Primary injection testing verifies the operation of protective devices by simulating real fault conditions. It’s the only way to confirm that circuit breakers respond correctly to high fault currents—ensuring fast, reliable tripping when it matters most.
Megger’s high-performance primary injection test sets are engineered for precision, speed, and repeatability. Whether you're validating tripping thresholds, checking protection relay coordination, or diagnosing performance issues, these instruments help you spot defects early and prevent critical failures.
Ideal for substations, switchgear, and industrial systems, Megger’s solutions boost safety, confirm compliance, and give you total confidence in your protection scheme.
Primary injection testing involves applying high current—typically hundreds to several thousands of amps—into a circuit to simulate real operating conditions. This method is used primarily for testing low-voltage circuit breakers and current transformers (CTs).
By injecting current on the primary side, you test the entire protection chain: the CT, wiring, connections, relay, and in some cases the breaker itself. It’s a full-path test that verifies not just individual components, but how they perform together under load.
Primary injection is usually performed during commissioning and requires the system to be offline. For circuit breakers, the test measures trip times across different protection settings (long-time, short-time, and instantaneous). For CTs, it confirms ratio, polarity, and performance.
It remains the only method to prove a direct-acting breaker will trip properly under real-world fault conditions.
Primary injection tests are more comprehensive than secondary injection tests. Secondary testing only verifies the relay, not the rest of the protection chain—leaving out the CT and its connections.
In contrast, primary injection pushes current through the entire circuit, verifying that the CT, wiring, relay, and breaker all work together. It confirms not just that components are functioning, but that the whole protection path is intact and properly coordinated.
Another key benefit is that it better mimics real-world fault conditions, stressing components with high current. This can expose issues that would otherwise go undetected with lower-current secondary testing.
While secondary injection is more portable and convenient, primary injection provides greater confidence in the protection system as a whole.
Primary injection testing of CTs involves sending high current through the primary side of the CT and measuring the resulting current on the secondary side. This approach simulates real-world loading and gives a true picture of CT performance—including ratio, phase angle, and polarity.
Megger’s ODEN unit features a built-in ammeter for this type of testing. With the optional BH-90130 switchbox, you can connect to multiple secondary taps and switch between them easily (while de-energised).
By contrast, secondary injection applies voltage to the secondary side and measures the response. It uses smaller, portable instruments like Megger’s MRCT or MVCT to evaluate CT ratio, polarity, and knee points.
Secondary injection is ideal for routine checks, but only primary injection tests the CT under real operating conditions.
Primary injection testing is commonly performed during commissioning of new systems, after protection scheme changes, or when verifying the operation of circuit breakers and relays.
It’s essential when you need full-path verification—ensuring that CTs, wiring, protection relays, and breakers all function correctly together. This is especially important in critical infrastructure such as substations, industrial plants, and power distribution systems.
You should also use primary injection when investigating protection system issues or validating repairs. Since it simulates real fault conditions, it's the most reliable way to confirm that protection devices will operate correctly when it counts.
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