IDMT Relay Operating Time Calculator
Calculate the trip time of an inverse definite minimum time (IDMT) overcurrent relay for the four IEC 60255 curve types, and see the curve at standard multiples of setting.
- M = If ÷ Is = 4,000 ÷ 400 = 10
- t = TMS × 0.14 ÷ (M0.02 − 1) = 0.2971 s
Operating time at multiples of setting
| Multiple | Current | Time |
|---|---|---|
| 1.5 × | 600 A | 1.719 s |
| 2 × | 800 A | 1.003 s |
| 3 × | 1,200 A | 0.6302 s |
| 5 × | 2,000 A | 0.428 s |
| 7 × | 2,800 A | 0.3528 s |
| 10 × | 4,000 A | 0.2971 s |
| 15 × | 6,000 A | 0.2516 s |
| 20 × | 8,000 A | 0.2267 s |
IEC 60255 IDMT Curves
SI: k = 0.14, α = 0.02 VI: k = 13.5, α = 1
EI: k = 80, α = 2 LTI: k = 120, α = 1
M is the fault current divided by the pickup setting (plug setting multiplier) and TMS the time multiplier setting.
Worked example
Standard inverse, pickup 400 A, TMS 0.1, fault 4000 A (M = 10): t = 0.1 × 0.14 ÷ (100.02 − 1) = 0.30 s.
Frequently Asked Questions
What is TMS in an IDMT relay?
The time multiplier setting scales the whole curve vertically. TMS 0.1 gives one tenth of the time at TMS 1.0.
Which IDMT curve should I use?
Standard inverse suits most networks. Very and extremely inverse grade better with fuses and where fault current falls sharply with distance.
What is plug setting multiplier?
The ratio of fault current to relay pickup current. The relay only operates when it is greater than 1.
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