Protection Selectivity & Grading

Compare upstream/downstream inverse-time trips (IEC 60255-151) and check grading margin.

Inputs

Upstream setting Iₛ A

Upstream curve —

Upstream TMS s

Downstream setting Iₛ A

Downstream curve —

Downstream TMS s

Fault current I_f A

Grading margin Δt s

Results

Note — Representative IEC 60255-151 inverse-time model. Selectivity requires the upstream relay to trip clearly later than the downstream one. Verify relay settings and curves with the manufacturer data and a certified engineer.