Thermal cutoffs and thermal protectors are nonresetting, thermally-sensitive devices that are designed to protect electrical appliances and industrial equipment from fire. They are sometimes called thermal one-shot fuses. When ambient temperature is increased to an abnormal level, the thermal cutoff senses the temperature change and breaks the electrical circuit. This is accomplished when an internal organic pellet experiences a phase change, allowing spring-activated contacts to permanently open the circuit. Consequently, cutoff temperature is one of the most important specifications to consider when selecting thermal cutoffs and thermal protectors. Other important considerations included cutoff temperature accuracy, voltage, alternating current (AC), and direct current (DC).
Thermal cutoffs and thermal protectors (one-shot fuses) differ in terms of lead material, lead style, case style and physical parameters. Tin-plated copper wire and silver-plated copper wire are common choices for lead materials. There are two basic lead styles: axial and radial. With axial leads, the thermal fuse is designed so that one lead extends from each end of the case. With radial leads, the thermal fuse is designed so that both leads extend from only one end of the case. Cases for thermal cutoffs and thermal protectors are made from ceramics or phenolics. Ceramic materials can withstand high temperatures without degradation. At ambient temperatures, phenolics have a comparative strength of 30,000 lbs. Physical parameters for thermal cutoffs and thermal protectors include lead length, maximum case diameter, and case assembly length. Some suppliers specify an additional lead length that can be added to the specified length of the thermal cutoff or thermal protector.
Applications and Approvals
Thermal cutoffs and thermal protectors are used in many consumer products and bear various marks, certifications and approvals. Common applications include hair dryers, irons, electric motors, microwave ovens, refrigerators, hot coffee makers, dishwashers, and battery chargers. Products that bear the CSA Mark meet safety requirements from the Canadian Standards Association (CSA). Thermal cutoffs, thermal protectors and thermal fuses that bear the UL Mark meet published standards for safety from Underwriters Laboratories (UL), an independent product testing agency. In Europe, thermal cutoffs and thermal protectors bear the CE Mark to indicate compliance with applicable European Union (EU) safety directives.