Two IEC Standard Inverse relays look correctly graded at one fault current — until the full-range sweep reveals the margin collapses at higher currents.
| Relay 1 (downstream) | Pickup 100 A, CT 1:1, TMS 0.1, IEC Standard Inverse (SI) |
| Relay 2 (upstream) | Pickup 150 A, CT 1:1, TMS 0.25, IEC Standard Inverse (SI) |
| Test fault current | 2000 A |
| Full sweep range | 105 A – 10,000 A (the calculator's default range) |
| Required grading margin (CTI) | 0.4 s |
Relay 1 protects a feeder; Relay 2 is the upstream backup device one bus further up. Both must trip in the right order with enough time separation, at every current either relay could realistically see.
IEC 60255-151 defines the Standard Inverse (SI) curve with k = 0.14 and alpha = 0.02.
This is the check most hand calculations stop at. It looks fine.
Real faults land anywhere in the range a relay can see, not just at one convenient test current. Recomputing the margin at every point across 105 A-10,000 A finds where it is smallest.
| Fault current | t1 (Relay 1) | t2 (Relay 2) | Margin |
|---|---|---|---|
| 2000 A | 0.227 s | 0.658 s | 0.432 s |
| 10,000 A (worst case) | 0.145 s | 0.399 s | 0.254 s |
IEC inverse curves flatten out at high multiples of pickup, which is why the two relays' trip times converge — and the margin shrinks — as fault current rises.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Margin at the single 2000 A test point | ≥ 0.4 s | 0.432 s | ✓ PASS |
| Worst-case margin, full 105 A–10,000 A sweep | ≥ 0.4 s | 0.254 s at 10,000 A | ✗ FAIL |
Key insight: A comfortable margin at one arbitrarily chosen fault current does not guarantee coordination everywhere. IEC inverse curves compress at high current multiples, so the worst-case point is often at the top of the range, not the bottom — which is exactly why this calculator sweeps the whole range by default instead of checking a single current.
Every input in this example is editable in the live calculator — free, no signup.
Open IDMT Relay Coordination calculator →Raising Relay 2's time multiplier setting (TMS) from 0.25 to roughly 0.34 restores at least 0.4 s of margin across the entire 105 A–10,000 A range, without affecting Relay 1's own trip times.
The IEC inverse-time equation t = TMS x (k / (M^alpha - 1)) approaches TMS x k / M^alpha as M grows, so at high multiples of pickup the trip time is dominated by the curve's shape rather than the TMS offset — two relays on the same curve family converge toward each other's trip times as current rises.
0.4 s (400 ms) is a commonly used CTI (coordination time interval) for electromechanical relays, covering breaker interrupting time, relay overtravel and a safety margin. Modern numerical relays with no overtravel are sometimes graded to 0.2-0.3 s — always confirm against your utility's or plant's own protection philosophy document.