Part 2 of the protection series. The margin over a fuse grows with the fuse time (0.4 t + 0.15 s). The steep part of the fuse curve at low current sets the relay TMS.
| Downstream | 200 A gG fuse on an outgoing feeder |
| Fuse total clearing times (manufacturer's curve, values for this example) | 1.5 s at 1.5 kA; 0.25 s at 2 kA; 0.02 s at 4 kA; 0.01 s at 15 kA (maximum fault) |
| Upstream relay | IEC Standard Inverse, pickup 1,000 A |
| Margin rule | t_relay ≥ t_fuse + (0.4 t_fuse + 0.15 s) |
| Current | Fuse (s) | Required relay (s) |
|---|---|---|
| 1.5 kA | 1.50 | 2.25 |
| 2 kA | 0.25 | 0.50 |
| 4 kA | 0.02 | 0.178 |
| 15 kA | 0.01 | 0.164 |
EI would need only TMS 0.035 at 1.5 kA, but at 15 kA its time falls so fast that TMS 0.46 is needed to keep 0.164 s. With these fuse data SI gives the better overall result; with other fuses EI often wins. Check the whole range either way.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Margin at 1.5 kA | ≥ 2.25 s | 2.41 s | ✓ PASS |
| Margin at 15 kA | ≥ 0.164 s | 0.35 s | ✓ PASS |
Key insight: Fuses have no fixed margin: the relay must allow for fuse tolerance, which grows with the fuse operating time. Use the manufacturer's total-clearing curve, not the pre-arcing curve.
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Open IDMT Relay Coordination calculator →It is a commonly used rule for an upstream relay over a downstream fuse: 0.4 t covers fuse tolerance and 0.15 s covers relay overshoot and breaker time.
Then the fuse must not melt before the relay and breaker clear, so compare the fuse's minimum pre-arcing curve with relay time plus breaker time, with margin.