Add up grandmaster, transparent clock and end-device errors, check the result against the time class sampled values need, and estimate how long the merging units stay accurate if GPS is lost.
| Function | Sampled values for protection — class T4, ±4 µs |
| Method | PTP power profile (IEC 61850-9-3) |
| Error contributions | Grandmaster 250 ns, 4 transparent clocks of 50 ns, no boundary clocks, end device 200 ns |
| Holdover | Device oscillator TCXO, 1 ppm |
IEC 61850-5 time performance classes: T3 ±25 µs, T4 ±4 µs, T5 ±1 µs.
IEC 61850-9-3 allows 250 ns at the grandmaster, 50 ns per transparent clock and 200 ns per boundary clock, so that 1 µs is held after 15 transparent clocks: 250 + 15 × 50 = 1 000 ns.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Time error | ≤ ±4 µs (T4) | ±0.65 µs | ✓ PASS |
| Holdover with TCXO | Long enough to ride through a GPS outage | ≈ 3.4 s | ✗ FAIL |
Key insight: The accuracy budget is usually easy to meet with PTP; holdover is where designs fall short. Check what happens when the GPS antenna fails, not only when it works.
Every input in this example is editable in the live calculator — no signup needed.
Open SCL NetStudio (IEC 61850 network design) calculator →The worst-case sum is conservative. The tool can also show the root-sum-square, which is closer to what is measured when errors are independent.
In each sampled-value frame smpSynch shows whether the merging unit is synchronised to a global source (2), a local one (1) or none (0); the capture analyser flags frames that are not synchronised.