🔥 IEC 60575: Thermal-Links — Requirements and Application Guide

IEC 60575:2021 (Ed.4) | Active | Technical Committee TC 72

📌 Scope and Thermal-Link Operating Principle

IEC 60575 is the international standard for thermal-links (Thermal Cutoffs, TCOs), developed under IEC/TC 72 (Automatic Electrical Controls). A thermal-link is a non-self-resetting thermal protection device: when the temperature of the protected equipment exceeds a predetermined threshold, the fusible alloy inside the thermal-link melts and, under surface tension, separates to permanently open the circuit. Unlike bimetallic thermostats, the thermal-link is a one-shot protective element — once activated, it cannot be reset and must be replaced.

Thermal-links are ubiquitously employed in household appliances (rice cookers, coffee makers, hair dryers, space heaters), power tools, electronic equipment (transformers, motors, UPS supplies), and automotive electronics, serving as the last line of defence in terminal thermal protection. Their design philosophy is “fail-safe” — under the most adverse fault conditions (e.g., cooling-fan stoppage, welded thermostat contacts), the thermal-link must reliably actuate to prevent equipment over-temperature from causing a fire. Per IEC 60575, the rated functioning temperature (Tf) typically spans 60°C to 280°C, with rated voltages up to 250 V AC and rated currents up to 25 A.

📊 Key Parameters and Classification of Thermal-Links

Parameter	Standard Requirement	Typical Values	Test Method
Rated Functioning Temp Tf	Nominal temperature at which the alloy reaches low-enough viscosity to open the circuit	76°C – 280°C	Oil-bath ramp method (0.5–1°C/min), detection current ≤10 mA
Holding Temperature Th	Max temperature at which the link shall not operate for a specified period (168 h)	Tf – 30°C (typical)	Thermostatic chamber aging
Maximum Limit Temp Tm	Maximum temperature the link can withstand for 15 min without mechanical failure after opening	Tf + 50°C (typical)	High-temperature endurance test
Rated Voltage	Dielectric test voltage withstandable in the open state	125 / 250 V AC	2×Un + 1000 V, 1 min
Rated Current	Current carried continuously under normal operation	1 A – 25 A	Constant-current load-life test (1000 h)
Post-Opening Creepage	Insulation distance between link terminals after opening	≥1.5 mm (functional insulation)	Visual + gauge

🔧 Selection and Installation Application Guide

Correct thermal-link selection is a prerequisite for effective thermal protection. The foremost selection parameter is the rated functioning temperature Tf. The selection principle: Tf must be higher than the equipment’s maximum normal operating temperature (including tolerances) but lower than the limiting withstand temperature of the protected materials (e.g., winding insulation, plastic enclosure). Engineering safety margins typically dictate that Tf should be 20–30°C above the equipment’s maximum normal operating temperature, and simultaneously 15–25°C below the thermal endurance class of the insulating material (e.g., Class F = 155°C, Class H = 180°C). The thermal-link must be positioned as close as possible to the heat source, commonly secured by stainless-steel spring clips or riveting onto motor winding end-turns, transformer cores, or heat-sink surfaces.

Electrical load characteristics are another critical selection factor. The thermal-link must interrupt circuit current upon opening; the arc generated at the moment of interruption can damage the link structure or cause re-closure via arcing-weld effect. The standard categorizes loads as resistive (AC-1), inductive (AC-7a), and capacitive, with each category corresponding to a different rated breaking capacity. For inductive loads such as motor windings, a thermal-link model must be selected whose rated breaking capacity exceeds the actual circuit prospective short-circuit current. Furthermore, when a thermal-link is used in conjunction with a thermostat, a minimum temperature differential (typically ≥15°C) must be maintained between their actuation temperatures, so that under normal operating conditions only the thermostat cycles, and the thermal-link triggers solely upon abnormal overheating.

⚠️ Engineering Design Insight: The thermal response time of a thermal-link is an easily overlooked yet crucially important engineering parameter. Unlike conventional fuses, the thermal-link’s operation depends not only on Joule heating from the current flowing through it, but also — and often predominantly — on the heat transferred from the heat source via the thermal path (air, metal bracket, potting compound). Consequently, the mounting method has a decisive influence on protection effectiveness. For example, a thermal-link potted in thermally conductive silicone directly within a transformer winding (direct thermal coupling) responds 5–10 times faster than one relying solely on air convection. In both UL and IEC certification testing, thermal-links must pass operational reliability verification under both “overheat protection” and “temperature-limiting protection” conditions. Designers must ensure that the thermal impedance of the actual installation structure does not exceed that present during the certification test.

🔑 Bottom Line: IEC 60575 is the global foundational standard for the design, manufacture, testing, and selection of thermal-links. Although simple in construction and low in cost, the thermal-link is the final physical barrier preventing electrical equipment from overheating-induced fire. For product safety engineers, correct Tf selection, thorough evaluation of the thermal coupling path, and rigorous verification of breaking capacity are essential requirements for obtaining certification under IEC 60335 (Household Appliance Safety) or IEC 60730 (Automatic Electrical Controls). Any thermal-link selection error can, under extreme conditions, lead to catastrophic safety incidents.