Introduction to Temperature Rise Testing for Low-Voltage Switchgear Assemblies

Temperature rise refers to the difference between the operating temperature of components (such as switching devices and busbars) within the assembly and the ambient air temperature. The temperature rise of components is influenced by multiple factors, including:

Selection of switching devices

Phase arrangement

Busbar material and fabrication

Density of functional units

Installation of switching devices

Degree of protection (IP rating) of the assembly

Temperature rise is typically measured using thermocouples, which are widely used in electrical equipment and assembly testing due to their small dimensions, minimal influence on the measured point, and ease of use.

1. Requirements for the Assembly Under Test

Before measurement, the assembly shall be positioned as in normal service, with all covers in place.

Coils of relays, contactors, and releases shall be energized at rated voltage.

The assembly under test shall be representative. If the assembly system comprises several configurations, the one with the most severe arrangement shall typically be selected for testing.

2. Determination of Test Current

The test shall be conducted on one or more representative circuit combinations that reflect the primary application of the assembly.

The selected circuits shall allow sufficiently accurate determination of the highest expected temperature rises.

Incoming and outgoing circuits shall carry the current specified in GB7251.1-2013.

If the assembly includes fuses, fuse-links shall be installed according to the manufacturer's specifications. Power losses of the fuse-links used in the test shall be recorded in the test report.

3. Temperature Rise Test Circuits

1) Single-Phase Test Circuit:

Applicable for test currents ≤ 400A.

Single-phase AC may be used for multi-phase assemblies only if the effects of electromagnetic fields are negligible.

Note: Temperature rise differs between three-phase and single-phase tests due to phase differences and varying proximity effects between conductors.

2) Three-Phase Test Circuit:

Mandatory for test currents > 400A.

3) Testing with Heating Resistors Simulating Equivalent Power Losses:

For enclosed assemblies with low-rated main/auxiliary circuit currents, power losses may be simulated using heating resistors generating equivalent heat.

Resistors shall be mounted at appropriate positions within the enclosure.

Lead cross-sections connected to the resistors shall not cause significant heat dissipation outside the enclosure.

4. Determination of Temperature Rise Measurement Points

Measurement points shall include:

Terminals of internal components for incoming/outgoing connections.

Terminals for connecting external insulated conductors.

Busbar joints and busbar-to-conductor connections.

Plug-in contacts of removable and withdrawable parts.

Insulating materials adjacent to or in contact with heat-generating components/conductors.

Operating handles.

5. Temperature Rise Measurement

Thermocouples shall be used, protected from air currents and thermal radiation.

The test duration shall be sufficient for temperatures to stabilize (generally ≤ 8 hours). Stability is considered achieved when the temperature rise does not exceed 1 K/h.

6. Ambient Temperature Measurement

Ambient temperature shall be measured during the last quarter of the test period.

At least two thermocouples or thermometers shall be uniformly distributed around the assembly at a height approximately half the assembly height and 1 m away.

Instruments shall be protected from air currents and thermal radiation.

7. Evaluation of Test Results

At test completion, temperature rises shall not exceed limits specified in the standard (e.g., GB7251.1-2013/IEC 61439-1).

Components shall operate correctly within the internal temperature of the assembly and their specified voltage limits.