Temperature rise testing is a fundamental safety evaluation for household electrical appliances, verifying that accessible surfaces, internal components, and windings do not exceed safe limits during normal and abnormal operation. This article explains the technical principles, applicable standards, test procedures, and compliance requirements of temperature rise testing, providing manufacturers and engineers with a practical reference for product safety assessment.
Technical Overview
Temperature rise testing () measures the increase in temperature of an appliance and its components relative to the ambient temperature while the appliance operates under specified conditions. Excessive heating is one of the most common causes of electrical appliance failure, capable of degrading insulation, melting plastic enclosures, damaging electronic components, igniting adjacent materials, and causing burns to users. The test therefore serves as a core safety evaluation required by virtually all household appliance safety standards.
The primary international standard governing this test is IEC 60335-1, "Household and similar electrical appliances – Safety – Part 1: General requirements," which specifies temperature rise limits and measurement procedures during normal operation (Clause 11) and abnormal or faulty operation (Clause 19). In China, the equivalent national standard is GB 4706.1; in Europe it is adopted as EN 60335-1; and in North America, UL standards such as UL 60335-1 apply corresponding requirements. Product-specific requirements are defined in the Part 2 standards of the IEC 60335 series, for example IEC 60335-2-30 for room heaters, IEC 60335-2-15 for appliances for heating liquids, and IEC 60335-2-24 for refrigerating appliances.
Temperature rise is expressed as a difference rather than an absolute temperature: Δt = Tmeasured − Tambient. This approach normalizes results so that test data obtained in different laboratory ambient conditions can be compared against standardized limits. Measurements are performed using thermocouples conforming to IEC 60584 for surfaces, components, and air, while winding temperatures are typically determined by the change-of-resistance method, which calculates average winding temperature from the increase in electrical resistance.
Testing Samples
Heating Appliances
Products that generate heat as their primary function, such as electric kettles, toaster ovens, room heaters, hair dryers, irons, rice cookers, and electric frying pans. These appliances are tested at rated power input under normal operating conditions, with special attention to heating elements, accessible metal surfaces, handles, and knobs.
Motor-Operated Appliances
Products such as vacuum cleaners, food mixers, blenders, washing machines, fans, and power tools for household use. Testing focuses on motor winding temperature rise, bearing temperatures, and enclosure surfaces, and includes stall and overload conditions to simulate abnormal operation.
Combined and Electronic Appliances
Appliances incorporating both heating and motor functions or containing power electronics, such as microwave ovens, air conditioners, refrigerators, induction cookers, and dishwashers. Testing evaluates transformers, PCB traces, electrolytic capacitors, internal wiring insulation, and electronic component temperatures in addition to accessible surfaces.
Components and Materials
Individual parts used within appliances, including switches, thermostats, connectors, power supply cords, lampholders, and insulation systems. Component temperature data is used to verify that each part operates within its marked temperature rating (such as T-markings on capacitors and switches) when installed in the end product.
Testing Items
Temperature rise testing typically evaluates the following parameters:
- Accessible surface temperature rise: enclosures, handles, knobs, grips, and other parts likely to be touched during normal use, with limits varying according to material type (metal, ceramic, plastic, glass) and contact duration.
- Winding temperature rise: motor stator and rotor windings, transformer windings, solenoid coils, and relay coils, compared against insulation class limits for Class A (105°C), Class E (120°C), Class B (130°C), Class F (155°C), and Class H (180°C) systems.
- Internal wiring and cord insulation temperature rise: verified against the rated temperature of the insulation, typically 75°C, 90°C, 105°C, or higher for heat-resistant wires.
- Component temperature rise: switches, thermostats, thermal cutouts, capacitors, lampholders, and terminal blocks, checked against their individual component ratings.
- Temperature rise of surrounding surfaces: the test corner walls and floor made of painted plywood on which the appliance is mounted, ensuring the appliance does not overheat adjacent mounting surfaces.
- Temperature of insulating materials in contact with live parts: supports and enclosures of current-carrying components, confirming the material's thermal endurance is not exceeded.
Testing Methods
Test Environment and Conditions
Testing is conducted in a room free from draughts and external heat sources, with an ambient temperature maintained typically between 20°C and 23°C (permitted range usually 15°C to 25°C, with limits referenced to 25°C). Appliances are operated at rated voltage and frequency; for normal operation tests under Clause 11, heating appliances are tested at input powers corresponding to 0.94 and 1.06 times rated voltage to account for supply variations. Portable appliances are placed on a test corner consisting of plywood panels coated with matte black paint, positioned as specified in the relevant Part 2 standard.
Thermocouple Measurement
Thin-wire thermocouples (commonly Type K, wire diameter not exceeding 0.3 mm) are attached to measurement points using a method that does not significantly alter the thermal characteristics of the surface, such as adhesive tape, thermal cement, or cyanoacrylate adhesive. Thermocouples are fixed securely to ensure good thermal contact. For surface measurements on accessible parts, the junction is attached at the point of maximum expected temperature, often determined by preliminary infrared thermal imaging surveys.
Resistance Method for Windings
For motor and transformer windings, the average winding temperature is calculated from resistance change using the relationship for copper windings:
t₂ = (R₂ − R₁)/R₁ × (234.5 + t₁) + t₁
where R₁ is the initial (cold) resistance at temperature t₁, R₂ is the resistance measured immediately after the test, and 234.5 is the reciprocal of the temperature coefficient of copper resistance. The winding resistance must be measured as quickly as possible after switching off the appliance, with extrapolation back to the moment of switch-off if cooling occurs during measurement.
Test Duration and Stabilization
The appliance operates until steady-state conditions are reached, defined as the point at which temperature changes of less than 1 K occur over consecutive measurement intervals. Typical intervals include 30 minutes for small heating appliances, but some products require extended testing: refrigerating appliances may need multi-day cyclic tests, and washing machines are tested through complete programmed cycles. During the test, temperatures are recorded continuously or at regular intervals by a data acquisition system.
Abnormal and Fault Condition Testing
Complementary tests under Clause 19 evaluate temperature rise under single-fault and abnormal conditions, such as stalled motors, restricted airflow, short-circuited heating elements, failed fan operation, and thermally cut-off electronic controls. The appliance must not release flames, molten metal, or ignite surrounding materials, and temperatures must remain within the elevated limits permitted for fault conditions.
Evaluation Against Limits
Recorded temperature rise values are added to the reference ambient temperature (25°C unless otherwise specified, or 35°C for tropical climate rating T) and compared with the maximum permissible values tabulated in Clause 11 of IEC 60335-1 and the applicable Part 2 standard. Common reference limits include 60 K rise for bare metal handles touched briefly, 45 K to 70 K for continuously held surfaces depending on material, and insulation class limits (such as 75 K rise for Class A windings in certain duty applications). Any exceedance constitutes a non-compliance requiring design modification.
Applications
- Certification and market access: mandatory for national certification schemes such as CCC in China, CE marking under the EU Low Voltage Directive, and safety approvals required by retailers and regulators worldwide.
- Product development and design verification: engineers use temperature rise data to select insulation materials, size motors and transformers, design airflow paths, and position thermal cutouts before production release.
- Quality control in manufacturing: routine production checks detect component substitution, winding defects, or assembly errors that cause abnormal heating.
- Failure analysis and complaint investigation: reproducing overheating conditions helps identify root causes of field failures, burnt components, or reported burn injuries.
- Material and component qualification: verifying that plastic enclosures, wiring, switches, and capacitors operate within their thermal ratings in the intended application environment.
FAQ
What is the difference between temperature and temperature rise?
Temperature is the absolute measured value of a component, while temperature rise is the difference between the measured temperature and the ambient temperature at the time of test. Standards specify limits as temperature rise values so that test results obtained under different ambient conditions remain comparable; the measured rise is referenced to a standardized ambient of 25°C (or 35°C for tropical-rated appliances).
Why is winding temperature measured by the resistance method instead of thermocouples?
The resistance method provides the average temperature of the entire winding, including internal hot spots that are inaccessible to surface-mounted thermocouples. It is non-invasive and does not alter the winding insulation, making it the preferred and standardized approach for evaluating motor and transformer windings against insulation class limits.
How long does a temperature rise test take?
The test continues until thermal equilibrium is reached, when temperature variation drops below 1 K between successive readings. Small heating appliances typically stabilize within 1 to 2 hours; motor appliances may be tested to steady state or through complete duty cycles; and refrigerating appliances can require 24-hour or multi-day cyclic testing. Total time including setup and instrumentation may range from several hours to several days.
What ambient conditions are required for the test?
The test room must be free from draughts, direct sunlight, and external heat radiation, with ambient temperature generally maintained between 15°C and 25°C (preferably 20°C to 23°C) and stable within ±1 K during measurement. Relative humidity and air movement are controlled to avoid influencing convection cooling of the sample.
What happens if temperature rise limits are exceeded?
Exceeding specified limits constitutes non-compliance with the safety standard. Corrective actions may include upgrading insulation class, increasing conductor cross-sections, improving ventilation, adding heat shields or thermal cutouts, relocating temperature-sensitive components, or selecting components with higher temperature ratings. The modified product must then be retested to demonstrate compliance.
Can infrared thermal imaging replace thermocouples in compliance testing?
Infrared imaging is a valuable screening tool for locating maximum temperature points during pre-compliance investigation, but it measures surface radiance influenced by emissivity and reflections. Regulatory compliance testing relies on contact thermocouple measurement per the standard, supplemented by the resistance method for windings. Accredited third-party laboratories use both techniques appropriately, with thermocouples providing the certified results.