These three tests should be in your preventive maintenance program for medium-voltage circuit breakers:

1. Contact resistance test. Use a megohmmeter, not a digital multimeter (DMM), so that you can measure the resistance at a relevant test voltage instead of at 9VDC. Test the line to load terminals for each phase; the readings should not vary by more than 50 percent.

2. Insulation resistance testing. Some instruments have both the megohmmeter and insulation resistance testing functions. Test phase to phase and phase to ground with the contacts open. Use the manufacturer’s recommended testing voltages and acceptance values, if available.

3. Dielectric withstand voltage. Perform this test with a high-potential meter (hi-pot) from the line side of each phase with the breaker open. Also perform it on the load side of one phase, with the line and load sides of the other phases tied together to ground. As with insulation resistance testing, Use the manufacturer’s recommended testing voltages and acceptance values if available.

Other tests will be appropriate depending upon the application and the circumstances. In addition, you should use thermography as a predictive maintenance tool for all of your circuit breakers.