Modified Primary Injection Testing: When and Why It’s the Right Choice

Published by Electrical Test Tech

In the world of electrical acceptance and maintenance testing, field conditions don’t always support a textbook approach. One such adaptation is Modified Primary Injection Testing—a hybrid method that uses primary current injection only where absolutely necessary (such as ground fault verification), while relying on secondary injection to test the remainder of the trip unit’s protective functions.

What Is Modified Primary Injection Testing?

Traditional primary injection testing requires a high-current source—often over 1,000 lbs in weight—and a power supply capable of delivering 150A at 480V. This can be a logistical hurdle for indoor switchgear, remote job sites, or buildings lacking sufficient temporary power.

Modified primary injection offers a practical alternative:

• Primary current is used only for validating the ground fault protection function, which cannot be reliably simulated using secondary current.
• All other protective functions—such as long time, short time, and instantaneous trips—are tested using calibrated secondary injection methods.

ETI PI-6000

Why Use It?

• Logistical Limitations – Large injection sets are not always practical for indoor panels or elevated installations.
• Power Supply Constraints – Some job sites cannot provide the necessary 3-phase 480V power for full-scale primary testing.
• Cost Efficiency – Reduces setup time, equipment rental, and overall project duration.
• Targeted Accuracy – Ground fault protection is validated with real primary current to ensure functional reliability.

When Is It Acceptable?

This method is commonly accepted for:

• Retrofit installations with existing or upgraded electronic trip units
• Maintenance testing of breakers previously verified by full primary injection
• Sites with access or power limitations, as long as methods are properly documented

Electrical Test Tech ensures all results are traceable and backed by calibrated instruments, with test reports that clearly specify the testing method used.

Closing Thoughts

Modified Primary Injection Testing is not a shortcut—it’s a practical solution tailored to challenging environments. At Electrical Test Tech, we apply this method when it offers the best balance of compliance, accuracy, and efficiency. Clients are always kept informed of the approach and confident in the reliability of the results.