Computes the trip time of an earth-fault (50N/51N) IDMT relay at a given earth-fault current, using the same verified IEC 60255-151 / IEEE C37.112 inverse-time curve mathematics as the phase-overcurrent IDMT calculator — only the CT arrangement (residual/core-balance) and typical pickup levels differ, not the underlying curve physics.
Standards followed
IEC 60255-151 (measuring relays and protection equipment)IEEE C37.112 (inverse-time characteristics for overcurrent relays)
What it can do
- Plug setting multiplier (PSM) and IDMT operating time from the earth-fault relay's curve family, type, pickup, time dial and CT ratio.
- Optional high-set instantaneous earth-fault stage (Ie>>) with its own definite-time delay.
Example problem
Relay: IEC SI, pickup 0.2A (secondary), CT ratio 200:1, TMS 0.2. Earth fault current 1500A primary.
- Relay current = 1500/200 = 7.5A.
- PSM = 7.5/0.2 = 37.5×.
- t = TMS × (0.14 / (PSM^0.02 − 1)) = 0.2 × (0.14 / (37.5^0.02 − 1)) ≈ 0.37s.
≈0.37s IDMT operating time — hand-checked against the same verified curve equation used by the phase IDMT calculator.
Want the full step-by-step version?
Reuses the already-verified idmt.ts curve engine directly rather than re-deriving the equations, to avoid a second, potentially-inconsistent implementation.