Overcurrent Protection Coordination Checker

Radial protection coordination means the device closest to a fault (downstream) should clear it before the next device upstream, with enough time margin — the coordination time interval (CTI) — that the upstream device only operates as a backup if the downstream device fails. Standard inverse-time curves are defined by closed-form equations: IEC 60255 uses t = TMS · k / ((I/Is)^α − 1), and IEEE C37.112 uses t = TD · [A / ((I/I_pickup)^p − 1) + B], with k, α or A, B, p set by the curve shape (standard/moderately, very, or extremely inverse).

This calculator evaluates both devices' trip time at a fault current you specify, reports the resulting CTI, and flags whether it clears the target margin you set — commonly 0.3 seconds for electromechanical relays or 0.2-0.25 seconds for microprocessor-based relays with no intentional delay. It plots both curves on time-current axes so you can see the full margin across the current range, not just at the one evaluation point.

This checks coordination at a single fault current only — a complete coordination study checks the full overlapping current range between the two devices' pickup settings, verifies both devices clear before equipment damage curves (cable withstand, transformer through-fault curves), and accounts for instantaneous trip elements if present. Treat this as a fast check of one operating point, not a substitute for a full protective device coordination study.