From Example 4.4, the pickup setting of Rb is 10.0 A, and the time dial setting is 1
2 . Theoretically,
to ensure that Rab backs up Rb it should be set for the same pickup current, i.e. it sees the same faults,
but is set at a slower (higher) time dial.∗ The operating time of Rb is determined from Figure 4.5 at
the maximum fault current at bus B (1500 A) divided by its pickup setting (20 × 10 A) or 7.5 × pu
and the 1
2 time dial. This is seen to be 0.25 s. Add 0.3 s coordinating time and Rab operating time
should be 0.55 s. If we make reference again to Figure 4.5, at the same maximum fault current of
1500 A at bus B and pu of 20 × 10 A, the multiple of the tap setting is 7.5 × pu and the operating
time is 0.55 s. Interpolating between the time dial setting curves of 1 and 2 gives a setting of 1.5.
(The actual time dial should be determined by test.)