Protection Coordination

Plot two devices' time-current curves and check selectivity up to the fault current (Lessons 46–47).

Lesson 47 cases:

Upstream device

In (A)Ir × Intr (s @ 6 Ir)

Short-time (S)

Isd × Irtsd (s)

Instantaneous (I)

Downstream device

In (A)Ir × Intr (s @ 6 Ir)

Short-time (S) Instantaneous (I)

Ii × In

Prospective fault at downstream device (kA)

Choose a Lesson 47 case or enter settings, then plot.

Idealized curves without tolerance bands. Long-time: t = tr·(6·Ir/I)². Instantaneous ≈ 20 ms. Final selectivity is confirmed with the manufacturer's selectivity tables and software.