The time-delay overcurrent relay 51 sees 352.5/80 or 4.4 secondary amperes. To set the relay,
the manufacturer’s instruction book and characteristic curves must be used. For our purposes,
however, assume there is a 5.0 A tap and the characteristics of Figure 4.5 apply. The time delay
must be set longer than the starting time of the motor. This assumes that the starting current lasts
for the full starting time. This is not strictly true. The starting current starts to decrease at about
90% of the starting time. However, this is a conservative setting that is often used to cover any
erratic motor behavior and to avoid false tripping during starting. The relay pickup during starting
is 1609/(80 × 5) = 4 × pu; the time delay must be at least 0.97 s which results in a time dial
setting of 1.5. Two overcurrent relays are usually used, one in phase 1 and the other in phase 3, on
the assumption that an overload is a balanced load condition. An ammeter (not shown) is usually
connected in phase 2.