Understanding Heat Run Testing with Primary Injection High Current Testers

Heat run testing is a critical procedure used to evaluate the thermal performance of electrical equipment, especially in power distribution systems. This test assesses how electrical components, such as circuit breakers, busbars, and switchgear, handle sustained high current over a period of time. It helps identify overheating issues, poor connections, and potential failures that could lead to equipment damage or electrical hazards.

One of the most effective ways to conduct a heat run test is by using a primary injection high current tester, which injects a controlled high current into the system, simulating real-world operational conditions. A heat run test, also known as a temperature rise test, is performed to ensure that electrical components operate within safe temperature limits under rated or overload conditions. The objective is to measure how much the temperature increases due to resistive losses in conductors, contacts, and connections when current flows through them for an extended duration.

Current density distribution in busbars (on the left), electromagnetic induction field distribution in the switchgear (on the right). Source - Szulborski, M.; Łapczyński, S.; Kolimas, Ł. Thermal Analysis of Heat Distribution in Busbars during Rated Current Flow in Low-Voltage Industrial Switchgear. Energies 2021, 14, 2427

This test is essential in verifying compliance with industry standards such as IEC 61439 (for switchgear) and IEC 60947 (for circuit breakers), which define the permissible temperature rise limits for various components.

A temperature-rise test evaluates an object by measuring its temperature until it reaches a steady-state condition, based on the rated current set by the manufacturer or customer. This test is crucial for all equipment and is considered successful if the recorded temperatures at various measurement points do not exceed the limits specified in the test requirements.

Temperature rise significantly impacts switchboard operation by increasing the electrical resistance of its conductive components, particularly copper conductors. According to Ohm's Law (V = IR), current (I) is inversely proportional to resistance (R) when voltage (V) is constant. As temperature rises, the molecular motion within the conductor intensifies, leading to more frequent electron collisions. This interference reduces the net flow of electrons, effectively increasing resistance.

In a switchboard, higher resistance results in reduced current flow, potential voltage drops, and increased heat generation, which can lead to overheating, inefficiencies, and even equipment failure. Over time, excessive heat can degrade insulation, loosen connections, and compromise the overall reliability and safety of the system. Therefore, managing temperature rise is essential for maintaining optimal switchboard performance and preventing electrical failures.

Why Use a Primary Injection Tester for Heat Run?

Primary injection testing is preferred for heat run tests because it allows direct high-current application through the primary conductors and connections of the electrical system. Unlike secondary injection testing, which only checks relay logic, primary injection tests the entire electrical path, including:

• Busbars and switchgear – Ensuring joints and terminations do not overheat.

• Circuit breakers – Checking contact resistance and heat dissipation under load.

• Power cables – Verifying cable heating characteristics under full load.

• Current transformers (CTs) and connections – Identifying potential loose connections or high-resistance joints.

All circuit within the assembly shall be individually capable of carrying their rated current (sec: IEC61439-1/sec.5.3.2). However, the current carrying capacity may be influenced by adjacent circuits. So, Test shall be conducted with maximum current of each incomer and outgoer.

And that is the critical point that sometimes is ignored. Interconnection cables must be takein into account and typically they are set to 2m length. If the outgoers load current is not matching with requirement it is possible to balance the load current to requirement by adding/removing the cables, thus affecting the total impedance of the circuit. Test systems like EuroSMC Raptor also give you the option to work with compliance voltage and current density by adjusting the number of turns and cables used in the test setup.

Below you will find a step-by-step guide of the heat run test preparation and execution.

How to Perform a Heat Run Test with a Primary Injection Tester

Step 1: Test Setup

• Select a high-current primary injection tester capable of delivering the required test current (typically 50% to 100% of the rated operational current).

• Ensure test connections are tight and proper safety precautions (such as insulation barriers and thermal monitoring) are in place.

• Attach thermocouples or infrared cameras to measure temperature rise at various points.

Step 2: Inject High Current

• Start with a lower current and gradually increase to the full test current based on the system’s operational rating.

• Maintain the current for a specified duration (commonly 30 minutes to several hours) to simulate real loading conditions.

Step 3: Monitor Temperature and Voltage Drop

• Use thermal sensors, infrared imaging, or thermographic cameras to monitor hot spots.

• Measure the voltage drop across connections to detect high-resistance joints.

• Record the steady-state temperature rise and compare it against the allowed limits in industry standards.

Step 4: Evaluate the Results

• If any component exceeds temperature limits, inspect and re-tighten connections, replace degraded components, or adjust ventilation.

• If resistance is too high, investigate contact wear, corrosion, or improper assembly.

• Repeat the test after corrective actions to ensure thermal stability.