Inverse Definite Minimum Time (IDMT) curves are used in both mechanical and electronic protection devices. These curves define the amount of time which should pass at a given current, before initiating an action, usually the tripping of a circuit breaker.
The IEC who develop international standards for all electrical, electronic and related technologies, have defined 4 curves to be used. The IEC standards are commonly used throughout Europe and also other parts of the world.
Two settings are used to determine the time for a relay to initiate a trip: current setting (Is) and time multiplier setting (TMS). For any of the curves and a current multiplier (I/Is), a time is calculated, this time is the point at which the relay sends a signal.
The lower the current multiplier, the longer the trip time, allowing for slight overloading of equipment like motor starting, transformer energisation, or a number of other functions. Inversely, a higher current multiplier will cause a shorter trip time, which helps in the case of faults or sever overloading of equipment.
The formula and curve constants used to determine the trip times are shown below:
| Curve type | k | α |
|---|---|---|
| Standard inverse | 0.140 | 0.020 |
| Very inverse | 13.5 | 1 |
| Extremely inverse | 80 | 2 |
| Long time inverse | 120 | 1 |
This convenient tool calculates the expected trip time for a given TMS and I/Is multiplier, and should give some guidance when testing a relay, as the usual method is to pick a multiple of Is and ensure that the actual trip time is close to the calculated value.
Trip times:
Standard Inverse - s Very Inverse - s Extremely Inverse - s Long Time Inverse - s