IDMT Curve Reference Card — IEC 60255-151 Standard, Very Inverse, and Extremely Inverse

Protection coordination relies on understanding IDMT (Inverse Definite Minimum Time) curves, yet many engineers have to look up the equations every time they set a relay. This A4 landscape reference card overlays all three IEC 60255-151 IDMT curve types — Standard Inverse (SI), Very Inverse (VI), and Extremely Inverse (EI) — on a single log-log time-current graph so you can visually compare their characteristics at any current multiple. Alongside the graph, each curve's time equation is presented in full: t = TMS × k / ((I/Is)^α - 1), with the specific constants (k and α) for each curve type clearly tabulated — SI (k=0.14, α=0.02), VI (k=13.5, α=1.0), EI (k=80.0, α=2.0). The reverse side covers practical grading guidance: current transformer ratio (CTI) grading margins of 0.3–0.5 seconds between upstream and downstream relays, TMS (Time Multiplier Setting) selection methodology with worked example, pickup current setting rules (1.05–1.3 × maximum load current), and a decision guide for choosing between SI, VI, and EI curves based on fault current characteristics and downstream device coordination requirements. A quick-reference table shows typical TMS values for common distribution network configurations. Scan the QR code for our online protection coordination calculator.