Electrical Tester online
December 2019
Protection testing the primary way

Protection testing the primary way

01 December 2019

Author - Marius Averitai

When you need to test a relay protection system, particularly during commissioning, you need to decide at an early stage whether to use primary or secondary injection. Both approaches have their merits, but Marius Averitai of Megger says that there are many cases where primary injection has the edge.

To test a relay protection scheme you can either inject large currents into the primary circuits or much smaller currents directly into the relays. On the face of it both approaches will give you similar results, so why would you lug around a large and heavy primary injection test set rather than its much smaller and lighter secondary injection counterpart?

There are actually a lot of good reasons why you might choose to go for primary injection but one of the most convincing is that it’s a much more comprehensive test. To illustrate this, think of a very simple example: a circuit breaker with an overcurrent trip relay that operates via a current transformer (CT).

You could inject your test current directly into the relay (secondary injection) and this would indeed show you whether or not the relay was operating correctly. But it would tell you nothing about the CT and the connections between the CT and the relay. If you inject your test current into the primary circuit (primary injection), however, your results will confirm not just that the relay is operating correctly, but also that the CT is serviceable and that the connections to it are present and correct.

Primary injection testing also has other valuable benefits. For example, it mimics the normal operating conditions of the equipment under test much more closely – the high test current will stress the equipment you’re testing just as it would be stressed in service. This can make a big difference to the test results.

Primary injection testing can also be less complicated to set up. The equipment under test has to be isolated and heavy current connections made to it, but it’s usually only one set of connections. For equivalent secondary injection testing, isolation is still needed but it will often be necessary to make several test connections to various parts of the equipment to carry out comprehensive testing.

It’s never all good news, of course, and you probably know already that primary injection testing is sometimes shunned because of the size and weight of the equipment needed. Primary injection tests typically involve currents of at least 100 A, and in some cases up to 20 000 A, whereas secondary injection currents are more likely to be a maximum of 50 A – the typical nominal rating of secondary circuits is 1 A or 5 A. This means primary injection test sets are never going to be as compact as their secondary injection counterparts, but what you may not know is that in recent years there has been a lot of progress in making them more portable.


If you look at the specifications for the latest primary injection test sets, you’ll find that weight and size are not the only areas where progress has been made. Another useful development is the introduction of test sets where the control unit can be connected to the current generator by a comparatively long control cable. This means that you can position the current generator very close to the equipment under test and in doing so minimise the length of the high current test leads, which makes testing easier and more practical.

If you decide to invest in a primary injection test set, you’ll want to be sure that it can cope with a wide range of burdens as, if it doesn’t, it may not be able to deliver the required test current even if this is within its nominal rating. A good test set will make provision for this by allowing the output voltage of the current generators to be raised at the expense of the output current, so that the power the test set needs to deliver isn’t increased unduly.

Another option you’re likely to find very useful is an integral timer that you can set to inject the test current for an accurately controlled time. This will make it easy for you to inject actual fault currents to perform tripping time tests that include the circuit breaker, the relay and the CTs. Your test set should also have auxiliary voltage and current measuring inputs so that it’s easy for you to test CTs, and the best test sets will provide you with a wide range of data, including impedance, resistance, virtual power, active power, reactive power, and power factor, together, of course, with CT ratio and polarity.

When you’re selecting a primary injection test set, you’ll quickly find that there are two main types – sophisticated fully automatic units, like the Megger SPI225, and simpler manually-operated units like those in Megger’s ODEN range. The right choice for you depends on your application. For tests requiring the recording of multiple results and automated comparison with standard tripping curves according to the settings of a protection relay, a smart computer controlled unit (SPI225) will make testing and reporting faster and easier. This is also the case when testing multiple MCCBs.

For straightforward applications, however, manual primary injection test sets have a lot to recommend them. First of all, very little training is needed to use them effectively and that their operation is largely intuitive. Megger has found that, on average, it takes engineers and technicians less than an hour to learn how to use a manual test set, and that the company’s technical help desk rarely gets queries about these instruments. Other benefits are that you don’t need to connect manual test sets to a computer to use them and, of course, they cost rather less than their automated counterparts.

As we’ve seen, primary injection testing has a lot going for it. It tests all of the components of the protection scheme at the same time, and it closely mimics real-life operating conditions, more so than secondary injection testing. The latest compact test sets are much more portable and manageable than their predecessors, and the best of these test sets offer wide-ranging functionality. So, if you’re thinking about investing in equipment for testing relay protection schemes, a modern primary injection test set could well be your best option!