Protection
Choosing a relay test set: channels, current and IEC 61850
A relay test set is a long-term purchase that your protection team will use on every commissioning and maintenance job. The right specification depends on the relays you test, not on the longest datasheet. These are the questions to answer before comparing models.
1. How many current channels?
- Three current channels are enough for overcurrent, distance and most feeder relays.
- Six current channels let you inject both sides of a transformer or busbar differential relay at the same time, which is the only practical way to test bias slopes and through-fault stability. If you test differential protection, plan for six.
- Some test sets can parallel channels to give more current on a single phase, which is useful for high-set instantaneous elements.
2. How much current, and how much power?
Work out the highest current you need from the relay's nominal current and its highest setting. A 5 A relay with an instantaneous stage set at 20× nominal needs 100 A to test above the setting, while a 1 A relay needs far less. Many modern numerical relays can be tested within a few tens of amperes per phase.
Output current is only half of the story. Electromechanical and static relays have a much higher burden than numerical ones, so check the output power in VA at the current you need — and the compliance voltage that drives it — not just the maximum current.
3. Voltage channels
Three phase voltages cover most tests. A fourth voltage channel is useful for synchrocheck, residual voltage and neutral displacement functions.
4. Accuracy and timing
- Amplitude and phase accuracy should be clearly better than the relay's own measuring accuracy, so the test set is never the limiting factor.
- Timer resolution in the sub-millisecond range, with binary inputs that accept both dry contacts and wetted (voltage) signals.
- Enough binary inputs and outputs to monitor trip, start and alarm contacts and to simulate breaker auxiliaries.
5. Digital substations: IEC 61850
If your network uses IEC 61850, the test set needs more than analogue outputs:
- GOOSE subscription and publication, to read trip signals and simulate interlocking messages over the station bus.
- Sampled Values publication, commonly to the IEC 61850-9-2 LE profile, to test relays fed by merging units instead of conventional CTs and VTs.
- Import of SCL files (SCD, CID) so you do not configure datasets by hand.
Even if your substations are conventional today, check that the test set can be upgraded later.
6. End-to-end and synchronisation
Line differential and teleprotection schemes are best proven with two test sets at both line ends injecting synchronised fault states. That requires GPS or IEEE 1588 time synchronisation and software that runs the same sequence at both ends.
7. Software and reports
Most of a test engineer's time goes into setting up tests and writing reports. Look for ready-made modules for each protection function, import of relay settings, automatic pass/fail assessment and report templates you can adapt to your utility's format.
8. Practicalities on site
- Supply voltage: confirm the test set runs on your site supply, 230 V or 110–120 V, 50 or 60 Hz, and on a generator if needed.
- Weight and transport case for crews who carry it between substations.
- Language of the software and manuals for your engineers.
- Calibration and service: calibration interval, local support and training on the first jobs.
Quick specification summary
| Your relays | Minimum to look for |
|---|---|
| Feeder overcurrent, numerical | 3 currents, 3–4 voltages, timer, binary I/O |
| Distance protection | 3 currents, 4 voltages, impedance test modules |
| Transformer / busbar differential | 6 currents, harmonic generation, differential module |
| Electromechanical relays | High output power in VA, not only high current |
| Digital substations | GOOSE, Sampled Values (9-2 LE), SCL import |
Our three-phase relay test set is configured for the relays and site supply in your market. Send us your relay types and the tests you run, and we will recommend a configuration. For the test sequence itself, see our secondary injection commissioning checklist.