Stationary and moving contacts are built from alloys that are formulated to endure the abuse of electrical arcing. However, if contacts are not maintained on a regular basis, their electrical resistance due to repeated arcing builds up. This resistance build-up results in a significant decrease in the contact’s ability to carry current. Excessive corrosion of contacts is detrimental to the breaker performance. One way to check contacts is to apply dc current and measure the contact resistance or voltage drop across the closed contacts. The breaker contact resistance should be measured from bushing terminal to bushing terminal with the breaker in the closed position. It is recommended that for medium and high voltages the resistance test be made with 100-Amps (or higher) direct current. The use of a higher current value gives more reliable results than using lower current values. The resistance value is usually measured in micro-ohms. The average resistance value for 15-kV-class air circuit breakers is approximately between 200-250 mico-ohms. The evaluation should be made on comparing resistance values of the three phases with each other, or with resistance values of similar breakers. A difference of more than 50 percent in resistance values among the three phases of a breaker should warrant further investigation.
Whether your needs are as simple as contact resistance testing or more advanced requiring a full regiment of breaker tests including First Trip, Protec Equipment Resources has a full inventory of micor-ohmmeters from Megger DLRO, Raytech, Vanguard and AEMC along with Circuit Breaker Timer Analyzers from Vanguard and Doble.