MEASURING MOLDED CASE CIRCUIT-BREAKER RESISTANCE

Molded case circuit breaker resistance was measured under various cond itions to demonstrate the need for more standardized procedures when p erforming millivolt drop measurements. Breakers, rated at 600 V(ac)/25 0 A(ac) and 120/240 V(ac)/50 A(ac), with silver-tungsten (Ag-W) contac ts were used to show that contact resistance can cause variability in circuit breaker resistance. These breakers were used to demonstrate th e influence of current, time, contact temperature, and applied voltage on contact resistance. These parameters each had a unique influence o n breaker resistance. Based on these results, guidelines were made for making reliable resistance measurements on molded case circuit breake rs (MCCB's) when using the millivolt drop technique.