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Primary Injection Testing

PI Service

Primary Injection for Current Sensing

“Why Primary Injection Tests?

This type of test involves the entire circuit – current transformer primary and secondary windings, relay coils, trip and alarm circuits, and all intervening wiring are checked. There is no need to disturb wiring, which obviates the hazard of open-circuiting current transformers, and there is generally no need for any switching in the current transformer or relay circuits.” Source: EEP

Validating current sensing equipment is critical for vessel owners to ensure safe operations. The most comprehensive method is primary injection testing. This involves using a specialized tool to inject a low-voltage, high-current signal directly into bus bars and cabling.

You can think of primary injection like testing a smoke detector. When you press the "test" button, you are checking if the detector can communicate a fire. But by actually injecting smoke, you confirm the sensor can properly detect a real fire. Similarly, primary injection injects a small "fault" to validate the entire sensing and protection chain.

Primary injection supports total system validation. It checks current transformers, Rogowski coils, wiring connections, and protective relays together. 

By injecting a controlled current, primary injection testing confirms:

  • Current transformer ratios match specifications

  • All devices measure current accurately within tolerance

  • Current flows through sensing equipment in the proper direction

  • Interconnected wiring is correct with no accidental transpositions


This validates the entire current measurement network. It ensures faults will be detected properly and protection systems will trip as designed. There is no possibility of misreading devices "agreeing" on an incorrect reading.

Primary injection delivers definitive validation. Owners gain confidence that current sensing equipment will perform when needed. This is vital for avoiding spurious system outages and ensuring fault clearance protects equipment.

Primary injection provides a high level of assurance without compromising safety or uptime as it can be done during shipyard.

When performed by trained technicians following procedures, it can assure owners and their clients of effective fault detection. Keeping vessels running safely and efficiently starts with robust power validation.

Applicable Rule sets

There is no guidance requiring this testing on vessels at this time. It is OneStep Power’s philosophy that primary injection does not need to be performed on a 5-yearly basis, if performed after commissioning and/or after major changes to the system, provided evidence of testing is available. It should be noted that Rogowski Coil (Rcoil) integrator units are an electronic device and the manufacturer's recommendations for testing period and type should be followed to ensure continued accurate operation.

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