A relay test set lets you verify a protection relay's behaviour — pickup current, operating time, directional sense, trip characteristic — without waiting for an actual fault to occur on the system. It injects controlled current and voltage into the relay's secondary circuit (the CT/PT-fed inputs the relay actually sees) and times how the relay responds. Choosing the right relay test set comes down to one central question: are you testing single-phase protection, or do you need three-phase, phase-angle-controlled injection? This guide walks through that decision.
What Secondary Injection Testing Actually Does
Protection relays don't see primary system voltage and current directly — they see scaled-down secondary values from current transformers (CTs) and potential transformers (PTs), typically 1 A or 5 A CT secondaries and 110 V PT secondaries. A relay test set reproduces those secondary conditions directly at the relay's terminals: it injects a controlled test current (simulating a fault current level) and, where required, a controlled test voltage and phase angle, then times precisely how long the relay takes to operate. This proves the relay itself — its settings, its calibration, its mechanical/electronic operating time — completely independent of whether the CTs, wiring, or the rest of the protection scheme are involved.
Single-Phase vs. Three-Phase Relay Test Sets
| Relay type | What it needs from the test set | Test set class |
|---|---|---|
| Overcurrent (OC) relay | A single controlled current, ramped or stepped, with an accurate operate-time measurement | Single-phase (RTS-5, RTS-5A) |
| Earth fault (EF) relay | A single controlled current representing the residual/earth fault path | Single-phase (RTS-5, RTS-5A) |
| Directional relay | Current and voltage together, with a controlled phase angle between them, to verify the relay only trips for the correct fault direction | Three-phase, phase-angle capable (RTS-33) |
| Distance (impedance) relay | Simultaneous multi-phase current and voltage with precise phase angle control, to simulate an apparent impedance at a specific reach point | Three-phase, phase-angle capable (RTS-33) |
| Differential relay | Independently controlled currents into two or more relay inputs to simulate through-fault vs. internal-fault conditions | Three-phase, multi-source (RTS-33) |
Key Specifications to Compare
- Current output range and burden capacity: The test set must supply enough current at the relay's rated burden (VA) to reach the pickup and multiples of pickup you need to test — high-set instantaneous elements are often tested at several times pickup current, which demands more output capacity than a simple pickup check.
- Timer resolution and range: Operating-time measurement accuracy matters most for time-graded overcurrent schemes, where discrimination between relay stages can depend on differences of tens of milliseconds. Look for millisecond-resolution timing.
- Phase angle control: Only relevant for directional and distance relays — but essential for them. A single-phase test set physically cannot verify directional or distance characteristics correctly, regardless of how it's connected.
- Fixed auxiliary voltages: Useful for powering relay trip circuits or auxiliary contacts during a test without a separate DC supply.
- Portability and construction: Site commissioning work — especially at remote substations — favours a rugged, trolley-portable single-unit design over a bench instrument.
Match the test set to the protection scheme, not just the relay
What a Commissioning Test Actually Verifies
- Pickup value: The minimum current (or voltage, for voltage relays) at which the relay just begins to operate — confirmed against the relay's setting.
- Operating time: How long the relay takes to trip at a given multiple of pickup — critical for time-graded discrimination between relays at different points in the network.
- Reset ratio: The ratio between pickup and dropout (reset) current — confirms the relay resets cleanly once the fault condition is removed, without chattering.
- Directional/distance characteristic (where applicable): Confirms the relay only operates for faults in the intended direction or within the intended reach/zone.
CIE manufactures the RTS-5 and RTS-5A single-phase relay test sets for overcurrent and earth fault relay testing, and the RTS-33 — a two-unit, three-phase system with full 0–360° phase angle control — for distance, directional, and differential relay commissioning. Contact us to match a test set to your protection scheme.