operating voltage of the thermal fuse.

II. Types of Thermal Fuses

1. Organic-Type Thermal Fuse

An organic-type thermal fuse consists of 10 components: isolated lead, sealing resin (epoxy), ceramic bushing, trip spring, star contact, disc (2x), barrel spring, thermal pellet, and case lead.

Under normal operation, current flows from the isolated lead to the star contact, then through the case lead. When the ambient temperature exceeds the rated temperature, the organic thermal pellet melts, releasing the trip spring. The star contact is then pushed away from the isolated lead, opening the circuit and cutting off the current flow, thereby providing over-temperature protection.

Typical manufacturers of this type: MICROTEMP, SCHOTT, ARLIN, SWC, ELMWOOD, etc.

2. Fusible-Alloy Type Thermal Fuse

A fusible-alloy type thermal fuse consists of 5 components: leads, fusible alloy, flux, shell, and potting compound.

As the ambient temperature rises, the specially formulated flux begins to liquefy. When the fusible alloy reaches its melting point, the flux increases the surface tension of the molten alloy, causing it to rapidly contract and form balls at both leads. This opens the circuit, cutting off the current and providing over-temperature protection.

Typical manufacturers of this type: SCHOTT, UCHIHASHI, SWC, SUPFUSE, etc.

III. Industry Standards

Region	Standard
China	GB/T 9816.1, GB/T 9816.2
EU	IEC 60691, EN 60691
USA	UL 60691
Canada	CSA E60691-08
Japan	J 60691 (H28)
Korea	KC 60691 (2016-08)

IV. Safety Certifications

Certification	Description
CCC	China Compulsory Certification – demonstrates compliance with Chinese safety standards.
UL	Issued by UL Solutions – demonstrates compliance with North American safety standards.
cUL	Issued by UL Solutions – demonstrates compliance with Canadian safety standards.
VDE	Issued by the German Association for Electrical, Electronic & Information Technologies – widely recognized in Europe and global markets.
TUV	Issued by TUV – highly recognized in Europe and global markets.
PSE	Mandatory safety certification for Japan.
KC	Mandatory safety certification for South Korea.