Static Contact Resistance Measurement

The modern High Voltage SF6 circuit breakers have two parallel contact sets. The main contacts are low resistance contacts, which are silver plated, whereas arcing contacts are of tungsten-copper which helps in initiating arc quenching and current interruption. Measurement of the static contact resistance with the breaker in closed position gives the resistance of main contacts only because the arcing contacts are bypassed. Healthiness of main contacts only gets checked during this test. The minimum dc test current should be used according to manufactures specification; however, the IEC and ANSI recommended levels are: 50 A IEC 60694 and 100 A ANSI. 

Static contact resistance is measured by injecting a dc current through the breaker and measuring the milli-voltage drop. A four wire measurement method is used. The breaker must be in the closed position. If low resistance readings are obtained when testing the breaker contact resistance using a low current, then it is recommended to re-test the contacts at a higher current. A higher current will have the ability to overcome connection issues and oxidation on terminals, where a lower current may produce higher readings under these conditions