The purpose of this thesis is to present procedures and results of field testing of molded case circuit breakers and bimetallic relays. Regular field testing is necessary, since the properties of the protective devices vary in time and this affects their time-current characteristics. The measurements are performed according to the recommendations and directions of the organization NEMA (National Electrical Manufacturers Association) standard AB 4.
Initially, molded case circuit breakers and bimetallic relays are described. Their tripping mechanism to detect fault currents and tripping characteristic are presented in detail. Additionally, procedures for testing electrical characteristics of the circuit breakers and bimetal relays are demonstrated. The measurements include measuring of the contact resistance and testing tripping characteristic in which short-circuit and overload tripping mechanisms are tested. Finally, results of field testing with the most common origins of faults are reported.
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