Testing Low Voltage Circuit Breakers

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Solid-state units can be tested by either secondary or primary current injection. The secondary injection test is performed using a specially designed power supply unit. Keep in mind that the secondary injection method only tests the solid-state trip unit and does not test the current sensors, wiring, or the breaker current handling components.  Most solid-state trip units have terminal blocks that are equipped with test plug terminals for making the test. The test set allows checking of the solid-state trip unit operation without using primary current.  The test set passes enough current to check any desired calibration point.  The breaker must be de-energized before checking the operation of the solid-state trip units.  If the test set shows that the solid-state trip unit is not functioning properly, the trip unit should be replaced. 


The primary current injection method is usually preferred because this method verifies the sensors and wiring, as well as the conduction path in the breaker.  It is recommended that the primary injection test be performed on all three phases when testing breakers with solid state-trip units.  If three phase primary injection testing is not practical, then it is recommended that the sensors and wiring should be tested separately.  This testing should be performed per NETA and NEMA procedures in accordance with manufacturer’s recommendations.

• Measure contact voltage drop at rated current, (finds loose connections, improper contact pressure, or damaged main contacts).
• Test instantaneous trip function, (tests short circuit protection capability).
• Measure overload tripping time at 300% rated current.
• Check mechanical operation. Inspect for broken parts.
• Test operation of ground fault, short time pickup and delay, shunt trips, under-voltage releases, alarm functions, electrical spring charging mechanism, etc.
• Check insulation resistance phase-to-phase and phase-to-ground, closed and across open contacts.

Protec Equipment Resources carries a vast line of both secondary and primary current injection test sets.  Boasting perhaps the largest rental inventory of secondary injection test sets from Cutler Hammer, Square D, GE, Siemens and ABB just to mention a few.  And primary current test sets from Electrical Test Instruments and Megger.  We also offer insulation resistance test sets and micro-ohmmeters for insulation resistance and contact resistance testing.