A load-end, mid-feeder and source relay all see the same fault — checking whether each step up the chain has enough time separation from the one below it.
| Relay 1 (load end) | Pickup 1 A, CT 200:1, TMS 0.1, IEC SI |
| Relay 2 (mid-feeder) | Pickup 1.2 A, CT 300:1, TMS 0.2, IEC SI |
| Relay 3 (source) | Pickup 1.5 A, CT 400:1, TMS 0.3, IEC SI |
| Fault current (seen by all three) | 5000 A primary |
| Required grading margin | 0.4 s at every step |
| Step | Downstream time | Upstream time | Margin | Requirement |
|---|---|---|---|---|
| Relay 1 → Relay 2 | 0.211 s | 0.518 s | 0.308 s | 0.4 s — FAIL |
| Relay 2 → Relay 3 | 0.518 s | 0.970 s | 0.451 s | 0.4 s — PASS |
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Relay 1 → Relay 2 margin | ≥ 0.4 s | 0.308 s | ✗ FAIL |
| Relay 2 → Relay 3 margin | ≥ 0.4 s | 0.451 s | ✓ PASS |
Key insight: A grading chain is only as strong as its weakest step — one under-margined pair anywhere in the chain fails the whole study, regardless of how much margin exists elsewhere. Raising Relay 2's TMS to about 0.24 restores the required 0.4 s at this fault level without touching Relay 1 or Relay 3.
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Open IDMT Relay Coordination calculator →Yes — raising Relay 2's TMS increases its own trip time everywhere, which widens the Relay 1 → Relay 2 margin but narrows the Relay 2 → Relay 3 margin. In this case there's enough headroom (0.451 s against a 0.4 s requirement) to absorb the small increase, but it should always be rechecked, not assumed.
Yes — a production grading study checks the full range of currents each relay could see (minimum fault at the far end of the protected zone through to maximum fault close to the source), not just one value, for the same reason the two-relay coordination example on this site shows a single-point check can hide a full-range failure.