Protection
Secondary injection feeds a small simulated current directly into an electronic trip unit to exercise its long-time, short-time, instantaneous and ground fault functions without a high-current source. It proves the unit's settings and timing, but not the sensors or the breaker mechanism, so it complements primary injection.
Secondary injection is a test of the electronic trip unit that sits inside or on a low-voltage molded-case, insulated-case or power circuit breaker. A portable test set supplies a low-level signal to the trip unit, usually through a test port or at the sensor input terminals, that the unit reads as if it came from the breaker's current sensors. The technician then confirms that each protective function responds at its setting: long-time, short-time, instantaneous and, where fitted, ground fault, together with any adjustable delay bands. The test can also exercise the trip actuator that opens the breaker. It is applied to solid-state trip units on breakers of many frame sizes, and it is specified as an optional test in the standards where primary injection is impractical or as a convenient complement to it. The breaker is out of service during the work, and the trip unit itself is the item under test.
A trip unit is the brain of the breaker. If its settings are wrong, its electronics have drifted or its wiring is faulty, the breaker may not trip when a fault or overload arrives, or it may trip too early and interrupt healthy load. Because the unit sits idle for years between events, nothing shows these defects until protection is needed, and then the consequences are severe: damaged cable and equipment, arc flash energy that lasts longer than designed, and loss of coordination with upstream devices. Tests also catch a unit left at factory defaults that no longer match the coordination study, and settings changed in service without a record. Secondary injection is quick, needs no heavy current, and can be run on a bench or in the cell, which is why it is so widely used for periodic verification. It cannot replace the proof that primary injection provides, but it gives a fast, repeatable look at the electronics.
A modern electronic trip unit measures current from sensors, converts it to a digital value and compares it with stored curves. The long-time function follows an inverse curve, so higher current trips faster, and the short-time function has a defined delay, often with an optional inverse shape. The instantaneous function trips without intentional delay, and the ground fault function uses a residual or direct sensing path with its own pickup and delay. The unit reacts to the signal at its input, not to the physical origin of that signal, so a test set that reproduces a sensor output at the right magnitude and waveform produces the same response as real current. That is what makes secondary injection possible. What it cannot test is everything on the other side of the input: the current sensors or transformers, their wiring, the connection to the bus, and the mechanical operation of the breaker under load. Those elements need primary injection or separate checks, so the two methods are used together where the full chain must be proven.
With the breaker out of service, de-energized and locked out under the owner's safety plan, the technician records the breaker identification, the type of trip unit, its rating plug and its settings. The settings are compared with the coordination study, and the position of every adjustable control is noted before anything is changed. The test set is connected to the unit's test port or to the specified terminals, following the manufacturer's instructions, because connections and signal levels differ between models. The set supplies a signal corresponding to a chosen multiple of the rating, held until the unit trips, and the elapsed time is measured. The long-time function is tested at a point on its curve, the short-time and instantaneous functions at points that straddle their pickups, and the ground fault function with the appropriate injection, being careful about whether the unit uses residual or source sensing. The test set is also used to find the pickup by raising the signal gradually, and the trip is confirmed at the breaker or the actuator, with an indicator or flag noted. After each trip the unit is reset and the settings are restored to the as-found values. Any differences between as-found and as-left settings are recorded. If the unit includes zone interlocking or trip indicators, the outputs are checked.
The record notes the breaker and trip unit identification, the rating plug, the settings found and left, the type of signal injected and the points tested for each function, the measured pickup values and trip times, the response of any indicator, the test equipment and its calibration status, and any observations about the condition of the unit, its wiring and the breaker.
Results are judged against the trip unit's own time-current characteristic and the manufacturer's published tolerance bands, which describe the allowed spread around each curve. Measured trip times should fall inside the band for the setting that was applied, pickups should occur in the range the manufacturer specifies, and ground fault and instantaneous functions should respond without the delay that belongs to the other functions. The settings found should agree with the coordination study, and any difference is a finding. Results from earlier tests of the same unit are compared as well. The test is described among the low-voltage circuit breaker tests in ANSI/NETA ATS-2025 and ANSI/NETA MTS-2023, in both cases as an optional alternative or complement to primary injection; refer to your licensed copies of both and to the manufacturer's literature for values and criteria.
Acceptance testing compares the settings with the study and establishes the as-found baseline, while maintenance testing looks for drift and for settings that were changed. Molded-case breakers with electronic trip units are often tested with a manufacturer's dedicated test kit that plugs into the unit, whereas power breakers and insulated-case breakers usually accept a general-purpose test set through a test connector. Some trip units have a built-in self-test or a test mode, which is useful but does not replace a measured result. Thermal-magnetic breakers have no electronics and cannot be tested this way; they are checked by primary injection. Where a trip unit supports arc energy reduction or zone selective interlocking, the maintenance mode and the interlock connections get their own checks. The choice between secondary and primary injection depends on the access, the available current source and the question being asked: secondary injection checks the electronics, primary injection checks the whole chain.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
Primary injection passes real current through the breaker and its sensors, so it proves the complete chain, including the sensors and the mechanism. Secondary injection feeds a simulated signal to the trip unit, proving the electronics, settings and timing, but not the sensors or the breaker mechanism.
Not entirely. It is a convenient and repeatable check of the trip unit and is accepted as an alternative where primary injection is impractical, but it leaves the current sensors, wiring and mechanical operation unproven. Where the full chain must be demonstrated, primary injection is still needed.
Testing often involves changing settings, and the unit must be restored exactly to what was found. Recording the as-found state also lets you compare it with the coordination study and spot settings that were altered or left at defaults.
No. Secondary injection applies only to electronic trip units that read a signal from sensors. A thermal-magnetic breaker responds to heat and magnetic force in the conductor itself, so it is tested by primary injection of real current.
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