Commissioning a protective relay proves on site that the device is healthy, that it measures correctly, that the approved settings are loaded, that every function and its logic operate as designed and that the trip reaches the right breaker. It differs from acceptance testing, which validates a relay model, and from maintenance testing, which confirms that nothing has changed. It relies on secondary injection and ends with functional tests of the complete scheme.
What relay commissioning is
Commissioning a protective relay is the on-site verification that this relay, with its actual wiring and settings, protects what it is supposed to protect. Working on the bench is not enough: it has to measure the right currents and voltages, operate with the settings of the approved study and trip the right breaker, in the expected time.
IEEE C37.233, the IEEE guide for power system protection testing, separates test types by timing and purpose. It follows a bottom-up approach: first the behavior of each component of the scheme is verified, then the behavior of the connected components as a function-oriented whole. CIGRE TB 637 covers acceptance, commissioning and field testing of protection and automation systems in the same spirit.
| Test type | When | What it proves |
|---|---|---|
| Acceptance (type or application) | Once per relay model and application | That the model and its logic are fit for the intended use |
| Commissioning | Before each relay is placed in service | That this relay, with its wiring and settings, works within its scheme |
| Maintenance | Periodically or after an event | That the relay and the scheme still work as they did at commissioning |
What is verified
Relay commissioning goes through the device from the outside in, and back out again to the breaker.
Hardware and power supply
Inspection of the relay and its mounting, identification and firmware version, DC auxiliary supply and relay self-supervision (watchdog contact). The firmware version matters: the settings and logic were prepared for a specific version.
Inputs and outputs
Each binary input is operated from its real source (breaker or disconnect switch auxiliary contact, local/remote selector, signal from another relay) and each output is checked all the way to its destination: trip coil, close coil, alarm or signal to the control system. This is where most wiring errors show up.
Measurements
Secondary injection proves that the relay measures magnitudes, angles and sequences as expected, and that the configured CT and VT ratios are correct. Later, with the installation energized, the real quantities confirm it; that part is described in cold and hot commissioning.
Protection functions
Each enabled function is tested at its pickup points and times: the overcurrent characteristic with its curve, the distance zones, the differential slope, directionality. You can check the expected behavior of a curve with our free inverse-time curves tool.
Logic and schemes
Reclosing, breaker failure, blocking, permissives, teleprotection and the logic programmed by the integrator. The whole logic is tested, not only the function: that reclosing respects its conditions, that breaker failure trips the right bays, that blocking signals reset.
Actual trips
With the test switches in the right position and the clearances in place, each trip command is checked to open the right breaker. This is the test that closes the chain from the relay to the power system.
Communications and records
Signals and events to SCADA, time stamping and synchronization, oscillography, GOOSE messages in IEC 61850 systems and, when there is a scheme between substations, end-to-end testing. In IEC 61850 it relies on test mode and simulation.
- 01Hardware and firmwareIdentification, power supply and self-supervision
- 02Inputs and outputsFrom their real source to their destination
- 03MeasurementsCT and VT ratios, magnitudes and angles
- 04Functions and logicPickup points, times, reclosing, 50BF
- 05Tripping and communicationsActual breaker, SCADA, GOOSE and end to end
Conventional relays and the digital substation
In a conventional bay, the relay receives currents and voltages through copper wiring and trips through a contact. In a digital substation with a process bus, the quantities arrive as sampled values from a merging unit and the trip leaves as a GOOSE message. The goal of commissioning is the same, but the tools change: instead of injecting at the terminals, simulated sampled values are published, and the SCL file configuration, time synchronization and data quality are verified. CIGRE TB 760 proposes a test strategy for this case.
Settings: the part that fails most
In practice, a good share of commissioning problems is not in the relay but in its settings: the wrong file version, a settings group that is not the active one, a mistyped CT ratio, a function left enabled by default. That is why commissioning must end with a copy of the loaded settings, compared against the approved study, which goes into the commissioning dossier.
What must not be skipped
- Test with the approved settings, not with test values.
- Verify the actual trip to the breaker, not only the output contact.
- Leave the relay in normal service: test switches closed, test modes off, alarms reset.
- Record everything in a test report with measured values and acceptance criteria.
The specific test plan for each relay depends on its model, its functions and its scheme, and is defined with the project specification.
We commission your relays
If your project needs the relay settings loaded, reviewed or corrected before energization, we prepare and load settings for the main brands through remote relay configuration, worldwide. In the field, our crew performs secondary injection, functional tests and end-to-end tests in medium, high and extra-high voltage substations and in industry, in Colombia, or elsewhere on request; contact us.
Related guides
- Secondary injection test set: what it is
- Test switches and bay isolation
- Relay test plan
- Substation commissioning: stages and tests
- IEC 61850 test mode: Mod, Beh, Sim and the test bit
References
- IEEE C37.233-2023, IEEE Guide for Power System Protection Testing.
- CIGRE TB 637, Acceptance, commissioning and field testing techniques for protection and automation systems, 2015.
- CIGRE TB 760, Test strategy for Protection, Automation and Control (PAC) functions in a fully digital substation based on IEC 61850 applications, 2019.
- ANSI/NETA ATS-2025, Standard for Acceptance Testing Specifications for Electrical Power Equipment and Systems.
Frequently asked questions
What is the difference between acceptance, commissioning and maintenance testing of a relay?
Acceptance testing validates that a relay model and its application are fit for the intended use, usually once per type. Commissioning testing checks each installed relay on site, with its actual settings and wiring. Maintenance testing confirms periodically that the relay and its scheme are still working.
Is it enough to test the relay functions with the test set?
No. Injection proves the relay, but commissioning must also prove that the real currents and voltages arrive correctly from the CTs and VTs, that the trips open the right breaker, that the signals reach SCADA and, where applicable, that the scheme works with the remote end of the line.
Should every relay function be tested, or only the enabled ones?
The ones enabled in the approved settings, with their pickup points, times and logic. It is also good practice to check that the functions that must stay disabled do not operate, because a forgotten default setting can cause unwanted trips.