The conversion of the electric command signal to a mechanical signal, typically to move the tripping latch, turns out to be a very critical part of the operating sequence of a circuitbreaker.
Fig shows a typical coil current signature. As indicated in the figure, the different parts of the process have their very characteristic features, and an experienced user can obtain an impressive amount of information about the condition of windings, lubrication, latches etc. from such traces.
For example, the time from the coil armature starts moving until it stops, is a measure of how swift the release mechanism is operating, while the magnitude of the coil current at the plateau before the auxiliary contact opens gives the DC resistance of the coil. Short-circuits in the coil windings will reduce this resistance.
Examination of the coil current is a widely applied diagnostic method for testing the release mechanism of circuit breakers. It is efficient in detecting the most frequently occurring problems in the release mechanism and is rather simple in use.
