The susceptibility to sags is also shown in Figure 3. Note that the faults at nodes 3 and 4 have the highest susceptibility level because they include the greatest length of line. The results are calculated for bolted faults. Sag Duration Sag duration is controlled by the circuit breaker reclosing relays and the operating time of the circuit breaker itself. Short Operating times result from relays set to trip on overcurrent instantaneously.
Longer times result from relays set to trip on inverse time .
Reclosing relays are usually set to trip instantaneously on the first fault. On reclosing, the relay trips inverse time is used to allow a downstream circuit breaker or fuse to clear the first and so minimize the number of customers affected by the fault. In the new system, the downstream circuit breaker has been eliminated and replacement for its fault interruption and sectionalizing function is rarely provided. Conclusions on Utility Related Factors Sags of magnitude exceeding the PQE are much more likely to occur from faults anywhere on the distribution system than they previously were. Therefore, a high level of preventive maintenance is now more important and also more difficult because all the voltage levels are also higher than before.