Low-voltage circuit breaker testing: ACB and MCCB

What is tested in a low-voltage circuit breaker

The low-voltage circuit breaker is both the switching device and the protection of the circuit. Unlike a medium- or high-voltage breaker, which depends on an external CT and relay, it carries the protection inside: its trip unit. That is why its tests cover both functions:

This applies to the two main types: the air circuit breaker (ACB), used for switchboard mains and bus ties, and the molded case circuit breaker (MCCB), used for feeders. Their characteristics are described in low-voltage circuit breakers: ACB, MCCB and MCB.

The trip unit: L, S, I and G

The electronic unit protects with four functions, each with its current and time setting:

• L (long time): overload, with inverse time.
• S (short time): short circuit with a delay, to coordinate selectively with downstream breakers.
• I (instantaneous): short circuit with no intentional delay.
• G (ground fault): residual or neutral current.

On ACBs and some electronic MCCBs, zone selective interlocking (ZSI) can be added, which links the trip units of different levels so that the breaker closest to the fault trips first. If the switchboard uses it, its wiring and logic are tested too, because a broken link makes the upstream breaker trip with its minimum delay and selectivity is lost.

The test verifies that each function picks up and trips within the band of the time-current curve published by the manufacturer for the loaded settings. Since the tolerance is a band and not a point, each result is compared with the model’s curve, not with a single value. You can see how these curves combine with those of other devices in low-voltage selectivity.

Secondary or primary injection

The general difference between the two methods, also for relays, is in primary vs secondary injection testing.

• Secondary injection. It enters through the test connector of the electronic unit with the manufacturer’s kit or a compatible source. It is fast and needs no high currents. Its limit is that it does not test the sensors or their connection: Schneider, for example, documents that its secondary injection test kit for Masterpact NT and NW does not test the current transformers or their connections. In recent generations of several brands, the test is done with the manufacturer’s software and a computer, instead of a dedicated test kit.
• Primary injection. It drives a high current through each pole of the breaker, as a fault would. It tests sensors, internal wiring, trip unit and opening in a single test. It needs a high-current test set, more time, and it adds operations to the breaker.

The same difference exists in medium- and high-voltage protection: secondary injection tests the relay and primary injection tests the chain from the CT.

When to test

• At switchboard commissioning, after the factory tests and before energizing. It is the moment to confirm that the loaded settings are those of the coordination study.
• In periodic maintenance, at the interval set by the plant’s program and the manufacturer’s recommendation.
• After a short-circuit trip, especially if the breaker cleared a current close to its interrupting rating.
• When the trip unit or its settings are changed.

Reference standards

• IEC 60947-2 is the product standard for low-voltage circuit breakers: it defines their characteristics and the type tests the manufacturer performs. In North America, molded case breakers are covered by UL 489 and low-voltage power circuit breakers by IEEE C37.13.
• IEC 61439 governs the low-voltage switchboards that house them; see low-voltage switchboard and MCC.
• ANSI/NETA ATS includes low-voltage circuit breakers among the equipment in its acceptance testing specifications, with inspection, electrical tests and trip unit verification.

The acceptance values of each test come from the manufacturer and the applicable standard. We do not publish generic values because they depend on the model.

What not to do

• Test the trip unit without first confirming which settings it should have.
• Consider the breaker tested only because the unit’s push-to-test button worked.
• Leave the breaker in service with the G function disabled “to avoid trips” without a study that supports it.
• Forget to return the test settings to those of the study when finished.

We test your breakers

At Tiza Group we test low-voltage circuit breakers in industrial switchboards and in substation station service, as part of commissioning or maintenance: inspection, mechanical operation, contacts, insulation and trip unit, in Colombia, or elsewhere on request. For field work, see testing and commissioning and our circuit breaker testing service.

Related guides

• Low-voltage trip unit: L, S, I and G functions
• Low-voltage circuit breakers: ACB, MCCB and MCB, parts and tests
• LV air circuit breakers: Emax 2, Masterpact MTZ and 3WL
• Low-voltage selectivity: breakers and fuses
• Contact resistance test: what it measures and why
• Interlocking of low-voltage breakers and bus ties
• Switchboard and MCC commissioning: tests
• Secondary injection relay test set: what it is

References

• IEC 60947-2, Low-voltage switchgear and controlgear - Part 2: Circuit-breakers.
• IEC 61439-1 and 61439-2, Low-voltage switchgear and controlgear assemblies.
• UL 489, Molded-Case Circuit Breakers, Molded-Case Switches and Circuit-Breaker Enclosures.
• IEEE C37.13, Standard for Low-Voltage AC Power Circuit Breakers Used in Enclosures.
• ANSI/NETA ATS-2025, Standard for Acceptance Testing Specifications for Electrical Power Equipment and Systems.
• Schneider Electric, MasterPact NT and NW catalog, test equipment. https://www.productinfo.schneider-electric.com/0613ct0001mp/
• Schneider Electric, MicroLogic Active user guide (primary injection testing).

Frequently asked questions