Before choosing breaker kA ratings: build a complete factory fault-case register

A breaker catalogue may offer several interrupting ratings for the same nominal current. Selecting the model by comparing one transformer nameplate or one remembered short-circuit number is not a defendable fault-duty study. The prospective short-circuit current depends on the grid supply, transformer impedances, network topology, connected rotating machines, converter contributions and which sources are allowed to operate together. The minimum fault current at the remote end of a feeder matters too, because a protection device must detect and clear credible faults under adverse supply conditions.

This guide explains how an FDI factory owner or electrical package manager can prepare a case register for an IEC 60909 engineering study before asking bidders to nominate ACB/MCCB or MV switchgear interrupting ratings. It supplies an example network, independent screening calculations, data boundaries and an approval checklist. This is a teaching example—not a complete IEC calculation, certified setting sheet or permission to energize equipment.

Prepared for QuangAnhcons · Editorial focus: short-circuit input cases and switchgear procurement · Sources reviewed: 10 October 2026. Every example impedance and source value is an explicit hypothesis.

Define the network and its switching states before discussing fault current

Draw the real electrical network from the contractual point of common coupling to the machine terminals. A common industrial arrangement contains a 22 kV supply feeding MV protection, one or more 22/0.4 kV transformers, an LV main switchboard, optional normally open bus ties, an emergency generator, motor feeders and—on some projects—PV or BESS inverters. The diagram needs an equipment ID at each bus, the approved normal switching position and every operationally permitted contingency. A drawing without operating states cannot establish the fault sources which may actually be connected.

A factory with two transformers and a bus tie is especially sensitive to incorrect assumptions. If the tie remains open, a fault on one LV section sees its own transformer and contributions from connected motors or converters. If an authorised operating mode closes the tie, contributions from another section may become possible. Conversely, a generator feeding an electrically isolated emergency board through a transfer switch may never operate in parallel with the utility. Drawing both sources on the same page does not mean their short-circuit contributions can be added: the ATS interlocking, approved sequence and actual fault path govern.

Bus or feeder	Upstream source paths	Allowed switch states	Required evidence
22 kV incoming PCC	Utility or licensed industrial-park distributor	Normal supply, declared alternatives	Maximum/minimum short-circuit power or Thevenin source data and X/R
Transformer A LV terminals	MV source via transformer A	MV device closed, LV ACB closed	Transformer rated data, guaranteed impedance, tap position
Transformer B LV section	Transformer B and possibly A through tie	Tie open versus authorized transfer	Electrical interlocking and closed-tie permission
Generator emergency bus	Generator and emergency motor feeders	Isolated island; no assumed grid parallel	Generator subtransient and control data, ATS diagram
PV/BESS feeder	Current-limited converters under real modes	Grid-connected, islanding if genuinely supported	Verified fault contribution and operating mode documents
Remote production DB	Feeder cable and local connected loads	Longest/lowest-fault path	Cable impedance, temperature, neutral/PE path and protection time

A case register is not just a matrix of switches. It must say why the case exists and which equipment it is intended to stress. The maximum prospective current may govern breaking capacity and initial peak making duty; a minimum source case may govern protection sensitivity or earth-fault disconnection. Arc-flash assessments, touch-voltage safety, motor withstand and specific insulation duties require additional analyses. Do not conflate those acceptance questions into one column labelled “fault current OK”.

Gather source impedance—not merely transformer kVA

For an initial order-of-magnitude check at the transformer LV terminals, engineers sometimes use the simplified relation I_k ≈ I_n × (100/u_k%) under a stiff-source approximation. Here I_n = S_n /(sqrt(3) × U_LL) and u_k% is transformer short-circuit impedance expressed in percent. This rule of thumb intentionally ignores the upstream grid impedance, voltage factors, resistance/reactance separation, motor and converter contributions, and the detailed maximum/minimum modelling required by IEC 60909-0. It therefore cannot justify a breaker rating or protection setting by itself.

Take a hypothetical 1,600 kVA, 22/0.4 kV transformer with a declared impedance of 6%. The idealized rated LV current is 1,600,000 / (sqrt(3) × 400) = 2,309 A. The stiff-source transformer-only screening result is 2,309 / 0.06 = 38.49 kA. It would be unsafe to state that the factory’s maximum available three-phase fault current is exactly 38.49 kA. The actual number is conditioned by source short-circuit strength, maximum voltage, tap position, conductor and busbar impedances, contributions from motors and converters, and the calculation method used by a competent engineer.

As a sensitivity illustration, applying the same approximation at 5% and 7% transformer impedance gives approximately 46.19 kA and 32.99 kA. The large movement across this narrow input band shows why a transformer procurement datasheet needs the supplier’s guaranteed impedance and tolerances, rather than a default number inserted by the estimator. Even this three-case comparison is insufficient for minimum-fault protection checks because it does not model remote feeder impedance or line-to-earth return paths.

Distinguish breaking duty, making duty and short-time withstand

A prospective initial symmetrical RMS short-circuit value and a breaker catalogue’s rated ultimate breaking capacity are not a complete coordination proof. The project must separately check the relevant breaking and service capacities at the correct voltage, the peak making/withstand duty, operating time, upstream/downstream selectivity or back-up protection, and any constraints imposed by the switchboard assembly verification. An MCCB with an impressive kA label may still be unsuitable for a particular circuit due to installation conditions, product series, coordination assumptions or application details.

Ask switchgear suppliers for exact model references and corresponding technical documents. Many device ratings change with applied voltage, pole configuration, AC/DC application, temperature and product variants. An upstream ACB and downstream MCCB combination may have tested selectivity or cascading tables, but those claims are valid only for the specified pair and arrangement. A generic statement “all breakers are IEC compliant” is not a substitute for the documented device/assembly combination that will actually be installed.

Example case register for a factory with utility, generator and PV

The following register describes a fictitious factory with one 1,600 kVA transformer, an LV MSB, an emergency diesel generator isolated from the grid under approved ATS controls, and a rooftop PV inverter feeder. These are case definitions, not verified numerical outputs. Replace every “to be obtained” field with actual utility and vendor data before the system study is signed off.

Case ID	Configuration	Fault location / type	Engineering question	Evidence still needed
SC-01	Utility max, production motors running, PV grid-connected	MSB three-phase	Peak/breaking duty at transformer LV bus	Utility maximum source, R/X, exact motors and converter current data
SC-02	Utility min, low voltage, feeders hot	Remote DB L–L and L–PE as applicable	Can the feeder protection detect/clear minimum credible faults?	Minimum source, cable R at temperature, PE/neutral loop, trip curves
SC-03	Utility absent; diesel island via ATS	Emergency DB three-phase and earth faults	Is available generator fault current sufficient for the selected protection?	Alternator subtransient and decrement curves, AVR support, ATS neutral
SC-04	Utility max, PV offline for maintenance	MSB three-phase	Sensitivity to converter connection	Validated inverter contribution model and switching constraints
SC-05	Allowed maintenance/transfer topology	Tie or alternative feeder boundary	Does one additional source path change duty?	Signed one-line, mechanical/electrical interlocks and authorization
SC-06	Minimum-source conditions plus long route	Final production DB	Disconnection under the least favourable credible conditions	Actual route length, conductor material/cross-section and return path

Crucially, do not activate a hypothetical “generator + utility closed tie” case simply because the spreadsheet offers such a combination. If parallel operation is not part of the approved design, mark it prohibited, not a valid running state. Conversely, if the plant has a closed-transition transfer or intentional parallel arrangement, omission of that real operating case creates a dangerous evidence gap.

Calculate independently without pretending a screening example is a study

At 400 V and 1,600 kVA, the rated current calculation should be independently reproduced to ensure the units are correct. Checking sqrt(3) × 400 V × 2,309 A ≈ 1.60 MVA catches common thousand-fold errors when one spreadsheet cell uses kV and another uses V. A second check varies the transformer impedance input across the expected procurement tolerance, and a third check compares the simplified terminal screening current with the independently modelled detailed result. These checks can reveal bad inputs; they do not validate the entire model.

A remote fault is not necessarily the worst current for all purposes. Cable impedance usually reduces current as distance increases, whereas low enough current can challenge an overcurrent protective device’s sensitivity. In a TN system, the phase-to-protective conductor return route and disconnection requirements are important; a TT arrangement has a different earth-fault protection basis. Do not make a design claim about the PE or RCD arrangement without an approved earthing schematic and measured or validated parameters.

For a grid-following inverter, the power-electronics controller may limit current and alter behaviour within cycles. The source cannot be modelled as if it were a synchronous alternator with the same rated kVA. The manufacturer must provide current-limiting and ride-through behaviour for the model, firmware and operating mode actually being supplied. The resulting treatment belongs in the project’s selected version of the applicable short-circuit methodology and related device verification.

Connect the study cases to procurement and commissioning

The project engineer should build a decision table connecting each case to required engineering deliverables. Use one row per device duty: for example MSB ACB — SC-01 — calculated fault quantity — selected model — voltage-specific breaking data — panel assembly evidence — reviewer — status. For a remote DB the controlling case may be SC-02 or SC-06, with device trip curve and earthing loop information. For the generator feeder another case may govern. Keep the database updateable as the machine supplier changes motor data or the owner revises the switching arrangement.

A purchase order should not release final switchgear ratings solely because a preliminary load-flow chart was accepted. Load current, voltage drop, short-circuit withstand and protection discrimination are separate acceptance paths. Identify a hold point for the fault study, another for device/assembly submittal approval, a site test plan for instrument/relay functionality, and a witness record for the approved switching interlocks. A planned factory date is not permission to energize an unverified network.

Hold point	Responsible producer	Required evidence	Decision
FP-01	Owner / supply interface	Current utility data and valid topology	Freeze source envelope or record limitation
FP-02	Electrical designer	Versioned maximum and minimum case results	Approve study basis for equipment selections
FP-03	Equipment vendor / designer	Model-specific Icu/Ics/Icw/peak and panel verification as applicable	Accept or reject candidate equipment
FP-04	Protection engineer	Discrimination/disconnection and setting report	Review coordination with actual curves
FP-05	Test/commissioning parties	Approved procedures, calibration and site witness records	Authorize the next commissioning hold point

If one of these records is missing, the matrix should identify who owns the unresolved item and the date required for a commercial decision. A status labelled “provisional for budget” should not be recast as a released construction design.

Standards and primary source register

Reference	Verified catalogue scope, edition	What has not been claimed
IEC 60909-0:2026	IEC official publication dated 23 July 2026, Edition 3.0; short-circuit current calculation in AC systems	Full standard text was not used to certify example values; no clause numbers invented
IEC 60076-1:2011	Transformer general requirements, IEC catalogue Edition 3.0	No thermal/impedance compliance statement about an unknown transformer
IEC 60364-5-52:2009+A1:2024	LV wiring systems, official consolidated catalogue	No unverified conductor ampacity or voltage-drop limit applied
Manufacturer breaker and assembly guides	Model-specific interrupting, making, withstand and coordination data	Catalogue family names do not prove an actual device/combination is selected
Applicable Vietnamese TCVN/QCVN and connection agreement	Local design, safety and distribution interface	Must be confirmed for named project, seller and equipment before design release

Practical RFQ, related work and next steps

When issuing an RFQ to electrical contractors or equipment suppliers, include the case register, the known source data, what operating arrangements are permitted, the list of missing values and the deadline for clarification. Ask bidders to quote an engineering deliverable, not only a breaker catalogue. A helpful tender return should include the assumptions file, fault-case list, equipment-specific duties, coordination/verification records and a list of calculations to be re-run after actual OEM datasheets arrive.

For the responsibility split, consult the factory electrical design responsibility matrix. For procurement release points, see the long-lead equipment schedule guide. For physical LV cable interfaces and installation methods, our industrial electrical installation overview describes the surrounding work package. These links address different aspects of the decision; they are not substitutes for signed project data.

The QuangAnhcons factory energization video archive provides context about real substation installation and handover. It does not demonstrate that the hypothetical fault network above is safe or that the numerical screening results meet IEC 60909-0.

To review a source/fault study scope for a factory in Vietnam, send the single-line drawing, agreed supply interface, transformer and generator guaranteed data, machine list and protected-feeder schedule to projects@quanganhcons.com. Any proposed device duty or setting remains subject to project-specific electrical engineering and the approved commissioning process.

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