Injection Test: What are Primary and Secondary Injection Tests?

Every circuit breaker has a trip unit for the protection of the device upon detecting overcurrent. Trip units allow accommodation of load circuit and use field interchangeable rating plugs. However, these units may become faulty, and overcurrent protective devices may fail at some point. To maintain the reliability and safety of such systems, you must execute well-planned injection tests in these air circuit breakers.

Injection Testing for Circuit Breakers

Many electricians use injection testing to check the characteristics of protective devices, including protective relays, power transformers, etc. For the circuit breakers, you can make use of the same injection test sets. You will conduct this test regarding tripping timeinsulation capacity, resistance, operational behavior, etc.

There are two types of commonly done Injection Test sets. 

Injection Testing

Primary Injection Testing

Primary current injection testing is a detailed analysis of the system that takes time. You can test both solid-state trip units and electromagnetic trip units through a primary injection test. In addition to trip units, you can test a low voltage circuit breaker through this method. It can verify that you have connected the sensors and wirings properly.

Though it is a beneficial method, yet it has one major drawback. That is, you can test only one phase at a time and detect no polarity problems. You have to test all the wirings separately with Polarity Test.

However, primary injection testing also requires more money and is an expensive maintenance program. The price is because it used large testing equipment, time-consuming single-phase tests, and proper circuit breaker removal. Therefore, the maintenance prices go up when all these steps are incorporated.

In most cases, you also need to occasionally follow the primary injection test sets with a secondary one.

Secondary Injection Testing

On the other hand, you can only conduct the secondary current injection test on the solid-state trip units. Hence, you cannot perform this test on molded case circuit breakers or older ones. In secondary current injection testing, you inject a three-phase current into the trip unit. This allows you to verify the trip unit polarities at the same time. 

However, one major drawback of this testing is that you can inspect only the trip unit logic and the system’s wiring. Unlike primary tests, you cannot check all the current-carrying components at a time. This fact gives an edge to the primary injection test sets over the secondary one.

Suppose either test fails and you detect anomalies in the functionality. In that case, you have to change the trip unit as soon as possible and mark it as defective. 

Why Do we Primary & Secondary Injection Tests?

Due to the following reasons, you may want to use Injection Test Sets over a specific period.

  • For personal protection; It is vital to maintain the trip unit logic. If the device cannot trip within the given time, it may cause arc flashes and electrical fires. As a result, it may damage your assets, and your life will be in danger.
  • For device protection; Again, if the device cannot trip with a given time, it will damage the equipment downstream more than you expect.

To avoid these events from happening, you have to conduct the current injection method. That way, your electrical devices will give you a good performance in the longer run.

Secondary Injection Tests

How do Primary & Secondary Injection Tests Work?

Primary injection test and secondary injection test sets work identically to each other. For both the electrical tests, you will inject the calculated amount of current through the breaker. Next, measure the time it will take to trip the breaker.

You can choose different injection tests sets according to the functions you are testing. For instance, you will use long-time and short-time delays to test the circuit breaker’s overcurrent protection features.

Long-time delay and pickup

The feature of long-time delay provides overload protection. For this, you will set the pickup time according to the continuous current rating. Then, you will inject the test current that is three times the long-time pickup. Moreover, apply the current from each pole and record the trip time.

Record the results and create a time-current curve out of the found data. Compare the readings with the data manufacturers have provided. If the values you found are near the standard ratings, you are good to go. Otherwise, you have to replace the equipment.

Remember, you will record the trip time in seconds.

Short-time delay and pickup

The short-term delay feature protects the equipment from short-circuiting or any other faults provided by an intentional delay. Here, you will inject a test current 1.5 or 2.5 times of short-time pickup, multiple long-time pickups. Moreover, apply the current from each pole and record the trip time.

Next, compare the readings with the data manufacturers have given in the manual. Record the results and create a time-current curve out of the found data. In this case, you will record the trip time in milliseconds or cycles.

Instantaneous pickup

The instantaneous trip feature also protects the equipment from short-circuit or fault conditions. Still, you do not need to provide any time delay. In this test, you will inject instant current pulses with increasing magnitude. Apply the current from each pole until the circuit breaker trips.

You will keep the starting pulse on about 70% of the expected pick-up and record the value when the circuit breaker trips. In this case, manufacturers typically provide tolerance of about 10% to 25% on the pickup.

Ground-fault delay and pickup

The ground fault pickup protects against ground faults. Here, you will inject a test current 1.5 or 2.5 times the pickup. Additionally, the test current is a 20-60% fraction of the continuous current rating value.

In a real environment, you will perform this test for service entrances by the National Electrical Code (NEC).


As the breakers may be magnetic or thermal, you will perform some electrical tests. However, the method will differ slightly. You can choose primary injection tests sets or secondary inject tests sets. Both to test the trip functions of the breakers. Here at Cloom, we offer custom wiring assembly with attention to each detail so that your wiring is easy to check and maintain. Please fill out the form and contact our team now.

Hey, I am John, General manager of Cloom and OurPCB.

I am a responsible, intelligent and experienced business professional with an extensive background in the electronics industry.

Reach me at [email protected] to get a quote for your projects.


Fill in your details and we’ll get back to you in 24 Hours.

contact us


Fill in your details and we’ll get back to you in 24 Hours.