When a problem is detected on an electrical power system, fast and reliable protection is extremely important. If a circuit breaker fails to clear the fault at that critical moment, the resulting damage can be serious, in terms of both personnel injury and equipment damage.
While circuit breakers tend to be very reliable, over time, they gather dirt, moisture, and contaminants. Furthermore, breakers used in hostile environments can be exposed to a number of corrosive contaminants. This can damage not only the insulation system but also the metal components. Due to these reasons, it is crucial that circuit breakers be maintained and tested to ensure proper operation during electrical faults.
There are two main types of electrical tests that can be performed on medium-voltage circuit brakes as part of a preventive maintenance program.
Also known as contact resistance test, the ductor test measures the resistance of electrical connections, such as terminations, joints, connectors, etc. These can be connections between any two conductors, for instance, cable connections or busbar sections. To perform this test a ductor tester equipment is needed to measure the resistance at the micro- or milli-ohm level. The equipment is used mainly to check that electrical connections are made properly.
The criteria for evaluating the contact resistance of electrical connections will depend on the type of connections, the contact pressure, the metallic contact surface, etc. It will also differ by the type of ductor test equipment and manufacture and there is no standard or code that dictates minimum contact resistances. With that said, the manufacturer recommendations should be checked.
Contact resistance can vary depending on the circuit breaker’s continuos current rating. It is recommended for the contact resistance to be kept as low and as even as possible across the three phases of the circuit breaker.
Insulation Resistance Test
The insulation system of a circuit breaker is a crucial area that calls regular testing and evaluation as it tends to weaken due to the heat created by the arc interruption, especially when it is not routinely maintained. The insulation resistance test is not only important to detect major defects in the system but can also be used as an extra safety check before returning the breaker to service.
A megohmmeter phase-to-ground and phase-to-phase is used to perform the test. Again, use the manufacturer recommended test voltage and acceptable values for this test.
Don’t forget to use the right testing label for an indication of the last maintenance date and overall condition of the devices that have been tested and maintained.