CONTACT RESISTANCE CHARACTERISTICS OF AG MATERIAL IN BREAKING LOW-LOAD DC ARCS

Contact resistance of Ag contacts was determined from nonload to 4 A u nder switching inductive load conditions. The tested results show that three characteristic contact resistace variations corresponding to th ree types of contact morphologies exist, and there are a critical tran sition load current and a critical transition operation cycle. When th e load current is less than 0.2 A, which is called critical transition current, the contact resistance at the beginning of the operation, wh ich is less than critical transition operation cycle, is like that in nonload condition, i.e., low and stable. If the load current is larger than this critical value, the contact resistance is, however, high an d unstable during the whole test process. It is demonstrated that this phenomenon is attributable to arc occurrence probability and arc dura tion. Both mechanical factors (wear and mechanochemical reaction) and electrical factors (arc erosion and heat corrosion) are considered to be dominant in different load currents and different operation periods .