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The Fatal Errors of Primary and Secondary Current Injection Testing

One question we often see in the industry revolves around current injection testing – namely, what is the difference between primary and secondary current injection. How do you know which test unit you should request, so that you can avoid spending time and resources ordering a test kit that either offers too much or too little for your testing needs?

Both primary and secondary current injection testing is typically conducted in order to monitor the operation of a circuit breaker and its protective relays and devices. In either test, the process often involves injecting the actual current required in order to operate a protective device through the circuit breaker. Though primary and secondary current injection tests are essential to the electrical engineering industry, there are some key differences between both types of testing. Pay close attention to how these forms of testing differ – you don’t want to waste money and time renting the wrong unit.


Primary Current Injection Testing

Primary current injection testing is applied to thermal magnetic and electronic breakers and high current, high voltage power distribution systems. Commonly found in electrical substations or large industrial installations, these systems are tested by injecting a test current into the primary side (commonly a protective scheme) to observe how the system behaves under varying current influence. Once a pre-determined current level is injected into the circuit breaker, engineers can then determine what level of current will cause the relay to fault, and how long it will take for a fault to transpire.

Some applications of primary current injection testing include: checking that the MV/HV/LV circuit breakers trip when probed with overcurrent; checking CT circuits, CT/VT/PV ratios and polarities, and ground grid installations; verifying the correct wiring and that all protective devices are connected; thermal testing busbars, switchgears and electrical panels; and more. Because it is disruptive and requires the plant/system being tested to be taken out of service/de-energized, primary current injection testing is often does as part of the commissioning process early on. Make sure you aware of the full extent of testing needed to be done before you order equipment. If it turns out that you need to test to primary current injection standards but wanted to cut costs by renting a less expensive secondary current injection unit, you will lose money in the long run when you end up having to rent a primary injector, too.

Secondary Current Injection Testing

Though just as widely used, secondary current injection testing does not check all components like primary testing does. It works by injecting a test current into the trip relay on the secondary side of the CT, and can be done on electronic breakers only. Much lower currents are needed to conduct secondary injection testing than are required of primary. Contrary to primary injection, which is generally time intensive, secondary current injection testing is relatively quick, and takes a little less expertise in order to complete.

The test kit for secondary current injection is relatively small and portable, often more so than primary testing due to its comparatively limited test capacity. Because of this, though, testing with secondary current injection methods is typically less expensive than testing via primary injection. If you know that your project will only require testing the trip unit alone or an electronic breaker exclusively, we’d recommend renting a secondary current injection test kit. Otherwise, you will end up with a primary current injector that is capable of testing beyond your needs, and costs more than what you’d spend renting a secondary current injector.

When testing a circuit breaker or other electrical system, its best to know how extensive your testing must be far ahead of purchasing or renting a test kit. We recommend renting injection current test equipment, particularly if you will end up needing a primary test kit, as their formidable testing capacity often translates into a big price tag when you go to buy. Similarly, even though a primary test kit can diagnose the same parameters as a secondary kit, if you only need to test to the capacity offered by a secondary current injector, there’s no need to waste money on a bigger, more comprehensive piece of equipment. If you’re proactive and get a clear plan for what testing you’ll need to do, you can save time and money by renting the exact right equipment for your job. Bottom line is this – do not rent a piece of equipment until you are confident in the scope of testing needed to complete your project. Jumping the gun and renting a current injector before you’re sure of how extensive your testing must be will cost you money, cut into your profits, and end up wasting your valuable time.

If you have further questions on the difference between primary or secondary current injection test kits or would like recommendations on which units to rent, please feel free to contact one of our electrical team leads. 
Posted: 5/24/2017 7:00:00 AM