Referring again to Figure 4.7 and to the time sequence shown in Figure 4.8, a fault at the end
of the line should be cleared by Rd and CB4. Rcd sees the same fault as Rd and starts to close its
contacts. If the circuit breaker clears the fault, assuming electromechanical relays, Rcd will reset
after some overtravel. The exact overtravel time can be obtained for specific relays but the normal
assumption is that the disc will reset in the same time that it took to move in the contact-closing
direction. Contact overtravel is included in the margin of safety. Overtravel does not apply to
solid-state or digital relays which do not have moving parts although some safety margin would be
provided. If the fault is not cleared, Rcd continues to close its contacts and initiates a trip of CB3
at the end of its operating time. To be sure that Rcd does not close its contacts before the fault is
cleared by CB4 it must be set longer than Rd operating time (U), plus CB4 clearing time (V ), and