Part 4 of the protection series. The margin is not a fixed 0.4 s: it is relay error, CT error, breaker time, overshoot and a safety allowance. Using it properly makes the upstream relays faster.
| R1 — MCC feeder (LV) | IEC SI, pickup 400 A, TMS 0.05; fault at MCC 22.9 kA |
| R2 — LV incomer | IEC SI, pickup 2,500 A; fault at LV board 37.4 kA |
| R3 — transformer HV relay | IEC SI, pickup 100 A at 11 kV (LV board fault = 1,360 A at 11 kV) |
| Margin components (numeric relays) | Relay error 5 %, CT error 10 %, breaker 50 ms, overshoot 20 ms, safety 30 ms |
With 0.4 s, R2 would need TMS 0.16 (0.40 s at the LV board), and R3 would be slower again — adding arc-flash energy at the LV board.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| R2 over R1 at 22.9 kA | ≥ 0.200 s | 0.216 s | ✓ PASS |
| R3 over R2 at LV board fault | ≥ 0.312 s | TMS 0.12 → 0.313 s | ✓ PASS |
Key insight: The margin should match the equipment. Numeric relays and fast breakers justify 0.2–0.3 s; older electromechanical relays need about 0.35–0.4 s because their errors and overshoot are larger.
Every input in this example is editable in the live calculator — no signup needed.
Open IDMT Relay Coordination calculator →It is a widely used engineering method (see protection application guides). IEC 60255 defines relay accuracy; the allowances for breaker time and overshoot come from the equipment data.
Check it at the maximum current that both relays see, and at any point where the curves come closest.