A time-current curve (TCC) plots the operating time of a protective device against the fault current it must clear. Devices with lower pickup (I_s) operate faster at high fault currents. On a log-log plot, inverse-time curves slope downward — higher current means faster operation. The coordination engineer places upstream and downstream device curves on the same plot to verify a minimum 0.2-second margin between them at all fault levels.
IEC 60255-151 defines four standard inverse-time characteristic curves: SI (Standard Inverse, k=0.14, α=0.02) — moderately inverse, used for radial feeders; VI (Very Inverse, k=13.5, α=1.0) — steeper, gives better discrimination in time-delayed coordination; EI (Extremely Inverse, k=80, α=2.0) — very steep, ideal for transformer backup protection where coordination with downstream fuses is required; D (Definite Time) — operates at a fixed time regardless of current magnitude, used as a backup.
Selectivity (coordination) means the downstream device (fuse or relay) trips first for a fault on its side of the protection zone, while the upstream device acts as a backup and only trips if the downstream device fails. The coordination margin is the time difference between upstream and downstream device operating times at the maximum fault level. IEC 60255 recommends ≥0.2 s margin at all fault levels for relays, and for fuse-relay coordination, the fuse should clear before the relay's backup function operates.
IEC 60269 gG general-purpose fuses follow t_melt = k · I²t_nom / I². The constant k (≈1.0–1.8) depends on the fuse design. At low currents near the minimum melting current (1.05×I_n), melting time can be many hours. As fault current increases, the I² factor dominates and melting time drops rapidly. The total clearing time (melt + arcing) is approximately 1.35× the melting time. For coordination, the fuse's minimum melt curve (not the total clear curve) is plotted as the upstream device characteristic.
Both relays and fuses appear as curves on the TCC. Relays ( protectors) have adjustable pickup I_s and time-dial setting (which scales the curve up or down). Fuses have a fixed characteristic determined by their rated current I_n — the higher the fault current, the faster the fuse clears. On the plot, a fuse curve is steeper than a relay curve. For relay-fuse coordination, the fuse curve must be to the right (slower) than the relay curve at all fault levels within the fuse's zone.
The CSV export contains I (A), device label, and t operating (s) columns. You can open this in Excel, ETAP, or AutoCAD to produce a professional coordination study document. The CSV includes all devices at the same current points so you can directly compare operating times. The export is useful for adding curves to a documentation package or for import into power system analysis software that accepts ASCII curve data.