Application of numeric protective relay circuit breaker duty monitoring

Most numeric or digital protective overcurrent systems on the market today state that they have some ability to provide an indication, based on parameters that are input by the user, that circuit breaker maintenance may be needed soon. This paper will look at variables in a protective relay's ability to provide the user with an indication of its associated circuit breaker's health based upon the amount of current the circuit breaker has interrupted and the associated energy dissipation in the circuit breaker contact surfaces. The variables in an arc energy formula will be examined and the complications related to estimating the associated wear or circuit breaker duty level from interrupted current will be reviewed. Practical examples will include calculations of circuit breaker wear at load currents and for full rated fault interruptions.