Primary Injection Test Sets … Getting the Best Performance

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Primary injection test sets are high current units designed for testing and adjusting low voltage circuit breakers and other current actuated devices.  Typical test sets incorporate a variable high current ac output along with circuitry to measure the reactions of the breaker under test.  In concept these are very simple test sets.  Inject the desired current level and measure the breaker’s reaction time … simple, right?


It can be that simple, but first we need to specify the proper test set for the breaker class we wish to test.  Can the instrument produce the appropriate current levels to meet all aspects of testing; continuous, long-term, short term and instantaneous?  Do we have an appropriate input voltage source and does it have sufficient capacity to maintain the rated input voltage required by the primary injection test set?  What kind of connection will be required to connect onto the breaker under test?  Does the test set have sufficient voltage to “push” the desired test current through the device?

Test sets are typically designed to operate on a single phase voltage source.  Input leads should offer a minimal amount of voltage drop from the input source to the input terminals of the test set.  Although most test sets are designed to operate satisfactorily at 95% to 105% of rated voltage, any drop in voltage below rated at the input terminals will result in a proportional decrease in the maximum available output on the test set.

Many test sets will provide two output connections, parallel and series, to provide various voltage and current ratings so that the test set can adapt to a wide variety of test circuit impedances.  When reviewing a test set’s performance specifications pay special attention on how the manufacturer derived the output current values.  Many times the values given on the data sheet are based upon a no load situation.  If this is the case special attention must be given to the VA rating of the test set and the maximum voltage at the test set’s output.  If test leads are to be used to connect to the device under test special care must be taken on the type of test leads.  Due to the voltage drop from the inductive reactance of the test circuit, a significant loss of current will result for each inch of test lead.  Therefore, when choosing test leads, the length and size of the leads chosen will determine the maximum available test current.  The use of 4/0 welding cable is convenient for constructing test leads.  Paralleling of sufficient cables provides higher test currents.  Each cable can be fitted with a compression lug on each end, and then bolted to the output terminals or stab board of the test set and the breaker.  Also, the two cables between the test set and the breaker should be twisted together or bundled with tape or cord to maintain the close proximity which minimizes inductive reactance.

It is sometimes necessary to use bus bar in order to obtain the desired maximum current.  When using bus bar, the buses should be run in parallel and kept as close to one another as possible.

Protec Equipment Resources offers a wide range of choices in primary injection test equipment to accommodate a variety of breaker classes.  We feature high performance test equipment from Electrical Test Instruments and Megger.  We also provide the technical expertise to consult with our clients and ensure the right test set goes to the job site!