An 11 kV phase-to-earth VT feeding a relay and a meter — sized to a standard VA rating that keeps it loaded in the specific percentage window where accuracy class is actually guaranteed.
| System voltage | 11 kV, phase-to-earth connection, directly earthed neutral |
| Secondary voltage | 110 V (line convention) |
| Burden | Protection relay 5 VA + multifunction meter 3 VA = 8 VA total |
| Lead run | 30 m one-way, 2.5 mm² Cu, go-and-return loop |
Every VT is rated 1.2 × continuously. For a phase-to-earth VT on an effectively (solidly) earthed system, IEC 61869-3 Table 302 adds a short-time factor of 1.5 for 30 s to cover the voltage rise on the healthy phases until the earth fault is cleared.
Accuracy class is only guaranteed within a specific loading window — too lightly loaded and error behavior isn't validated by the standard either.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| VT loading within accuracy-maintaining band | 25%–100% of rated VA | 80% of 10 VA | ✓ PASS |
| Lead voltage drop | n/a (informational — should stay small) | 0.083% | ✓ PASS |
Key insight: Oversizing a VT's VA rating 'to be safe' can actually make accuracy worse, not better — a VT loaded well under 25% of its rated burden falls outside the range the accuracy class is validated for, the same way a VT overloaded above 100% does. The right target is comfortably inside that window, not simply as much headroom as possible.
Every input in this example is editable in the live calculator — free, no signup.
Open VT Sizing calculator →The same 8 VA burden on a 50 VA-rated VT would only load it to 16% — below the 25% floor where the standard's accuracy class guarantee applies, meaning the VT's ratio and phase error at that light loading are technically unspecified by the accuracy class, even though nothing is being overloaded. Matching the standard VA rating reasonably closely to the actual expected burden, not just picking the largest available size, is what keeps the VT inside its guaranteed accuracy window.
The short-time voltage factor increases — from 1.5 for 30 s on an effectively earthed system to 1.9 for 30 s on a non-effectively earthed system with automatic earth-fault clearance, and 1.9 for 8 hours on an isolated or resonant-earthed (Petersen coil) system without automatic clearance, because the VT has to survive a sustained earth fault where the healthy phases rise toward full line-to-line voltage rather than being cleared quickly. This changes the VT's required insulation and thermal rating, not just its burden calculation.