Dynamic (contact) resistance measurements (DRM) test on circuit breakers

This test serves to ascertain the length and condition of the current - carrying section of the arcing contacts within SF6 circuit breakers. DRM (Dynamic Resistance Measurement) tests are carried out by passing DC current through the breaker's main contacts during the breaker's operation. Subsequently, the breaker analyser computes and plots the resistance as a function of time. When contact movement is concurrently recorded, the resistance at each contact position can be determined. Through DRM measurement, the length of the arcing contact can be accurately estimated. The only alternative method for this is to disassemble the circuit breaker.

In SF6 breakers, the arcing contact is typically composed of a tungsten/copper alloy. With each interruption of current, it burns off and becomes shorter. A circuit breaker experiences arcing contact wear not only during normal operation but also when interrupting short - circuit currents. If the arcing contact is overly short or in poor condition, the main contact surfaces can be damaged by arcing. This leads to increased resistance, excessive heating, and in the most severe cases, an explosion. As shown in the figure, the main contact resistance is measured dynamically during an open or close operation in DRM.