Measuring molded case circuit breaker resistance

Measurements of molded case circuit breaker resistance under various conditions carried out to demonstrate the need for more standardized procedures when performing millivolt drop measurements are discussed. Breakers rated at 600 V_ac/250 A_ac and 120/240 V_ac/50 A_ac, with silver-tungsten contacts, were used to show that contact resistance can cause variability in circuit breaker resistance. They were also used to demonstrate the unique influence that current, time, contact temperature, and applied voltage each had on contact resistance. Based on these results, guidelines were developed for improving the reliability of the millivolt drop test