An overcurrent relay opens a circuit. A larger fault makes it open faster. Two relays in series must open in the right order. The one nearest the fault must open first. This page shows how the curve works and how to grade two relays.
The inverse-time curve
An IDMT relay has an inverse definite minimum time characteristic. The trip time falls as the current rises. A small overload gives a long delay. A large fault gives a short one.
The curve has one formula: t = TMS × k / (M^α − 1).
- t is the trip time in seconds.
- M is the fault current divided by the pickup current.
- TMS is the time multiplier setting.
- k and α are constants of the curve.
The tool offers seven curves. The IEC curves come from IEC 60255-151:2009. The standard inverse curve, IEC-SI, uses k of 0.14 and α of 0.02.
Below the pickup current the relay does not trip. M must be above 1.
The three settings
Pickup current. The relay starts timing above this current. Set it above the normal load.
Time multiplier. It scales the whole curve. A larger value gives a slower relay.
Instantaneous pickup. An optional element. Above this current the relay trips at once. The tool uses 0.01 s.
Grading
Grading sets the settings so the downstream relay opens first. The upstream relay waits. It acts only if the downstream relay fails.
The gap between the two trip times is the grading margin. The tool warns when the margin is below 0.3 s. This is common practice, not a value from the standard.
A worked pair
A fault of 1200 A flows through two relays. Both use the IEC-SI curve.
| Relay | Pickup | TMS |
|---|---|---|
| Downstream | 100 A | 0.1 |
| Upstream | 200 A | 0.25 |
The downstream relay sees M = 1200 / 100 = 12. The upstream relay sees M = 1200 / 200 = 6.
- Downstream trip time: 0.27 s
- Upstream trip time: 0.96 s
- Grading margin: 0.68 s
The margin is above 0.3 s. The downstream relay opens first with room to spare.
Open the IDMT tool with these relays.
A pair that fails
Reduce the upstream TMS from 0.25 to 0.12. Keep the rest.
- Downstream trip time: 0.27 s
- Upstream trip time: 0.46 s
- Grading margin: 0.19 s
The tool warns that the margin is below 0.3 s. The two relays are too close. A small timing error could open both.
Open the IDMT tool with the faster upstream relay.
One fault current is one point
A grading check at one fault current is not enough. The two curves can cross at another current. The tool computes both curves across a range of multiples. Check the gap at the highest and the lowest fault current.
The fault current comes from the supply. Use the fault level tool to find it at each relay position.
Where it goes wrong
The margin is checked at one current only. Check the lowest and the highest fault.
Different curve families are mixed. IEC and IEEE curves use different formulas. An IEEE curve at time dial 1 is not the same as an IEC curve at TMS 1.
The pickup is set too close to the load. A relay that starts timing at normal load trips on a surge.
The instantaneous element is ignored. Above its setting the trip time is 0.01 s. That can remove the margin.
Maker data is not used. The tool follows the standard curve. A real relay has its own tolerance. Check the maker data.
This page is a design aid. Verify every value against the relay maker data.
See also: IDMT curve grading for a 100 A feeder, IEC 60255-151 IDMT curve constants and Prospective fault current at a board.