When a project electrical engineer in Rotterdam encountered repeated protection-study exceptions during a data-centre expansion, she approved the switchgear schedule and asked the contractor to release the medium-voltage breakers. The visible failure came quickly: the quoted interrupting duty did not match the calculated three-phase fault current at the incoming bus. The reversal was not a defective breaker; it was a specification problem—fault level, voltage class, insulation coordination and time duty had not been translated into a complete circuit-breaker requirement.
Summary: Specify a VCB from the calculated maximum prospective short-circuit current at its installation point, then select a rated short-circuit breaking current in kA that meets or exceeds it at the applicable rated voltage. Also check rated short-circuit making current, short-time withstand current and duration, plus insulation levels. IEC 62271-100 separates AC circuit-breaker ratings and tests from the common switchgear requirements in IEC 62271-1; the recommended action is to issue a coordinated duty schedule, not simply request a “20 kA breaker.”
For commercial facilities, industrial plants and EPC packages, vacuum circuit breaker breaking capacity specifications convert a system study into procurement language. “Breaking capacity” is commonly used for the interrupting capability, but a usable specification needs more than one current figure. It must state the system conditions, the circuit-breaker ratings and the required evidence that the offered arrangement is suitable.
Start with the fault duty at the breaker location
The governing value is the maximum prospective short-circuit current that can flow where the VCB is installed. A fault-level calculation normally models the utility contribution, transformers, generators, motors, cables, parallel sources and network configuration. IEC 60909-0 is the widely used method for calculating short-circuit currents in three-phase AC systems; the protection study should identify the initial symmetrical short-circuit current and the assumptions used.
For an illustrative 12 kV bus with a calculated 18 kA prospective three-phase fault current, an offered breaker with a 20 kA rated short-circuit breaking current may be a candidate only after its rated voltage, making duty and short-time duty are verified. The 18 kA result is not a universal selection rule: a changed transformer impedance, utility fault contribution or tie-breaker operating state can raise the duty. Ask the study author for the maximum case and the minimum case needed for protection coordination.
Specify the ratings that carry the duty
Use a one-line duty schedule for every incomer, feeder, bus-section and transformer breaker. Rated normal current addresses continuous load and temperature rise; it is not the short-circuit interrupting rating. A 630 A marking, for example, describes normal-current duty, while a separate kA rating and time duration describe fault duty.
| Specification item | What it controls | Buyer check |
|---|---|---|
| Rated voltage and frequency | Network and test conditions | Match nominal system and applicable voltage class. |
| Rated short-circuit breaking current (kA RMS) | Interrupting prospective fault current | Meet or exceed the calculated maximum at that location. |
| Rated short-circuit making current | Closing onto a fault, including peak duty | Verify against the system’s calculated/asymmetric peak requirement. |
| Short-time withstand current and duration | Thermal and electrodynamic survival while protection clears | State kA and seconds; coordinate with relay clearing and backup time. |
| Insulation level | Ability to withstand specified dielectric stresses | Coordinate power-frequency and lightning-impulse requirements. |
Rated short-circuit making current is a peak current, whereas rated breaking current is normally expressed as an RMS AC value. The relationship cannot be replaced by a fixed mental conversion because asymmetry and applicable IEC conditions matter. Request the manufacturer’s declared values and test documentation; do not infer making performance from a single breaking-current number.
Coordinate insulation, switching duty and the installation
Breaking duty is only one side of the selection. BIL—basic impulse insulation level—is often used in project specifications to describe lightning-impulse withstand capability; IEC equipment documentation may instead use the relevant rated lightning impulse withstand voltage terminology. Coordinate that requirement with the system overvoltage study, arresters, altitude, earthing arrangement and the upstream/downstream equipment. Also state the rated power-frequency withstand voltage and any site conditions that change insulation performance.
Switching capacitors, unloaded transformers, motors and cable networks can introduce duties beyond a simple three-phase bus-fault check. IEC 62271-100 includes specified test duties for AC circuit breakers, but a buyer still needs to identify whether the application introduces a duty requiring additional assessment. This is especially important when switching conditions, protection settings or the system grounding method differ from the assumed test conditions.
| Application point | Duty information to obtain | Likely specification focus |
|---|---|---|
| Utility or plant incomer | Maximum/minimum source fault levels; transformer contribution | Breaking capacity, protection coordination and insulation coordination. |
| Bus-section breaker | Parallel-source and closed-tie operating cases | Highest credible fault level after network reconfiguration. |
| Motor or process feeder | Motor contribution, starting/switching sequence and clearing time | Interrupting duty plus application-specific switching review. |
| Outdoor distribution point | Voltage class, environmental conditions and surge exposure | Insulation levels, enclosure/application fit and documentation. |
Use IEC standards correctly in the tender
IEC 62271-1 provides common specifications for high-voltage switchgear and controlgear, including service conditions, dielectric requirements, temperature rise and general testing provisions. IEC 62271-100 addresses AC circuit breakers and their ratings, design and type-test requirements. They should be named with the applicable edition in the project documents, alongside the installation’s national rules and owner requirements.
Conformity with IEC 62271-1 or IEC 62271-100 is not a blanket product certification issued by IEC. The standards define requirements and tests; a supplier’s declaration, independent test report or certification may have a particular product, configuration, laboratory and scope. Require documents that identify the offered type and relevant rating, and avoid unsupported claims such as “IEC certified” when the evidence only covers a component or a different configuration.
A practical procurement checklist
- Obtain a current short-circuit and protection-coordination study, including all credible source and tie configurations.
- State rated voltage, frequency, normal current, breaking current, making current, short-time withstand current and its duration for each breaker position.
- Define power-frequency and impulse insulation requirements, then review site altitude, surge protection and earthing assumptions.
- Identify special switching duties and request a documented application review where the standard rating alone does not resolve them.
- Ask for traceable rating schedules, drawings and applicable type-test evidence before technical approval.
Fuerte can support buyers who need to align a configurable medium-voltage offer with a clear duty schedule; review the vacuum circuit breaker breaking capacity specifications alongside the required installation and documentation scope rather than selecting from a current rating alone.
Frequently asked questions
What breaking capacity should a vacuum circuit breaker have?
It should have a rated short-circuit breaking current at least equal to the maximum prospective fault current calculated at its location, at the stated rated voltage. Confirm the calculation cases, required making current and related test conditions before selecting the rating.
How are VCB interrupting ratings specified in kA?
They are normally stated as rated short-circuit breaking current in kA RMS, together with rated voltage and the applicable standard. A kA value alone is inadequate because recovery voltage, frequency and circuit-breaker test conditions affect what that rating represents.
What is the difference between rated breaking and making capacity?
Breaking duty describes interrupting a prospective short-circuit current; making duty describes closing onto a fault and includes peak current stress. Both must be compatible with the system study and the manufacturer’s declared ratings.
How does voltage affect VCB breaking capacity?
Voltage changes the dielectric and transient recovery-voltage conditions a breaker must withstand after interruption. Select a breaker at the correct rated voltage class and insulation level; do not assume the same kA rating has identical meaning across voltage classes.
What is short-time withstand current in a VCB?
It is the RMS current the breaker and relevant equipment can carry closed for a stated short duration without unacceptable thermal or mechanical damage. Coordinate both the kA value and duration with protection clearing time, including any backup delay.
How do I select a breaker for a commercial fault level?
Commission a short-circuit study for the actual commercial network, including utility data, transformers, generators and operating modes. Then match each breaker location to the full duty schedule and verify the switchgear assembly, protection scheme and documentation together.
References
- IEC 62271-1, High-voltage switchgear and controlgear—Part 1: Common specifications.
- IEC 62271-100, High-voltage switchgear and controlgear—Part 100: AC circuit-breakers.
- IEC 60909-0, Short-circuit currents in three-phase AC systems—Part 0: Calculation of currents.
A sound VCB choice is not the largest kA figure on a data sheet—it is the rating set that matches the system duty, insulation coordination and evidence required at the point of use. For a project-specific review of vacuum circuit breaker breaking capacity specifications and medium-voltage options, contact Fuerte’s team with your single-line diagram and study assumptions.