Part 5 of the protection series. LV earth faults do not appear as earth faults on the delta HV side, so the HV earth fault relay can be fast. The neutral standby earth fault relay grades with the LV feeders.
| Transformer | 1.6 MVA, 11/0.4 kV Dyn11, LV neutral solidly earthed with a neutral CT |
| LV feeder EF relay | IEC SI, pickup 100 A, TMS 0.10 |
| Standby earth fault (SBEF) on the neutral | IEC SI, pickup 600 A (about 25 % of LV FLC) |
| Earth fault at the far DB | 3.78 kA (from Part 5 of the short-circuit series) |
| Margin | 0.3 s |
For an earth fault on the board itself (38.8 kA) the SBEF is the main protection.
A Dyn transformer has a delta HV winding, which blocks zero-sequence current. An LV earth fault appears on the HV side only as phase current (815 A in two phases, see Part 3), never as residual current. The HV earth fault relay therefore does not need to grade with LV earth fault protection and can be set low and fast for HV cable and winding faults.
Faults inside the LV winding near the neutral produce little phase current. A REF relay compares neutral and phase CTs and trips instantaneously for winding faults only.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| SBEF over feeder EF at 3.78 kA | ≥ 0.486 s | 0.523 s (TMS 0.14) | ✓ PASS |
| HV EF grading with LV EF needed? | Zero sequence blocked by delta | No | ✓ PASS |
Key insight: Earth fault grading stops at a delta winding. Treat each side of a Dyn transformer as its own earth fault zone, with SBEF and REF covering the LV neutral and winding.
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Open IDMT Relay Coordination calculator →It backs up the feeder relays and protects the LV board, the transformer LV terminals and the neutral conductor, which no feeder relay covers.
Yes. With solid earthing check the neutral conductor and transformer short-time ratings; with resistance earthing check the resistor's time rating.