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.
Related electrical test equipment available for rent from Protec:
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Editor’s Note: This post was originally published January, 2015 and has been updated freshness, accuracy and comprehensiveness.