An earth-fault relay with both an inverse-time (51N) stage and a high-set instantaneous (50N) stage, checked at two different fault levels.
| Relay pickup (Ie>) | 0.2 A (relay/secondary side) |
| CT ratio | 200 : 1 |
| Time multiplier (TMS) | 0.2, IEC Standard Inverse (SI) |
| Instantaneous stage (Ie>>) | 4000 A primary, 0.05 s definite-time delay |
| Case A — primary earth fault current | 1500 A |
| Case B — primary earth fault current | 5000 A |
The high-set instantaneous stage bypasses the inverse-time curve entirely for severe faults, tripping on a short fixed delay instead.
| Case | Primary fault current | 50N pickup | Instantaneous stage |
|---|---|---|---|
| A | 1500 A | 4000 A | Restrained — below pickup |
| B | 5000 A | 4000 A | Operates in 0.05 s (definite time) |
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Case A (1500 A) — governing trip path | n/a | 51N inverse-time stage, 0.372 s | ✓ PASS |
| Case B (5000 A) — governing trip path | n/a | 50N instantaneous stage, 0.05 s (bypasses the 0.276 s IDMT time) | ✓ PASS |
Key insight: A dual-stage scheme gives you both worlds: the inverse-time stage keeps grading margin with downstream/upstream devices for moderate faults, while the instantaneous stage clears severe, close-in faults almost immediately, since there's no coordination need at currents that high — nothing downstream should still be carrying that much fault current.
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Open IDMT Earth Fault Relay (50N/51N) calculator →An instantaneous stage set too low would trip for faults further down the network that a downstream device should clear first, defeating coordination. The high-set pickup is deliberately set above any fault level the relay should see from a downstream fault, so it only operates for faults close to the relay itself.
The plug setting multiplier is simply how many times over pickup the relay is currently seeing. A PSM of 1.0 is right at the trip threshold (technically never trips, since the curve is asymptotic there); higher PSM means a more severe fault and a faster trip time.