Part 1 of the protection series. Step by step from the downstream relay time to the upstream TMS, checked at the maximum fault current — and why the margin only grows at lower currents.
| Downstream relay R1 (11 kV feeder) | IEC Standard Inverse, pickup 400 A, TMS 0.10 |
| Upstream relay R2 (11 kV incomer) | IEC Standard Inverse, pickup 1,000 A, TMS to be found |
| Maximum fault at the feeder | 13.12 kA (from IEC 60909) |
| Grading margin | 0.3 s (numeric relays, vacuum breakers) |
| Fault current | R1 (s) | R2 (s) | Margin (s) |
|---|---|---|---|
| 2 kA | 0.428 | 1.906 | 1.48 |
| 4 kA | 0.297 | 0.946 | 0.65 |
| 8 kA | 0.227 | 0.626 | 0.40 |
| 13.12 kA | 0.194 | 0.503 | 0.31 |
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Margin at maximum fault | ≥ 0.3 s | 0.31 s | ✓ PASS |
| Margin at 2 kA | ≥ 0.3 s | 1.48 s | ✓ PASS |
Key insight: Always set the margin at the maximum fault current the downstream relay sees. If the curves are of different types (for example SI over EI), check the whole range — they can cross.
Every input in this example is editable in the live calculator — no signup needed.
Open IDMT Relay Coordination calculator →0.4 s is a traditional margin for electromechanical relays and oil breakers. Numeric relays with fast breakers commonly use 0.25–0.3 s; Part 4 shows how the margin is built up.
Above the maximum load current (with margin) and below the minimum fault current the relay must detect, including its back-up zone.