IDMT Relay Calculator

IDMT Relay Curves for IEC 60255

Inverse Definite Minimum Time (IDMT) relays are the foundation of overcurrent protection coordination in industrial and utility electrical systems. Their defining characteristic is an operating time that decreases as the fault current increases: small overloads are tolerated for longer periods to avoid nuisance tripping, while large fault currents are cleared rapidly to prevent equipment damage. This calculator computes the operating time for all five standard IEC 60255 inverse curves at any given fault current and time multiplier setting (TMS), making it straightforward to verify relay coordination during protection design.

Key concepts

• IEC 60255 curve types. The five standard curves are Standard Inverse (SI), Very Inverse (VI), Extremely Inverse (EI), Long Time Inverse (LTI), and Definite Time (DT). Each follows a different mathematical formula relating the current multiple to operating time. SI is the most common for general distribution protection. VI and EI curves trip faster at high fault currents, making them suitable for transformer inrush coordination and fuse grading.
• Time multiplier setting (TMS). The TMS, sometimes called the time dial, scales the operating time of the relay curve up or down. A lower TMS produces a faster trip time at the same current. TMS is the primary adjustment used to achieve selectivity (grading) between upstream and downstream relays.
• Pickup current (plug setting). The minimum current at which the relay starts to operate. It is usually set as a multiple of the current transformer (CT) secondary rating. The relay does not operate below this threshold. Setting the pickup correctly ensures the relay ignores normal load current and only responds to fault or overload conditions.
• Grading margin. To ensure selectivity, the operating time of the upstream relay must exceed the downstream relay by a grading margin of at least 0.3 to 0.4 seconds at the maximum through-fault current. This margin accounts for relay timing errors, CT errors, and circuit breaker operating time.

Common scenarios

1. Distribution feeder protection grading. An industrial site with a main incomer relay and three downstream feeder relays. All use Standard Inverse curves. The feeder relays are set with TMS 0.1 and the incomer with TMS 0.3, providing a 0.4 second grading margin at the maximum fault level. If a fault occurs on one feeder, that feeder relay trips first while the incomer acts as backup.
2. Transformer protection coordination. A 1000 kVA distribution transformer with an upstream utility relay on the HV side and LV circuit breakers on the secondary. The HV relay uses a Very Inverse curve to ride through the transformer inrush current (which decays quickly) while still providing fast clearance for sustained faults. The LV breakers are set to trip before the HV relay for any fault on the secondary side.
3. Motor feeder relay setting. A large motor draws 6 to 8 times full load current during starting for up to 10 seconds. The relay pickup is set above the full load current, and the TMS is set high enough that the relay does not trip during the starting transient. An Extremely Inverse curve may be chosen so that the relay is slow at moderate overcurrents (motor starting) but very fast at high fault currents (cable or busbar fault).