Testing and Calibration of Circuit Breakers

A circuit breaker is a switching device that stops any excessive current flowing through a circuit thereby protecting the electrical system from severe damage. It is a mechanical device that disconnects the flow of high current and additionally can also perform as a switch.


Working Principle

The circuit breaker consists of a set of fixed and moving contacts. When the circuit is closed these contacts are pressed against each other under the pressure of a spring and they allow the current to pass through under normal conditions. However when the electrical current starts to flow at a level higher than a pre-set level then the circuit breaker opens the contacts automatically and thereby stops the flow of the current in the circuit. In normal operating conditions, the circuit breaker can be opened or closed as a switch easily as only slight manual pressure is required to operate it. Without circuit breakers household or industrial electricity supply would be dangerous because of the potential for fires and other issues resulting from wiring problems and equipment failures.


Testing of Circuit Breakers

Testing of circuit breakers is slightly more difficult as compared to other electrical equipment because the short circuit current is very large.


Type Tests of Circuit Breakers

Type tests are conducted to prove the capabilities and confirm the rated specifications of the circuit breaker. These tests are conducted in a qualified testing laboratory. Type tests may include short circuit tests, insulating tests, thermal tests, performance tests, and mechanical tests. Tests are also done for making capacity, breaking capacity, short time rating current and operating duty.


Circuit Breaker Testing

The circuit breakers carry out the critical protective function of connecting and disconnecting the electrical power whenever demanded by the operating conditions. They must prove their consistency and efficiency in field tests during installation and periodical maintenance tests to ensure so that they perform satisfactorily during their lifespan.



Small low voltage breakers that operate under 250 Volts are not calibrated. They are manufactured and tested at the factory. If a circuit breaker fails the test it is discarded.

Large low voltage breakers that operate at 400 Amps and over have a trip unit that can be set at pre-defined levels. When they are checked, a high current test set is used to inject current through the breaker to test their working. If a circuit breaker fails the test it is discarded.

Large high voltage breakers with a capacity of over 1200 Amps are tripped by protective relays. The relays operate on secondary transformer current which is about only 5 A at full load. These relays can be calibrated to a specific current level using precision test sets. It is recommended that the circuit breaker relays should be re-calibrated on regular basis.

The three main parameters that are calibrated for most circuit breakers are the timing, current, and contact resistance. There can be other tests that may be necessary to be performed depending on the type of each circuit breaker.

Timing measurements may be simulated by using a pulse generator or by using a switching circuit and by measuring the timing of the switch with an electronic counter. Current measurements are made by using a power supply to produce the required current and by using an appropriate current shunt to accurately measure the applied current.

The contact resistance measurement is tested using a set of low-ohm resistors or a standard resistor box for getting the standard resistance.  The exact value of the resistors can be obtained by calculating the resistance using the Ohms law to remove any errors during measurement.

If some of the values are found to be beyond the specifications given by the manufacturer, for most circuit breakers, the timings can be changed by using special calibration software without any hardware intervention.


Automatic Testing

Automatic testing equipment called Circuit Breaker Analyzer is available to test circuit breakers of various types and capacities. Such a unit is easy to use, either in stand-alone or computer-controlled mode.  It can operate either in Time Travel analyzer mode or in “on-line” timing mode. In Time -Travel mode, it can fully analyze a circuit breaker’s performance by evaluating its contact motion analysis.


Circuit breaker testing and calibration are vital to make sure that it is safe and has reliable performance.


e2b calibration is an ISO/IEC 17025 accredited calibration laboratory and has the necessary standards to test and calibrate various types of circuit breakers. Our verifiable services are unmatched in the industry. We are registered with ANAB. We are also ANSI/NCSL Z540-1-1994 certified. We have the NIST Traceable Wide scope of ISO/IEC 17025 accreditation. Contact e2b calibration for more information.



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