The design and construction of the electromechanical and solid-state instantaneous overcurrent relay
are described in Section 2.4 (Figure 2.7) and Section 2.5 (Figure 2.14). In order to properly apply
the instantaneous overcurrent relay, however, there must be a substantial reduction of short-circuit
current as the fault is moved away from the relay toward the far end of the line. The relay must
be set not to overreach the bus at the remote end of the line and there still must be enough of a
difference in the fault current between the near and far end faults to allow a setting for the near-end
fault. This will prevent the relay from operating for faults beyond the end of the line and, at the
same time, will provide high-speed protection for an appreciable portion of the circuit.
Figure 4.7 also shows why simple inverse-time overcurrent relays cannot be used without additional
help. The closer the fault is to the source, the greater the fault current magnitude, yet the
longer the tripping time. The addition of instantaneous overcurrent relays makes this system of
protection viable. If an instantaneous relay can be set to see almost up to, but not including, the
next bus, all of the fault-clearing times can be lowered, as shown in Figure 4.11.