Thermal Fuses

Thermal fuses, also known as thermal cutoff fuses or temperature fuses, are critical safety devices designed to protect electrical circuits and equipment from damage caused by overheating. Unlike standard electrical fuses that react to overcurrents, a thermal fuse responds specifically to excessive temperatures, acting as a one-shot, non-resettable thermal cutoff.

This makes them indispensable in applications where a rise in internal temperature, rather than just an electrical fault, poses a significant risk. From consumer appliances to industrial machinery, these components provide a vital layer of protection, preventing fires, equipment failure, and potential hazards by permanently interrupting the circuit when a predetermined temperature limit is exceeded.

How Thermal Fuses Work

A thermal fuse contains a heat-sensitive element—often a fusible alloy or pellet—that melts when ambient temperatures exceed a defined threshold. Once tripped, the circuit remains permanently open, halting power flow until the fuse is replaced. This irreversible action ensures faults are addressed before the system is restarted.

Common designs include the thermal cutoff fuse, thermal cutout fuse, and limitor thermal fuse, each offering precise trip temperatures. These devices are widely used to prevent overheating in critical systems such as transformers, power supplies, and heating appliances.

Industrial Applications of Thermal Fuses

Thermal fuses are essential safety components across a wide range of industrial applications, protecting equipment from hazardous overheating and ensuring operational integrity.

• Motor protection: In electric motors and pumps, a temperature fuse cuts off power if internal temperatures exceed safe limits due to jamming or overload.
• Heating systems: Ovens, boilers, and commercial water heaters use thermal cutoff fuses to prevent runaway temperatures from thermostat failures.
• Power supplies and transformers: Overheating in switching power supplies or transformers can be stopped early with a thermal fuse.
• Lighting: In HID lamps and LED drivers, a heat fuse prevents overheating caused by ventilation failure or internal faults.
• Battery packs: Thermal cutouts are used in battery banks for robotics and backup systems to prevent fires during charging or discharge cycles.
• Electronics: On control boards, limitor thermal fuses are installed near hot components to shut down systems before damage occurs.

Types of Thermal Fuse

Thermal fuses are categorised based on their resettability and the specific mechanism they employ to interrupt the circuit. Understanding these types is crucial for choosing the right protective device for a given application.

• Non-resettable thermal fuses (thermal cutoff fuse): The most common type. Once tripped, they must be replaced. Ideal for critical applications.
• Resettable thermal fuses: These resettable thermal fuses (e.g. PPTC devices) automatically restore function once the temperature drops, suitable for non-critical or temporary overheating conditions.
• Fusible alloy fuses: Designed to melt at a specific temperature, commonly used as heat fuses in power supplies and appliances.
• Pellet-type fuses: Triggered by a temperature-sensitive pellet. Often used in limitor thermal fuse designs for accurate thermal cut-off points.
• Thermal cycling or bimetal fuses: Also known as thermal cutouts, they reset automatically and are more like circuit breakers than single-use fuses.

The choice between a non-resettable or resettable thermal fuse depends heavily on the application's safety requirements, the severity of potential overheating, and maintenance considerations.

How to Choose the Right Thermal Fuse

Selecting the correct thermal fuse is critical for effective circuit protection against overheating. Consider the following key factors:

• Rated temperature (Tf): Choose a thermal cutoff temperature that suits your system’s maximum normal operating range.
• Voltage and current rating: Ensure the fuse can safely handle your circuit’s specs.
• Holding temperature (Th): For resettable thermal fuses, this is the max temperature before tripping.
• Size and mounting type: Options include axial, radial, and surface mount formats, depending on where the fuse is installed.
• Compliance: Always select components certified to relevant standards like UL or VDE.
• Application: Use limitor thermal fuses for precise protection or heat fuses for general safety in heating systems.
• Brand: When you buy thermal fuses, choose trusted manufacturers for consistent trip points and reliability.

How to Install Thermal Fuse

Proper installation of a thermal fuse is paramount to its effective operation and the safety it provides. The general principle is to place the fuse in close thermal proximity to the component or area it is intended to protect.

• Power down: Always disconnect power before replacing or installing a thermal cutoff fuse.
• Position correctly: Mount the fuse close to the heat source (motor winding, transformer, PCB hot spot).
• Mounting methods:
  • Crimping/soldering: Solder leads to the board or wiring using minimal heat.
  • Mechanical mounting: Use clips or clamps if specified.
• Insulate: Use heat-shrink tubing or silicone covers to prevent shorts.
• Avoid bypassing: Never replace a thermal fuse with a standard fuse. Use an exact match for temperature and ratings.
• Check operation: After installation, test the equipment to ensure proper operation.

Get Your Thermal Fuses Online from RS Australia

RS Australia, a division of the RS Group, is committed to providing quality protective components at accessible prices to support the safety and longevity of your systems. We host a wide selection of industrial components sourced from reputable brands like Bourns, Limitor, and onsemi. Our user-friendly website is available at any time, making product discovery and procurement a breeze.

Beyond components like thermal fuses, we offer a diverse range of tools, including PCB electrical transformers, ball switches, and fuse kits. Recognising the importance of prompt delivery, we ensure dependable and fast nationwide shipping throughout Australia. For detailed information regarding our ordering procedures, shipping details, and associated charges, please consult our dedicated delivery page.