From the GridCircuits tools team: a feeder protection setting is a compromise. Trip too fast and a brief overload or a downstream fault can black out customers who did not need to lose supply. Trip too slowly and fault energy, equipment damage and safety risk all grow. Inverse definite minimum time (IDMT) protection is widely used because it balances those risks: the larger the fault current, the faster the relay trips, while a definite minimum time keeps operation predictable at very high currents.
What IDMT actually means
An IDMT relay compares measured current with its pickup setting, often written Is. Below pickup, it does not start timing. Above pickup, the operating time falls as the multiple of pickup, I/Is, rises. The time multiplier setting (TMS), also called the time dial, scales the whole curve up or down without changing its shape. That separation matters: the curve type sets how steeply time falls with current, while TMS sets how fast or slow that particular curve is for grading with upstream and downstream devices.
Standard inverse
Standard inverse is the usual starting point for many distribution feeders. Its time falls only gently as current rises, so it gives useful grading margins over a wide range of fault levels and tolerates load variation better than steeper curves. Use it where fault current does not change dramatically along the feeder, where downstream fuses or relays need a clear time margin, and where a moderate delay at high current is acceptable. Its gentler slope is also forgiving when fault levels are uncertain.
Very inverse
Very inverse falls more steeply. At low multiples of pickup it is slower than standard inverse for the same TMS, but at high multiples it becomes faster. That makes it useful where fault current varies a lot with fault location, for example on a long feeder where a fault near the source is much larger than one at the far end. It can improve discrimination with downstream devices that also respond more strongly to high current, but check the low-current end carefully so a modest overload does not take too long to clear.
Extremely inverse
Extremely inverse is the steepest of the three. It stays relatively slow close to pickup, then trips very quickly at high multiples. That shape suits protection that must coordinate with fuses, whose melting time also falls sharply with current, and feeders where very high fault currents must be cleared fast to limit damage. Do not choose it by default: at low multiples it can be slow, and small errors in pickup or current-transformer performance can have a larger effect on operating time.
Worked example
Take a feeder relay with a pickup of 500 A and a TMS of 0.1. For a 5,000 A fault, the multiple is 10. Using the IEC standard-inverse, very-inverse and extremely-inverse shapes, the operating times are about 0.30 seconds, 0.15 seconds and 0.08 seconds respectively. The same fault, the same TMS, and yet the extremely-inverse element is roughly four times faster than standard inverse. That difference is why curve choice is a coordination decision, not just a speed contest.
Now keep the fault and curve fixed and double TMS from 0.1 to 0.2. The standard-inverse time roughly doubles, from about 0.30 seconds to about 0.60 seconds. That is the practical use of TMS: once the curve shape gives the right grading behavior, adjust TMS in small steps to obtain the required margin above the downstream device and below the upstream device, then recheck both the maximum fault current and a lower, more distant fault.
Check before you set
Before applying any setting, confirm the pickup against normal load and motor starting, check current-transformer saturation at the highest fault current, and confirm grading margins at both ends of the feeder. To check how changing pickup, TMS and fault current moves the curve, try this IDMT relay curve calculator interactively before you finalize a setting. A curve that looks right at one fault level can still fail grading at another, so test the full range you expect in service.
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