Circuit breaker testing is more challenging compared to other electrical equipment, such as transformers or machines, due to the high short circuit current. Circuit breaker testing is typically categorized into two groups: type tests and routine tests.
Type Tests of Circuit Breaker
Type tests are performed to verify the rated characteristics and capabilities of circuit breakers. These tests are carried out in specialized testing laboratories. Type tests can be classified into four categories: mechanical performance test, thermal test, dielectric or insulating test, and short circuit test. The purpose of the test is to check the circuit breaker’s making capacity, breaking capacity, short-time rating current, and operating duty.
During a mechanical ability test, the circuit breaker should open and close at the correct speed and perform its designated operation without any mechanical failures.
Mechanical Test – During a mechanical ability test, the circuit breaker is opened and closed repeatedly to perform its designated operation. The breaker should perform without any mechanical failures.
Thermal Test – Thermal tests are performed to evaluate the thermal performance of circuit breakers. In this test, the breaker-rated current is passed through the poles till the pole temperature reaches the steady state value. The temperature rise must not exceed 40°C for current less than 800A and 50°C for 800A and above.
Dielectric Test – These tests are conducted to determine the power frequency and impulse voltage withstand capacity of a new circuit breaker. The voltage for power frequency testing varies based on the rated voltage of the circuit breaker.
The test voltage is applied in three ways: (1) between poles with the circuit breaker closed, (2) between pole and earth with the circuit breaker open, and (3) across terminals with the circuit breaker open. The test voltage frequency is between 15-100 Hz.
In impulse tests, an impulse voltage of a specified magnitude is applied to the breaker. Dry and wet tests are conducted for outdoor circuits.
Short-Circuit Test – Circuit breakers undergo short-circuit tests in laboratories to observe their behavior during switching, contact breaking, and arc extinction.
The oscillograms are studied to analyze the making and breaking currents and symmetrical and asymmetrical restriking voltages. Switchgear is tested at rated conditions.
Routine Tests of a Circuit Breaker
According to the recommendations of the Indian Engineering Service and Indian Standards, routine tests are carried out on the manufacturer’s premises to ensure the proper functioning of the circuit breaker. These tests are conducted to confirm that the circuit breaker is working as intended.
One Minute Dry Power Frequency Voltage Withstand Test– Overvoltage at power frequency is applied to the circuit breaker for one minute to check the insulation strength of the breaker’s part.
Resistance measurement and millivolt drop circuit-During a routine test, we measure the direct current (DC) voltage drop across the breaker contact when injecting current into the circuit. The injected current ranges from 100A to the maximum rated current of the circuit breaker.
Power Frequency Overvoltage Withstand Test – Manufacturers perform a test to confirm if the main circuit insulation is strong enough to withstand the abnormal overvoltage conditions of the system. To ensure that a circuit breaker can withstand all kinds of stresses and over-voltage, it undergoes various voltage or dielectric tests.
Operational Test– An operational test is then performed on the breaker by simulating its tripping through the artificial closing of the relay’s contacts.