Thermal Fuses

A thermal fuse, also called a thermal cutoff, is a safety device used in electrical circuits to provide protection against appliances overheating. Thermal fuses are normally supplied in an axial or radial package, although some thermal fuses are now available as a chip.

Like the circuit breakers found in consumer units, the thermal fuse acts as a non-resettable safety mechanism. Their simple yet effective design prevents excessive heat build-up in appliances, mitigating the risk of fire and equipment damage. For more information, refer to our guide to thermal fuses.

How do thermal fuses work?

As a thermal cutoff, thermal fuses work in a similar way to electrical cartridge fuses. When the conditions exceed a certain limit, the circuit will be broken. For thermal fuses, the limit is based on temperature. Thermal fuses typically contain a small pellet within them or have a thin piece of wire which melts at a certain temperature. When the pellet or piece of wire melts, the circuit breaks. This process is called 'blowing out' or 'burning out', which permanently stops the flow of current.

Types of Thermal Fuses

Axial-Lead Thermal Fuses

Axial-lead thermal fuses are defined by having two straight, flexible leads extending from opposite ends of the cylindrical fuse body. This common configuration makes them easy to mount directly onto printed circuit boards (PCBs) or wire terminals. They are widely used in domestic appliances, including coffee makers and small household equipment, where they provide reliable over-temperature protection.

Radial-Lead Thermal Fuses

Radial-lead thermal fuses feature two leads that emerge from the same end of the fuse body and are bent down, creating a compact footprint. This design is particularly useful for vertical mounting on PCBs or in confined spaces within power supplies and electronic modules. They are often used in surge protection devices and transformers where efficient use of space is required.

Chip / SMD Thermal Fuses

Chip or SMD (Surface Mount Device) thermal fuses are tiny, ceramic-based packages designed for direct soldering onto a circuit board's surface. Their distinguishing feature is their small size, which is critical for miniaturised modern electronics. They are applied in battery packs, charging circuits, and specialized industrial controls where high-density assembly is required.

Single-Use Thermal Fuses and Resettable Thermal Fuses

Most thermal fuses are single-use and non-resettable. Once the internal thermal element melts, that thermal cutoff remains open. The thermal fuse will need replacing before you can operate your appliance again. Non-resettable fuses, sometimes called a heat fuse, provide complete protection and removal of the fault before the device can be used.

Resettable thermal fuses are available but tend to have a higher upfront cost than single-use counterparts. These fuses are often polymer-based. For resettable fuses, when the wax pellet within melts, the circuit is broken. As the appliance cools, the wax pellet will cool and solidify. This will complete the circuit again, and your appliance can be used without intervention, making them ideal for devices where temporary heat spikes are common.

What Is the Difference Between Functioning Temperature and Holding Temperature?

All thermal fuses have a holding temperature rating and functioning temperature rating. Understanding this difference between the two is key when selecting the correct thermal fuse for your specific application.

The holding temperature, normally measured in Celsius, defines what temperature the thermal fuse can operate at continuously without blowing. This is known as the limit of the safe temperature range for the fuse. Operating continuously above this temperature will eventually cause the thermal cutoff to activate.

The functioning temperature is the lowest temperature rating (also in Celsius) at which the thermal fuse will blow and the circuit will break. This heat fuse rating guarantees the fuse will open the circuit quickly once this specific trip temperature is reached, preventing dangerous overheating and potential fire hazards in the appliance.

Factors to Consider When Choosing Thermal Fuses

Selecting the correct thermal fuse is important for effective protection and system integrity. Matching the fuse’s ratings to your specific appliance prevents both nuisance tripping and insufficient safety.

• Operating Temperature Range: The most critical factor is the functioning temperature—the lowest temperature at which the thermal fuse will blow. Choose a fuse whose functioning temperature is slightly above the maximum safe operating temperature of your appliance, but below the point of danger.
• Current and Voltage Rating: The thermal fuse must be rated to safely carry the normal operating current and voltage of the circuit without blowing prematurely. For example, ensure the 10A thermal fuse can handle your circuit's steady-state current.
• Mounting Style and Package Type: Thermal fuses are available in axial-lead, radial-lead, and surface-mount (chip) packages. Choose the mounting style that fits your circuit board layout (e.g., radial for vertical placement, or SMD for miniaturised electronics).
• Response Time: Consider the speed at which the fuse must react to a heat spike. Some thermal cutoffs are designed for rapid response, suitable for applications where temperature increases quickly.
• Single-shot vs. Resettable: Decide whether you need a single-use thermal fuse (which must be replaced after tripping) or a resettable fuse (which restores the circuit once cooled). Single-shot types offer permanent protection after a failure; resettable types suit temporary over-temperature issues.
• Compliance and Safety Standards: Verify that the thermal fuse complies with relevant New Zealand and international safety standards. Adherence ensures regulatory compliance and verifies the product's quality and reliability.

Applications of Heat Fuses

Thermal fuses are indispensable safety devices that prevent catastrophic overheating in various domestic and industrial applications by breaking the circuit when temperatures exceed safe limits.

• Household & Small Appliances: Thermal fuses are commonly found in hairdryers, coffee machines, toasters, and irons. They provide localised protection against appliance overheating, preventing fire and equipment damage if a heating element fails or ventilation is blocked.
• Power Supplies & Transformers: In power supplies and transformers, a thermal fuse acts as a backup safety feature. If heat builds up excessively due to an electrical fault or overload, the thermal cutoff activates, interrupting power before the component fails catastrophically.
• Heating Elements & Motor Windings: Equipment with integrated heating elements, such as tumble dryers, furnaces, and electric blankets, rely on thermal fuses to monitor their internal operating temperature. The heat fuse ensures the temperature does not reach a dangerous level, protecting both the product and the user.
• Automotive Electronics: In the automotive sector, thermal fuses safeguard complex electronic modules and wiring harnesses from thermal runaway. They are used in electric vehicle charging systems or heating elements to ensure safety and reliable performance.

Browse Quality Thermal Fuses at RS New Zealand

RS New Zealand is a trusted supplier and distributor of comprehensive thermal fuses and overheating safety solutions. We offer a wide range of thermal fuses and thermal cutoff options sourced from reputable manufacturers like Bourns, Limitor, and Onsemi, ensuring reliable heat fuse protection for every application.

Beyond thermal fuses, we also provide a wide array of other types of fuses, such as car fuses, bottle fuses, and cartridge fuses, catering to all your circuit protection and electrical safety requirements.

Explore our user-friendly online site for the full range of products and have them shipped to your door at your convenience. With our flexible delivery options, you can receive your products precisely when needed. For comprehensive details on delivery times and fees, please consult our Delivery page.