Inverse-Time Overcurrent

Set an inverse-time overcurrent relay per IEC 60255-151 standard curves: pick-up I_p, time multiplier T_p and curve type (NI / VI / EI / LTI); read the operating time for a fault current and the current ratio needed for a target time.

Inputs

Pick-up current I_p A

Time multiplier T_p s

Fault current I_f A

Curve type —

Grading time limit t_max s

Results

Note — Operating time t = T_p · A / ( (I_f/I_p)^p − 1 ) with IEC 60255-151 constants: NI A=0.14 p=0.02, VI A=13.5 p=1, EI A=80 p=2, LTI A=120 p=1. The relay operates only when I_f > I_p. The current ratio for a target time is solved from the same formula. Confirm the chosen curve, T_p and the grading margin against the coordination study; IEEE C37.112 constants differ and are not used here.