Solid-state trip unit of an electrical circuit breaker with contact wear indicator

Claim 1

A digital solid-state trip unit including an electrical circuit breaker with separable contacts, comprising: detection circuit means for generating an analog signal proportional to current flowing in a conductor protected by the circuit breaker; analog-to-digital convertor means, connected to said detection circuit means, for converting said analog signal into a sampled digitized signal; digital processing means connected to said analog-to-digital convertor means and generating a tripping order after at least one of a long time delay and short time delay when said sampled digitized signal exceeds respective predetermined thresholds, said tripping order being time delayed as a function of a magnitude of said sampled digitized signal; means, responsive to said tripping order, for opening said separable contact; said digital processing means comprising: means for detecting a maximum value of current broken each time said separable contacts open by comparison between the successive values of said digitized signals which are applied to said processing means between the time the circuit breaker tripping order is generated and effective opening of the contacts occur; means for generating, upon each opening of said separable contacts, a wear value representative of wear of said separable contacts as a function of a respective said maximum value of current; means for calculating a sum of a succession of said wear values generated from a succession of said contact openings; means for storing said sum in a memory; and means for displaying said sum to provide an indication of a degree of wear of said contacts.