Short answer: secondary injection proves the breaker's electronic trip unit makes the right decision at the right time. Primary injection pushes real test current through the breaker's main contacts, so it proves the whole chain: current sensors, trip unit, trip actuator and mechanism. The NETA maintenance testing standard (ANSI/NETA MTS-2023) specifies primary current injection for the pickup and delay tests, and lists secondary injection of the trip unit as an optional extra.
How a breaker decides to trip
A power circuit breaker with an electronic trip unit works as a small system. Current sensors on each phase measure the load current. The trip unit compares that signal with its settings: long-time, short-time, instantaneous and, where fitted, ground fault.
When a setting is exceeded for long enough, the trip unit fires an actuator that releases the mechanism, and the contacts open. A fault anywhere in that chain means the breaker may not open when it has to.
Smaller molded-case breakers often use a thermal-magnetic trip instead: a bimetal element for overloads and a magnetic element for short circuits, with no electronics at all.
Secondary injection: testing the trip unit
A secondary injection test set feeds a signal into the trip unit through its test connection or sensor circuit, as if the sensors were seeing a given current. The technician steps through each protective function and records the pickup and the trip time against the settings and the manufacturer's time-current data.
- What it proves: the trip unit's settings, pickup and timing for each function, and that it fires the actuator.
- What it doesn't prove: the current sensors, the wiring between sensors and trip unit, and how the main contacts carry current.
- Why it's used: the test set is portable, the test is quick, and it can usually be done with the breaker racked out in its cell or on a bench. It is useful between full tests and after a settings change.
Primary injection: testing the whole breaker
A primary injection test set drives high test current through the breaker's main contacts, pole by pole. The breaker sees what it would see in service: the sensors measure real current, the trip unit acts on it, and the mechanism opens the contacts.
- What it proves: the complete protection chain, from sensors to opened contacts.
- The only option for thermal-magnetic breakers: with no trip unit to inject into, the overload and instantaneous elements can only be tested with real current.
- The trade-off: the test set is heavy, the breaker usually has to come out, and each test takes longer.
When to use which
- Full maintenance tests to NETA MTS: primary current injection for long-time, short-time, instantaneous and ground-fault pickup and delay, as ANSI/NETA MTS-2023 section 7.6.1 lists them.
- Commissioning, retrofits and repairs: primary injection, so the whole breaker is proven after anything was changed: a new trip unit, new sensors, or a rebuilt mechanism.
- Checks between full tests, or after a settings change: secondary injection of the trip unit, where the manufacturer provides for it.
- Thermal-magnetic molded-case breakers: primary injection where trip testing is in scope.
- A breaker that tripped, or failed to trip, in service: primary injection to find out why.
Whichever method is used, NETA's acceptance rule is the same: pickup values as specified, and trip times inside the manufacturer's published time-current tolerance band. Settings are recorded as found and as left, and NETA expects them to match the coordination study.
What about medium-voltage breakers?
On most medium-voltage switchgear, protection lives in separate protective relays that read current transformers and send a trip signal to the breaker. Those relays are tested by secondary injection at the relay, and the breaker itself gets mechanism service, timing, contact resistance and insulation tests, plus a trip check from the relay. See medium-voltage breaker testing for critical facilities.
Injection testing is only part of the job
A trip test proves the breaker will open. It says nothing about how well the closed contacts carry current or how sound the insulation is. A complete breaker test also includes:
- Contact resistance with a digital low-resistance ohmmeter, poles compared with each other and the last test
- Insulation resistance phase to phase, phase to ground and across the open contacts
- Visual and mechanical inspection, cleaning, lubrication and exercising to the manufacturer's instructions
- Accessories such as shunt trips, undervoltage releases and charging motors operated and checked
How we do it
Our crews test low- and medium-voltage breakers during planned shutdowns across Canada, using primary and secondary injection as each breaker needs, and leave field test sheets for every breaker. See our circuit breaker testing service and the sample test sheets, or read what a breaker test report should include.