In this chapter we have examined the protection of a transmission line with time-delay and instantaneous
overcurrent relays using only the information available at the relay location. These are
the most commonly used relays for the protection of radial and low-voltage subtransmission lines,
industrial or commercial installations. Time-delay relays coordinate with each other by grading the
relay operating currents, in primary amperes, as the fault is moved from the load to the source end
and by allowing enough time between the relay operating times so that the relays closest to the fault
will operate first. Instantaneous relays, by definition, cannot coordinate with each other and must
be set so as not to overreach their intended zone of protection. In a networked system, relays must
be directional so they will operate only for faults in the desired direction and allow coordination
by current and time in that one direction. Coordination must be made by comparing the current
in each relay, taking due account of the distribution of current for a given fault. Directionality is
obtained by comparing the operating current with a voltage or current that is constant, regardless
of the location of the fault.