Electrical Factors
Insulation Resistance Measurement Test
This test is made to verify the condition of the insulation of the circuit breaker. A minimum of 1000 V test voltage should be used for low-voltage (600 V class) breakers for making this test. It would be preferable to use a DC test voltage that is at least 1.5–1.6 times the peak AC voltage of the circuit breaker. Tests should be made between pole to ground, between adjacent poles with circuit breaker contacts closed, and between phase-to-load terminal with breaker in open position. A minimum value of 1 MΩ is considered safe to prevent a flashover. Resistance values below 1 MΩ should be investigated for possible trouble.
illivolt Drop Test (Contact Resistance)
This is the primary circuit resistance test that was discussed in Section 8.7.2 for low-voltage power circuit breakers. This test consists of applying a DC across the closed circuit breaker contacts and measuring the voltage drop due to the contact resistance. Excessive voltage drop indicates abnormal conditions such as contact and/or connection erosion and contamination. This test is similar to the circuit breaker contact resistance measurement test described in Section 7.4.5 for medium-voltage breakers. The manufacturers of MCCBs should be consulted in order to find the acceptable millivolt drop values for particular breakers being tested. It is recommended that large breakers be tested with DC of at least 100 A and smaller breakers be tested at rated (or below rated) currents. The measured values should be compared among three phases of the breaker under test, or with values of breakers of similar size or with manufacturer’s recommended values to assess whether the contacts need to be replaced or dressed.
AC Dielectric Withstand Tests
This is the same test that was discussed in Section 8.7.2.2 for low-voltage power circuit breakers. This is a hi-pot test, and conducted to detect a gross failure of an insulation, the presence of a foreign object within an insulation system, or insufficient clearance between energized components and ground. This test maybe conducted to verify the MCCB withstand voltage capability.