Circuit breakers are one of the most intricate and essential mechanical components in the electrical power system. Their primary function is to interrupt both normal operating and short-circuit currents and manage routine adjustments in system configuration.

A range of tests can be performed on high-voltage circuit breakers to assess the performance of their internal mechanisms. Whether the breaker operates using air blast, oil, vacuum, or gas, it is crucial to conduct regular testing to ensure reliable performance during system faults or switching operations.

1. Contact Timing Test

As a rule of thumb, there are only 6 timing tests that are performed on a circuit breaker:

• Open – Simulates a short circuit trip. Also known as O.

• Close – Simulates a close on live circuit. Also known as C.

• Open, Close – Simulates a fast close after short circuit trip. Also known as O-C.

• Close, Open – Simulates a trip on short circuit after a close. Also known as C-O.

• Open, Close, Open – Simulates a reclose on a short circuit. Also known as O-C-O.

• Close, Open, Close, Open, Close, Open – Simulates a multiple close after short circuit trips.

The primary objective of the contact timing test is to measure the moment when the contacts change state precisely, as well as to verify the contact travel, speed, and detect any irregularities. According to IEC 56 3.3.1, all contact poles should separate within 1/6th of a cycle of one another.

The measured results are then compared to the tolerance limits provided by the manufacturer. Commissioning or acceptance test values are often used as reference points. Any deviation from these reference values can help determine the necessary course of action following a detailed analysis.

2. Mechanical Motion Test

High-voltage circuit breakers are engineered to interrupt short-circuit currents at a precise speed to prevent voltage re-strikes. If the circuit breaker operates too slowly, it can reduce the breaking capacity of the main contacts, whereas excessive speed can lead to mechanical damage to damping components and cause excessive vibration.

The velocity or acceleration curve of the circuit breaker is derived from the motion curve recorded by a transducer attached to the moving part of the operating mechanism. This curve helps identify any changes that could impact the mechanical performance of the circuit breaker.

Motion tests are performed using a circuit breaker motion analyzer equipped with a transducer kit to check the operating mechanism stroke, velocity, damping, and over-travel against the manufacturer’s specifications. The recorded motion is presented as a curve displaying distance vs. time.

3. Control Circuit Test (Coil Current Measurement)

These control components are usually powered by low-voltage substation batteries and can be prone to failure if their integrity is not regularly inspected. In the event of a system fault, a malfunctioning trip coil could lead to severe damage to the power grid and trigger unnecessary outages as upstream devices attempt to clear the fault.

External voltage and current measurements will let the engineer check batteries and current through the breaker’s engines. Be sure to confirm that your CB testing unit can work with both DC and AC values, as an improper connection could be problematic.

Circuit breaker control circuits can be evaluated by measuring the trip and close coil currents, along with the minimum pickup voltage. These measurements are compared against the manufacturer’s specifications to verify proper functionality.

The operating coil current waveform offers a visual representation of the mechanical and electrical condition of the coils. Any significant deviations from the baseline test results should be thoroughly investigated. Usually, you’ll see waveforms for both coils you are measuring on the same graph, and to have a deeper analysis a status bar of each pole of the breaker is shown alongside.

4. Contact Resistance Testing

This test is crucial for contacts that handle marginally high magnitudes of current, such as switchgear busbars, because increased contact resistance can reduce current-carrying capacity and lead to higher losses. Ductor testing is typically conducted with a micro/milli-ohmmeter or low ohmmeter.

Measuring contact resistance is essential for detecting fretting corrosion and diagnosing and preventing contact degradation. An increase in contact resistance can cause significant voltage drops in the system, which must be managed effectively.

The visual inspection involves examining the contacts for signs of arcing damage, such as pitting, as well as checking for any wear or deformation.

The second check is contact resistance measurement, which entails passing a fixed current through the contacts and measuring the resulting voltage drop. This test is conducted using a specialized contact resistance measuring instrument. The resistance is then calculated using Ohm’s law and compared to both the manufacturer’s specifications and previous records.

Both tests are necessary as they complement each other; contacts may exhibit good resistance but still be physically damaged. For a contact to be deemed in good condition, it must pass both the resistance measurement and the visual inspection.