Educational protection tools.

IEC 60255-151 (IEC curves): t = TMS · k / ((I / I>)^α − 1)

SI: k = 0.14, α = 0.02 · VI: k = 13.5, α = 1 · EI: k = 80, α = 2 · LTI: k = 120, α = 1

IEEE C37.112 (as also given in IEC 60255-151): t = TD · (A / ((I / I>)^p − 1) + B)

MI: A = 0.0515, B = 0.114, p = 0.02 · VI: A = 19.61, B = 0.491, p = 2 · EI: A = 28.2, B = 0.1217, p = 2

Currents are primary amperes; times are seconds. The multiplier is applied directly as in the equations above. Some relays use a different time-dial scale for IEEE curves, so a "TD" value from a relay may not be directly comparable.

• The inverse characteristic is applied from 1.05 × I>; below that the relay is shown as not operating. This threshold is an assumption of this tool — actual relays define their own start value (one manufacturer's implementation, for example, uses 1.1 × I>).
• Above 20 × I> the operating time is held constant, consistent with the end of the effective range of the characteristic described in IEC 60255-151. Individual relays may differ.
• A high-set stage, when enabled, operates after its definite time if the current is at or above I>>; the relay operates at whichever stage is faster.
• Relay timing tolerances, CT errors and saturation, breaker interrupting time, relay overshoot, reset characteristics, DC offset and directional or voltage-dependent functions are not modelled.
• Accepted inputs: pickup 10–20 000 A, multiplier 0.02–1.20, high-set 10–100 000 A and 0–5 s, required margin 0.10–1.00 s, fault current 100 A–40 kA. Out-of-range entries are flagged and the last valid value is kept.
• The equations and coefficients were checked against published IEC 60255-151 curve tables. The tool itself has not been validated against a specific relay and should not be used to set one.