A secondary injection test set, or relay test set, generates the currents and voltages that the current and voltage transformers would deliver, and measures when and how the relay responds. It is used to test pickups, operating times, logic and complete schemes during protection commissioning and maintenance. Current models also publish and subscribe to IEC 61850 messages.
What a secondary injection test set is
It is a portable instrument that replaces the current transformers (CTs) and voltage transformers (VTs) while a relay is tested. It generates currents and voltages at secondary level, with the magnitude, angle and frequency it is told, and at the same time reads the relay’s output contacts to measure when and how it responded. It is also called a relay test set, a protection test set or a secondary injection kit.
It is used at three moments:
- Acceptance testing and commissioning of new relays and schemes.
- Periodic maintenance testing of the protection.
- Trip analysis, when a real fault is played back to see how the relay responds.
What it generates and measures
| Part | What it does |
|---|---|
| Current outputs | Simulate the CT secondaries, normally as a three-phase system |
| Voltage outputs | Simulate the VT secondaries, including the residual voltage when needed |
| Binary inputs | Read the relay’s trip, pickup and alarm contacts and measure their times |
| Binary outputs | Simulate the breaker status (52a, 52b) and other signals the logic needs |
| Ethernet port | Communication with the software and, on models with IEC 61850, publishing and subscribing to GOOSE and Sampled Values |
| Time synchronization | Lets two test sets at opposite ends of a line be coordinated for end-to-end tests |
The test set is controlled with its software, where the test plan is built: which values to inject, in what sequence and what result is expected. It is used for steady-state tests (pickups), ramps, operating times, complete characteristics such as the mho circle of a 21 or the dual slope of an 87, and dynamic tests that play a fault as a pre-fault, fault and clearing sequence. COMTRADE records of a real fault can also be played back; to generate a test record you can use our free COMTRADE generator.
Types of test set
- Three-phase current and voltage. The most common for numerical feeder, transformer, line and generator relays.
- With more current channels. For transformer and bus differentials, which receive currents from several windings or bays at once.
- Single-phase or high power per channel. For electromechanical or high-burden relays and for high secondary current tests.
- With IEC 61850. Test sets that, besides the analog outputs, publish and subscribe to GOOSE and generate Sampled Values. There are models dedicated to IEC 61850 only, without conventional amplifiers, for substations with a process bus.
Market examples include the OMICRON CMC family, the Doble F6150e and the Megger SMRT and Freja; we describe them in OMICRON CMC and Test Universe and in Doble F6150e, Megger SMRT410 and Freja. We do not compare them: the choice depends on the use.
What to check when choosing or renting
In general terms:
- Enough current and voltage channels for the relays to be tested, without connecting and reconnecting.
- Current and power per channel against the relay burden and the test leads, especially with electromechanical relays.
- Binary inputs and outputs, their type (dry or wetted contact) and their time resolution.
- IEC 61850: whether GOOSE, Sampled Values or both are needed, and in which edition.
- Software and licenses: the test modules for each function are often sold separately.
- Time synchronization for end-to-end tests.
- A current calibration certificate, especially if the test report must be submitted to a utility or a client.
- Accessories: test leads, adapters for the panel’s test blocks and additional amplifiers.
What a well-done test leaves behind
The test set produces measurements; the value lies in how they are organized. A useful set of secondary injection results includes, for each relay:
- Identification of the relay, its firmware and the settings file tested.
- The test set used, with its serial number and calibration date.
- Each function tested, with the expected value, the measured value, the tolerance applied and the result.
- The logic tests and trip tests to the actual breakers, if they were done.
- The observations and any open items.
What is tested and with what criteria is set beforehand in the relay test plan. Without that traceability, the test cannot be repeated or compared at the next maintenance. We cover it in detail in what to ask for in a protection test report.
The test set does not test the whole chain
A secondary injection test set connects at the relay terminals or at the test block. So it does not verify the ratio or polarity of the CTs, nor the wiring between the CT and the panel. That part is covered by instrument transformer testing and by primary injection or load checks at energization. Nor does it replace the study: the injected values come from the relay settings, and the acceptance criteria from the standard and the manufacturer. The applicable standards are in standards for protection testing and secondary injection.
We do your protection testing
At Tiza Group we test protective relays and complete schemes with our own test sets, in medium, high and extra-high voltage substations and in industry, in Colombia, or elsewhere on request: secondary injection testing and end-to-end testing.
Related guides
- OMICRON CMC 356, CMC 430 and Test Universe
- Doble F6150e, Megger SMRT410 and Freja test sets
- Test blocks and bay isolation for testing
- Standards for protection testing and secondary injection
- What to ask for in a protection test report
- COMTRADE files: .cfg, .dat, .hdr and .inf
- Protection testing with GOOSE and Sampled Values
- Protective relay commissioning: what is verified
- IEC 61850 test mode: Mod, Beh, Sim and the test bit
References
- IEEE C37.233-2023, Guide for Power System Protection Testing. https://standards.ieee.org/ieee/C37.233/6676/
- IEC 60255-1:2022, Measuring relays and protection equipment - Part 1: Common requirements.
- IEC/IEEE 60255-24:2013 (IEEE C37.111), COMTRADE.
- ANSI/NETA ATS-2025, Standard for Acceptance Testing Specifications for Electrical Power Equipment and Systems.
- OMICRON, CMC 850 brochure (test set dedicated to IEC 61850, GOOSE and Sampled Values).
Frequently asked questions
What is the difference between a relay test set and a primary injection test set?
The secondary injection test set delivers currents and voltages at CT and VT secondary level directly to the relay. The primary injection test set drives high currents through the primary of the CT or of the low-voltage breaker and tests the whole chain. They are complementary: the first tests the relay and its logic; the second, the chain from the primary.
Can a three-phase test set test any relay?
It can test most numerical relays. The limits appear with high-burden electromechanical relays, which may need more power per channel, with differentials that need more current channels than the set has, and with IEC 61850 relays that receive currents as Sampled Values, which require a test set with that function.
Is it better to buy or rent a relay test set?
It depends on how often it is used, on the trained staff available and on the software licenses needed. Buying the test set alone is not enough: you need people who can plan and run the tests and interpret the results, and a calibration plan. That is why many companies hire the testing service.