Circuit breakers are used everywhere – in our homes, offices, and electric facilities. The degradation of the electrical insulation that is an integral component of these protective devices, can be influenced from both environmental and operational stresses. Additionally, oxidation and ware of circuit breaker contacts can lead to relatively high resistance connections and overheating of the circuit breaker. In the worst case, contacts can become welded together, thereby defeating the protective intent of the circuit breaker. This is why it is important to utilize the proper electrical test equipment when testing, to ensure that the circuit breaker is properly fulfilling its intended purpose.
Circuit breakers play a very important role in protecting electrical equipment and should be regularly tested and maintained. Doing so aides in avoiding unplanned outages or damage to circuits that these breakers are intended to protect. Listed below are some DC tests that can be performed to determine the efficiency and reliable performance of these breakers. These tests have been recommended by NETA – InterNational Electrical Testing Association, Maintenance Testing Specifications (MTS).
- DC overpotential tests are performed to verify the insulation's ability to withstand electrical stress.
- Insulation resistance tests are performed to characterize the insulation component of the breaker.
- Contact resistance tests are performed for evaluating the condition of the primary current path.
A very vital component that should be regularly tested and evaluated is the insulation system of the breaker. If there has been degradation of the insulation, a fault could occur during the process of arc interruption and/or compromise the safety of personnel working on or near this equipment. Control wiring of the breaker should also be regularly tested. Short circuits in control wiring can disable signals that are used to communicate how the breaker is to perform. If breakers have been in use for a long time, the factory lubrication may get dried out resulting in the binding of moving parts. These are some of the major issues that plague circuit breakers which have been in service for lengthy periods of time without proper care.
The three very common DC testing methods are explained in detail below.
Dielectric Withstand Test
This is a standard test that has been recommended by NETA MTS – 2015 air frame, medium and high voltage breakers. A high DC voltage is placed across the insulation system of the breaker. The intent of such a test is to verify the insulation's ability to withstand certain specified electrical stresses. While the insulation may not have faulted in service, this test can sometimes identify insulation that will fail if subjected to reasonable levels of abnormally high electrical stress.
A dielectric withstand test is essentially what the industry might refer to as a go-no-go test. If electrical stress is applied for a specified period of time, and the leakage current through the insulation does not exceed a specified value, then the test is passed, otherwise it fails.
Insulation Resistance Test
Unlike a dielectric withstand test, an insulation resistance test is intended to characterize the insulation rather than to stress the insulation. These tests may expose defects and contamination of electrical insulation. Industry standards and guideline also recommended to test the control wiring and instrument transformers if applicable during routine maintenance. This test will test the ground wall insulation of each conductor as well as the insulation that electrically isolates conductors from one another.
Care must be taken to not apply voltages beyond the insulation rating to over-current devices or the spring charging motor while testing controls wiring. Additionally, one must ensure that the circuit or device being tested is isolated.
Micro-Ohmmeter or Contact Resistance Test
Circuit breaker contacts and their supporting structures are provided as a means to continuously carry the load current and to interrupt fault current when the need arises. Poor connections can be a result of oxidation or contact pitting. When these kinds of phenomena occur, the resistance of the primary current path increases. As current is passed through these elevated values of resistance, it generates heat at the point of connection. The generated heat can lead to damaging fault conditions or the inability to respond to fault conditions if contacts were to weld together.
There may be many surfaces that carry current, and they should be properly maintained. Contact resistance testing is a good way to evaluate the condition of electrical contact points. Micro-ohmmeters are ideal for conducting contact resistance tests because of the requirement to accurately measure and display very low values of resistance.
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Editor’s Note: This post was originally published November, 2012 and has been updated freshness, accuracy and comprehensiveness.