connected in the residual of the CT secondary circuit so that it sees the vector sum of the secondary
current of all three phases. This is particularly applicable in motor installations where each phase
is energized at a different point of the voltage wave, resulting in different DC offsets and inrush
currents. Because of these differences there is a difference in the CT secondary output, and this
difference flows in the common residual circuit. Therefore, instantaneous relays cannot be used and
a very-inverse or short-time induction relay, or its solid-state or digital equivalent, is required. The
time delay prevents this type of relay from tripping falsely during startup. Typical pickup settings
of the time-delay relay are one-fifth to one-third of the minimum fault current with some time
delay. In power plants, to avoid incorrect trips due to vibration, the time delay is usually not set at
the lowest setting although virtually any time delay will be longer than the starting current error