The temperature rise test is one of the most critical type tests for low-voltage electrical apparatus, verifying that switching devices, controlgear, and busbar systems remain within safe thermal limits while carrying rated current. Performed in accordance with IEC 60947 and GB/T 14048 series standards, this test prevents insulation aging, contact deterioration, and potential fire hazards caused by excessive Joule heating. This article explains the test principle, sample types, measurement methods, acceptance criteria, and practical applications of temperature rise testing for low-voltage electrical equipment.
Technical Overview
A temperature rise test (also called a calorimetric or thermal test) evaluates the thermal behavior of low-voltage switchgear and controlgear when the device carries its rated current continuously. Because every current-carrying component—including terminals, contacts, busbars, and coils—possesses electrical resistance, power is dissipated as heat according to the Joule effect (P = I²R). If this heat cannot be dissipated effectively, the resulting temperature rise can degrade insulation, oxidize contact surfaces, increase connection resistance, and ultimately cause equipment failure or fire.
The measured parameter is temperature rise, defined as the difference between the temperature of a component and the ambient air temperature, expressed in kelvin (K). Testing is primarily governed by the IEC 60947 series for low-voltage switchgear and controlgear, with part-specific requirements in IEC 60947-2 (circuit breakers), IEC 60947-3 (switches and disconnectors), and IEC 60947-4 (contactors and motor starters). In China, the equivalent standards are the GB/T 14048 series, while related North American requirements appear in UL 489 and UL 508. The test is typically performed as a type test under strictly controlled ambient conditions, usually within an ambient temperature range of 10°C to 40°C.
Testing Samples
Switching and Protection Devices
This category includes miniature circuit breakers (MCB), molded case circuit breakers (MCCB), air circuit breakers (ACB), residual current circuit breakers (RCCB/RCBO), fuse-switch disconnectors, isolators, and automatic transfer switches. For these devices, the critical measurement points are the external terminals, internal current path joints, and accessible operating surfaces.
Electromechanical Control Devices
Contactors, electromagnetic relays, motor starters, and thermal overload relays are tested for both the current-carrying circuit temperature rise and the coil winding temperature rise. Coil heating is particularly significant for devices held energized for long periods, and winding limits depend on the insulation class of the coil.
Busbar and Distribution Systems
Busbars, bus ducts (busways), terminal blocks, distribution boards, and low-voltage switchgear assemblies are also subject to temperature rise verification. For complete assemblies, testing follows IEC 61439-1/2, which requires validation of the rated current of every circuit and the main busbar system under simultaneous full-load conditions.
Testing Items
Depending on the product standard and the device construction, the temperature rise test typically verifies the following items:
- Temperature rise of terminals intended for the connection of external conductors
- Temperature rise of internal busbars, flexible connections, and bolted or riveted joints
- Temperature rise of coil windings, measured by the resistance method
- Temperature rise of accessible surfaces, including handles, operating levers, and enclosure walls
- Temperature rise of adjacent insulating parts, which must not exceed the thermal endurance of the insulation material
- Ambient temperature monitoring throughout the test to ensure validity of the results
Testing Methods
Thermocouple Method
The thermocouple method is the standard technique for terminals, busbars, and accessible surfaces. Fine-gauge thermocouples (typically Type K or Type T, wire diameter commonly 0.25 mm to 0.5 mm) are bonded directly to the measurement points using thermally conductive adhesive or secured by drilling and peening, ensuring minimal disturbance of the heat flow. Thermocouple junctions must be electrically insulated from live parts where necessary to avoid measurement errors. A multi-channel data acquisition system records all channels continuously, and the ambient temperature is measured by at least two thermocouples placed approximately 1 m from the specimen, at half its height, protected from draughts and radiant heat.
Resistance Method for Windings
For coil windings, the average winding temperature is determined by the change in winding resistance. The cold resistance R1 is first measured at a stabilized ambient temperature T1. After thermal equilibrium is reached at the end of the test, the hot resistance R2 is measured as quickly as possible. For copper windings, the average winding temperature is calculated as T2 = (R2 / R1) x (T1 + 234.5) - 234.5, and the temperature rise equals T2 minus the ambient temperature at the end of the test. The constant 234.5 applies to copper, while 225 is used for aluminum windings.
Test Circuit and Current Injection
The specimen is supplied from a low-voltage, high-current source, typically a regulating transformer with a step-down current transformer, capable of delivering the rated current at rated frequency with a near-sinusoidal waveform. Connections use copper conductors of the cross-section specified in the relevant standard tables, with a minimum length generally of 1 m per side. To avoid introducing additional contact resistance at the supply connections, the circuit employs a four-terminal (Kelvin) configuration. All poles of the device carry rated current simultaneously, either by connecting the poles in series to a single-phase source, where permitted by the product standard, or by energizing the device from a polyphase supply.
Test Procedure and Duration
The test current is applied continuously until thermal equilibrium is reached. Equilibrium is generally considered achieved when the temperature rise of every measurement point varies by no more than 1 K per hour, although the exact criterion depends on the applicable product standard. In practice, tests last from two to eight hours depending on the thermal mass of the device. Current is monitored continuously and maintained within the tolerance band of the rated value (typically ±2% or better). Recordings are continued after switch-off where hot resistance measurement of windings is required.
Acceptance Criteria
The specimen passes the test if no temperature rise limit is exceeded and no component shows signs of damage, excessive deformation, or degradation that would impair its function. Typical limits based on IEC 60947-1 are summarized below; the applicable product standard and its edition always take precedence:
- Terminals with bare copper conductors: 60 K rise
- Terminals with bare brass or tin-plated surfaces: 65 K rise
- Terminals with silver-plated or nickel-plated surfaces: 70 K rise
- Coil windings by resistance method: 85 K (Class A), 100 K (Class E), 110 K (Class B), 135 K (Class F), 160 K (Class H)
- Hand-held metal operating parts: 15 K rise; hand-held insulating parts: 25 K rise
- Metal surfaces touched briefly in normal operation: 30 K rise; insulating surfaces: 40 K rise
Applications
Temperature rise testing serves multiple purposes across the product life cycle of low-voltage apparatus:
- Certification and type testing: mandatory verification for product certification under IEC, EN, UL, and GB standards before market access
- Design validation: confirming that contact geometry, conductor cross-sections, plating thickness, and ventilation design meet thermal targets during R&D
- Production quality control: routine or periodic sampling tests to detect manufacturing defects such as poor crimping, loose bolted joints, or contaminated contacts
- Assembly verification: proving the rated current of switchgear assemblies, distribution boards, and busway systems under realistic full-load conditions
- Failure analysis and derating studies: diagnosing overheating complaints in the field and establishing reliable derating curves for high ambient temperatures or enclosed installations
FAQ
Why is the test result expressed as temperature rise rather than absolute temperature?
Temperature rise (the difference between component temperature and ambient temperature) is used because heat dissipation depends on the temperature difference with the surrounding air. Expressing results as a rise allows a test performed at any ambient temperature within the specified range to be compared against fixed limits, and permits extrapolation to the maximum service ambient (typically 40°C including the average over 24 hours) defined by the standard.
How long does a temperature rise test take?
The test duration is determined by thermal equilibrium, not by a fixed time. Most devices reach equilibrium within two to eight hours; the test ends when all measured points change by no more than 1 K per hour. Large assemblies with heavy busbars may require longer stabilization periods.
Can a device pass the temperature rise test but still overheat in service?
Yes, in specific scenarios. Field overheating can result from undersized connecting cables, loose or corroded terminations, harmonic-rich loads that increase RMS heating, or ambient temperatures exceeding 40°C. This is why correct installation, conductor sizing, and periodic thermographic inspection remain essential even for certified products.
What happens if the measured temperature rise exceeds the limit?
The device fails the type test, and corrective actions are required, such as increasing conductor cross-sections, improving contact pressure or plating, reducing joint resistance, enhancing ventilation, or derating the device's rated current. The design is then modified and the test repeated until compliance is demonstrated.