Heating Tests for Household Electrical Appliances: Standards, Methods, and Temperature Rise Evaluati

Heating tests, also known as temperature rise tests, are essential safety evaluations for household electrical appliances, verifying that surfaces, windings, and insulation materials do not exceed safe temperature limits during normal operation. Conducted in accordance with international standards such as IEC 60335-1 and GB 4706.1, these tests protect users from burns and prevent premature degradation of insulation systems. Accredited third-party laboratories perform heating tests to support product certification, design verification, and regulatory compliance for global markets.

Technical Overview

A heating test, commonly referred to as a temperature rise test, is a fundamental safety assessment performed on household and similar electrical appliances. The purpose of the test is to measure the temperature of accessible surfaces, internal components, electrical insulation, and motor windings while the appliance operates under defined normal operating conditions, and to compare the measured temperature rises against the limits specified in the applicable safety standard.

Excessive heat is one of the most common causes of appliance failure and hazards. Sustained overheating accelerates the thermal aging of insulation materials, reduces the dielectric strength of windings, degrades switches and capacitors, softens plastic enclosures, and creates burn risks for users who touch handles, knobs, or outer surfaces. The heating test therefore serves as an early indicator of unsafe thermal design.

The primary reference standards for heating tests include:

• IEC 60335-1 – Household and similar electrical appliances – Safety – Part 1: General requirements, together with the applicable Part 2 standards (for example, IEC 60335-2-9 for grills, toasters, and similar portable cooking appliances, and IEC 60335-2-15 for liquid heating appliances)
• GB 4706.1 – The Chinese national standard, technically aligned with IEC 60335-1, mandatory for CCC certification
• EN 60335-1 – The European harmonized version used for CE marking and conformity assessment
• UL 60335-1 – The North American adoption of the IEC standard
• JIS C 9335-1 – The Japanese national series corresponding to IEC 60335

Insulation systems are classified by thermal class, including Class A (105°C), Class E (120°C), Class B (130°C), Class F (155°C), Class H (180°C), Class 200, Class 220, and Class 250. The permissible winding temperature rise depends directly on this classification.

Testing Samples

Heating Appliances

Appliances whose intended function is to generate heat typically present the most demanding thermal conditions. Representative samples include electric kettles, rice cookers, electric ovens, toasters, coffee makers, electric irons, room heaters, hair dryers, and hair straighteners. These appliances are tested with vessels or loads specified in the relevant Part 2 standard to simulate worst-case intended use.

Motor-Operated Appliances

Appliances driven by electric motors generate heat in windings, bearings, and electronic controls. Typical samples include electric fans, vacuum cleaners, food mixers, blenders, juicers, range hoods, and massage devices. Testing focuses on winding temperature rise under rated load and continuous or cyclic operation as defined by the standard.

Refrigeration and Combined Appliances

Refrigerators, freezers, dehumidifiers, air conditioners, and washer-dryers contain compressors, heating elements, and drive motors operating simultaneously. Heating tests verify that condenser tubing, motor-compressor windings, and electronic assemblies remain within safe limits under steady-state and cyclic operation.

Appliances with Electronic Controls

Induction cookers, microwave ovens, air fryers, and smart appliances incorporate power electronics whose components, such as transformers, inductors, and power semiconductors mounted on printed circuit boards, require temperature measurement at the hottest accessible points.

Testing Items

The heating test evaluates temperature rise at all locations where excessive temperature could compromise safety or functionality. Typical measurement items include:

• Accessible surfaces frequently touched by the user, such as handles, knobs, push buttons, and grips
• External enclosure surfaces that may be touched briefly during use or maintenance
• Winding temperatures of motors, transformers, solenoid valves, and relays
• Insulation materials in contact with current-carrying parts, including winding wire enamel and slot liners
• Internal wiring and supply cord insulation at entry points and near heat sources
• Switches, thermostats, thermal cutouts, and capacitors, particularly those marked with rated operating temperatures
• Printed circuit boards and electronic components such as power devices, optocouplers, and connectors
• Contact surfaces of lampholders and terminal blocks
• Plywood panels of the test corner, which simulate adjacent wooden furniture or building surfaces
• Ambient air temperature in the vicinity of the appliance

Representative temperature rise limits for windings under normal operation, referenced to a maximum ambient temperature of 25°C, are approximately 75 K for Class A insulation, 90 K for Class E, 95 K for Class B, 115 K for Class F, and 140 K for Class H. For surfaces held continuously in the hand, limits are typically around 30 K to 35 K for bare metal and 40 K to 45 K for plastic and glass, with more relaxed limits for surfaces touched only briefly. The exact values must always be confirmed against the applicable Part 2 standard.

Testing Methods

Heating tests are conducted in a controlled laboratory environment that reproduces the conditions specified by the standard, ensuring repeatable and comparable results.

Test Environment and Setup

The test is performed in a room free from drafts and external heat sources, with ambient temperature maintained at approximately 20°C ± 5°C. The appliance is placed in a test corner constructed of plywood panels painted matte black, positioned as specified in the relevant Part 2 standard to simulate installation against walls or on floors. The appliance is supplied at rated voltage and rated frequency with a tolerance of ±1%, and it operates under normal operating conditions defined for its category.

Thermocouple Measurement Method

Fine-wire thermocouples, typically Type K with wire diameter of 0.3 mm or smaller, are bonded to predetermined measurement points using a thermally conductive adhesive or high-temperature tape. The thermocouple junction must make intimate contact with the surface being measured without significantly altering heat dissipation. Thermocouples are attached to accessible surfaces, insulation systems, electronic components, cord insulation, and the plywood panels of the test corner. Temperatures are recorded continuously with a data acquisition system until steady state is reached.

Resistance Method for Windings

Winding temperatures are determined by the change-of-resistance method, which exploits the linear relationship between copper resistance and temperature. The cold resistance R1 and initial ambient temperature t1 are recorded before the test, and the hot resistance R2 and final ambient temperature t2 are measured immediately after de-energizing the appliance. For copper windings, the temperature rise is calculated as Δt = (R2 − R1)/R1 × (234.5 + t1) − (t2 − t1). Where possible, this method is supplemented by embedded thermocouples placed between winding layers.

Steady State and Duration

Heating appliances are operated until thermal equilibrium is established, indicated by temperature variations of less than 1 K over a defined period, or for a specified number of duty cycles. Motor-operated appliances may run continuously or undergo cyclic loading according to their Part 2 requirements. The final temperature rise at each measurement point is calculated as the measured temperature minus the ambient temperature, and where the test ambient differs from the reference value of 25°C, the measured rise is corrected accordingly before comparison with the limits in the standard.

Evaluation and Follow-Up

After the test, the appliance must not show damage affecting safety or compliance: protective devices shall not have operated during normal operation, insulation shall show no visible deterioration, and thermosetting materials shall not have softened to an unsafe degree. If any limit is exceeded, corrective actions such as improving ventilation, upgrading insulation class, increasing conductor cross-section, or modifying the thermal path are implemented before retesting. Heating tests under normal operation are complemented by abnormal operation and faulty condition tests covered by clause 19 of IEC 60335-1.

Applications

Heating test reports from accredited third-party laboratories are used across the entire product lifecycle:

• Certification and market access – Supporting CB Scheme certification, CE marking for the European Union, CCC certification for China, and North American listing programs based on UL 60335-1
• Design verification – Validating thermal simulation results and confirming that prototypes meet temperature rise limits before tooling investment
• Material selection – Demonstrating that plastics, insulation systems, adhesives, and surface coatings withstand operating temperatures without degradation
• Production quality control – Periodic verification that mass-produced units maintain the thermal performance of the certified design after component substitutions
• Regulatory compliance and surveillance – Providing objective evidence for market surveillance authorities and responding to customer complaints or field failure investigations
• Research and development – Benchmarking competitor thermal performance and optimizing cooling strategies such as heatsinks, fans, and airflow channeling

FAQ

What is the difference between a heating test and an abnormal operation test?

The heating test evaluates temperature rise under normal operating conditions as defined in clause 11 of IEC 60335-1, whereas abnormal operation tests in clause 19 subject the appliance to stalled motors, restricted airflow, short-circuited heating elements, and electronic circuit faults to verify that no fire or electric shock hazard results.

Why is the winding temperature measured by the resistance method instead of thermocouples alone?

The resistance method yields the average winding temperature non-invasively and is the internationally recognized reference technique. Embedded thermocouples measure localized hot spots but can disturb winding geometry, so both methods are often combined for a complete assessment.

How long does a heating test take?

Test duration depends on the appliance type. Small motor-operated appliances may reach steady state within one to two hours, while heating appliances such as ovens and dryers may require four or more operating cycles, resulting in test durations of several hours to a full day.

What happens if an appliance exceeds the temperature rise limits?

Non-compliant results require design modification, such as reducing power density, improving heat dissipation, upgrading to a higher insulation class, or adding thermal barriers, followed by retesting to confirm compliance before certification can be granted.

Can heating tests be performed at any ambient temperature?

No. The standard specifies an ambient temperature of approximately 20°C ± 5°C during the test, and the measured temperature rise is corrected if the ambient deviates from the 25°C reference condition used in the limit tables, ensuring consistent evaluation regardless of minor laboratory variations.