8.2 Primary vs Secondary Injection & Relay Pickup

8.2.1 Primary Injection

Primary injection drives a high current through the actual primary circuit — the breaker, bus, and CT primaries — and lets the CT secondaries feed the relay, which in turn trips the breaker. It verifies the complete loop: CT ratio and polarity, secondary wiring, relay setting, trip coil, and the breaker itself. Because the current is real (hundreds to thousands of amps), primary injection is performed at commissioning, after major repairs, and whenever the CT or wiring has been disturbed. Test sets include the Omicron CPC 100 with CPOL, Megger Biddle, SMC PTE, and Doble. It is slower, heavier, and more hazardous than secondary injection because of the energy involved.

Typical primary injection checks:

8.2.2 Secondary Injection

Secondary injection injects a calibrated current directly into the relay terminals (or into the CT secondary with the primary de-energized and shorted). It bypasses the CT and primary conductor and exercises only the relay, its settings, and the trip output. Modern secondary injection relay test sets (Doble F-series, Omicron CMC, Megger SMRT) generate programmable three-phase currents and voltages at arbitrary magnitudes, phase angles, and frequencies.

Secondary injection is the routine, periodic, and functional test — it is safer, cheaper, and faster than primary injection, and is sufficient to verify that the relay's settings, curves, and logic still match the coordination study. It is what most relay technicians run on a 1 to 4 year maintenance cycle.

When secondary injection alone is acceptable:

• Periodic relay calibration verification.
• Setting changes (re-test the relay only).
• Curve and timing verification.
• Relay firmware upgrade validation.

When primary injection is required:

• New installation commissioning (first energization).
• After CT replacement or rewiring.
• After breaker replacement or trip-coil work.
• Whenever the CT-to-relay loop integrity is in doubt.

8.2.3 Relay Pickup Test

The pickup test finds the minimum current at which the relay begins to operate — disc rotation in an electromechanical relay, pickup LED or assertion bit in a digital relay. For an ANSI 51 (time overcurrent) element set to pick up at 5.0 A secondary, the test ramps current until the element asserts; acceptance is typically +/- 5% of setpoint (so 4.75 to 5.25 A). ANSI 50 (instantaneous) is tested the same way but is expected to assert with no intentional time delay once pickup is exceeded.

Procedure:

1. Set the relay to the tested setting and disable any upstream trips or lockouts.
2. Ramp current slowly upward until the element asserts; record the pickup value.
3. Ramp back down; record the dropout value (typically 95 to 98% of pickup for electromechanical relays, programmable for digital).
4. Compare to the setpoint; pass if within +/- 5%.

A 51 element set to 5.0 A that picks up at 5.4 A is misadjusted high by 8% and will under-reach (it will not operate for a 5.2 A fault that should trip it). A pickup at 4.6 A is misadjusted low by 8% and will over-reach (nuisance trips on load).

8.2.4 Timing Test

The timing test verifies that the relay operates in the time dictated by its time-current characteristic (TCC) curve at a specified multiple of pickup. For an IDMT 51 element set at 5.0 A pickup, time dial 3, IEEE Very Inverse, a 25 A test current (5x pickup) should trip in the curve-predicted time per the IEEE C37.112 equation. Acceptance is typically +/- 5% of the curve time (or +/- 0.1 cycle, whichever is greater). The timing test is what proves coordination: the downstream feeder relay must trip before the upstream main relay for the same fault.

A TCC curve plots current on the X-axis (log scale) and trip time on the Y-axis (log scale). When the feeder curve sits below (faster than) the main curve across the worst-case fault-current region, the system is selectively coordinated — the feeder breaker trips first and only the faulted section is isolated.

8.2.5 Why Both Tests Belong in Acceptance

NETA acceptance specifications call for both primary and secondary injection as part of commissioning because they verify different things:

• Secondary injection proves the relay's settings, curves, and logic are correct in isolation.
• Primary injection proves that the CTs, wiring, relay, trip coil, and breaker all work together as a system.

A relay that passes secondary injection can still fail primary injection if the CT polarity is reversed, the wiring is open, or the trip coil is miswired. Conversely, primary injection alone cannot isolate a timing-curve defect from a CT ratio error — if the timing is off, you cannot tell whether the relay curve is wrong or the CT ratio is wrong without a separate secondary injection. Running both is how the technician proves the protection system is correct end-to-end.