Switchgear Testing

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

This test is used to determine the length and condition of the current carrying part of the arcing contacts in SF6 circuit breakers. DRM tests are conducted by injecting DC current through the breaker main contacts while the breaker is operated.
The breaker analyser then calculates and plots resistance as a
function of time. If contact movement is recorded simultaneously, the resistance can be determined at each contact position. With DRM measurement, the arcing contact length can be reliably estimated. The only other means to do this is by dismantling the circuit breaker. In SF6 breakers, the arcing contact is commonly made of a tungsten/copper alloy and is burned off and becomes shorter for each interruption of current.A circuitbreaker will have arcing contact wear by normal operation as well as when breaking short-circuit currents.
If the arcing contact is too short or in bad condition, the
main contact surfaces can be deteriorated by arching,
resulting in increased resistance, excessive heating and in
worst-case explosion.
The main contact resistance is measured dynamically over
an open or close operation in DRM as figure.

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Source
Megger company catalogue

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