Transfer and withstand
The non-fused PC switch compared here can make and withstand the declared short-circuit duty, but is not the fault interrupter. Coordinated protective devices clear faults.
Non-fused, switch-based comparison
Transfer switching / protection architecture
PC-Class vs CB-Class ATS: IEC 60947-6-1 Selection Guide
Choose the class by its short-circuit role. Then verify the protection, load duty and complete tested arrangement—not just the ampere rating or controller features.
Class defines the fault role
Transfer control is a separate question.
Protection is coordinated
Declared conditions must match the system.
The assembly matters
Component certificates are not the whole proof.
State the required edition
Current IEC publication is not automatic local adoption.
Generic distribution-room context. Photo: P199 / Wikimedia Commons, public domain. Display crop and dark overlay.
The direct answer
The difference is who interrupts the fault
PC-Class and CB-Class describe the short-circuit role of transfer switching equipment (TSE). They do not mean automatic versus manual operation.
Scope of this comparison: the PC path below is a non-fused, switch-based arrangement. Class PC also includes fuse-combination units with a fault-breaking role, so “PC never breaks a fault” is not a complete definition for every configuration.
Transfer with breaker protection
CB-Class TSE includes overcurrent releases and is intended to interrupt short-circuit current. Its making, breaking and any assigned short-time withstand ratings still need verification.
Fault interruption is part of the CB arrangementDefinition nuances: ABB IEC classification training (2022). Product-family examples: Schneider Electric source-changeover range. Apply the edition and exact configuration required by the project.
Selection rule: establish where and by what equipment fault current will be interrupted. Then confirm source data, protection coordination, load duty, transfer sequence and the exact marked TSE configuration. Neither class is a universal upgrade over the other.
Keep the labels separate
Four questions, four different answers
A technically complete specification needs more than “automatic ATS.” Class, operating mode, utilization category and controller scope describe different things.
Equipment class
PC and CB describe different short-circuit roles. Class CC contactor-based equipment also exists; this guide compares PC and CB, not every class.
Operating designation
MTSE is manual, RTSE is remote and ATSE is automatic transfer switching equipment. The command method does not establish the fault-interruption role.
Utilization category
The declared category connects switching duty to the load and operating conditions. It does not replace a short-circuit or coordination review.
Controller scope
Source sensing, commands, timing, alarms and communications control the transfer. A controller cannot confer a class on an unverified power assembly.
IEC 60947-6-1:2026Edition 4.0 · published 2 April 2026
The IEC record confirms that the fourth edition replaces the 2021 edition. Its scope includes manual, remote and automatic TSE, plus specified bypass/isolation, closed-transition and controller arrangements. State the edition adopted by the project and local authority; international publication does not make it legally adopted everywhere. Official IEC edition record.
Scope limit: the IEC record covers TSE at rated voltages up to 1,000 V AC or 1,500 V DC; that is not a rating assigned to every product. Class CC and controller distinctions are illustrated in Socomec’s ATS controller brochure.
Conceptual comparison
Two ways to assign the protection function
Both approaches need a verified system. The key difference is the duty assigned to the transfer equipment itself.
Non-fused PC-Class concept
Associated protection clears the fault
Normal source
Specified protective deviceCoordinated with the TSE
Alternate source
Specified protective deviceCoordinated with the TSE
Non-fused PC-Class transfer equipmentSelects one source · declared making/withstand duty
Fault interruption by associated protection
Fault interruption by associated protection
Selected source supplies the load
Evidence: the stated TSE ratings and exact associated protective-device combination.
CB-Class concept
Breaker protection is within the TSE
Normal source
Alternate source
COMPLETE CB-CLASS TSE
Breaker path AOvercurrent release
Breaker path BOvercurrent release
Documented interlocks + transfer control
Transfer + fault-interruption functionDeclared ratings, releases and settings apply
Selected source supplies the load
Evidence: complete TSE documentation, breaker/release data and coordinated settings.
Concept only—not a wiring diagram or a switching state. Protective-device locations are illustrative; the project design determines source- and load-side protection. Conductors, neutral arrangements, earthing and terminals are intentionally omitted. The two sources are not shown as permitted to operate in parallel.
| Decision point | Non-fused PC-Class TSE | CB-Class TSE |
|---|---|---|
| Fault-clearing role | Not intended to interrupt short-circuit current. | Intended to interrupt short-circuit current with overcurrent releases. |
| Typical device basis | Switch or switch-disconnector architecture. | Circuit-breaker-based architecture. |
| Critical evidence | Declared withstand/conditional ratings and associated protection. | Breaking capability, any assigned withstand duty, release settings and coordination. |
| Misleading shortcut | “There is a breaker somewhere, so the switch is protected.” | “The breaker frame is large enough, so the complete ATS is suitable.” |
| What both need | Exact configuration, load-duty review, interlocks and acceptance plan. | Exact configuration, load-duty review, interlocks and acceptance plan. |
Non-fused PC-Class application
Separate transfer duty from fault interruption
A non-fused PC-Class switch can fit a design that deliberately assigns fault clearing to coordinated protective devices.
Switch and switch-disconnector technologies are common PC-Class implementations. The transfer equipment handles its declared normal switching duty and specified short-circuit stresses; it is not the device selected to interrupt the fault.
Do not leave the protection requirement at “upstream breaker by others.” State the device type, relevant ratings and applicable settings, and obtain the manufacturer’s coordination evidence for the offered combination.
- Match the declared conditions
Conditional short-circuit performance applies only with the stated protective devices and test conditions. A number in a catalogue is not an unrestricted system rating.
- Keep the transfer requirements
Check isolation needs, poles, neutral treatment, interlocks, enclosure and controller arrangement independently of the class decision.
- Assign responsibility
Name who supplies, sets, verifies and records the associated protection: supplier, panel builder, designer or installer.
Product examples are not universal specifications. Schneider identifies EasyPact WATSN as switch-disconnector-type Class PC equipment. Socomec declares PC-Class and AC-33B for its ATyS a M range. Read the exact model and coordination documents; do not transfer those declarations to another product. Schneider example · Socomec example.
CB-Class application
Include breaker protection in the verified transfer arrangement
CB-Class can fit a design that requires overcurrent releases and a fault-interruption function in the TSE.
A breaker is not a complete ATS. Its frame, trip unit or interrupting rating does not by itself establish the class or suitability of the assembled source-changeover system.
Verify the arrangement, not just the frame
Check breaker frames, releases, protection settings, interlocks, controller behavior, transfer sequence and configuration limitations. Coordination and selectivity remain design tasks even when the equipment includes fault-clearing devices.
Do not assume every CB-Class assembly has the same short-time withstand capability. Any assigned withstand current and duration must be checked alongside the breaking and making ratings. A settings change can alter the coordination result.
Schneider’s source-changeover family distinguishes breaker-based Class CB and switch-disconnector-based Class PC solutions. This is an architecture distinction, not evidence that either one is inherently correct for every facility.
Product-family context: Schneider source-changeover equipment. The 2021 IEC record also describes CB-type performance alignment with IEC 60947-2; use the project-required edition for the actual acceptance basis.
Do not combine certificates and assume complete compliance. The current IEC scope excludes configurations not fully manufacturer type-tested or marked as a complete transfer switch under the document. Request evidence for the actual devices, controls, interlocks and configuration. IEC scope and exclusions.
For derived TSE, applicable component-test evidence can cover tests already addressed by the relevant device standards. This avoids unnecessary repeat testing; it does not remove complete-TSE verification and marking. See the IEC 2026 published sample, derived-TSE definition.
Evidence that drives selection
Match the declared rating to the real duty
Rated current is only one input. The fault level, clearing conditions and load behavior decide whether the proposed arrangement is suitable.
| Item to verify | Why it matters | Evidence to obtain |
|---|---|---|
| Prospective short-circuit current | Establishes fault duty at the TSE, including relevant source contributions. | Current study or calculation, source assumptions, location and revision date. |
| Protection architecture | Defines where faults are interrupted and which devices must coordinate. | One-line, device references, ratings, settings and manufacturer coordination data. |
| Utilization category and load | Matches declared switching duty to actual loads and operating frequency. | Load list, motor/transformer behavior, duty cycle and exact product category. |
| Transfer sequence | Open, delayed, closed and bypass functions change the equipment and controls. | Approved functional narrative and permissible load interruption. |
| Poles, neutral and earthing | Changes the power path and the review of source grounding. | Source topology, grounding diagram and project requirements. |
| Complete tested configuration | Individual component compliance does not prove the final TSE claim. | Manufacturer declaration, marking, manuals, wiring and configuration record. |
Keep rating meanings separate. Making capability, short-time withstand and conditional short-circuit performance describe different duties. Breaker breaking capability is another check. Compare values only with their voltage, duration, protective-device and test conditions—not simply the largest kA figure.
Short-time withstand is a manufacturer-assigned rating under stated test conditions, not a fault-breaking value. Schneider Electric: what Icw means.
A certificate labelled “CB” can refer to the IECEE CB certification scheme, not CB-Class TSE. For example, Socomec’s published ATyS a M CB Scheme certificate explicitly states equipment class PC. Read the class field rather than the certificate title.
Controls are a separate layer
A capable controller cannot change the power-path class
Compatibility, interlocking and transfer behavior still have to be demonstrated for the complete system.
Source sensing, voltage and frequency limits, timing, generator-start commands, alarms and communications belong in the control specification. They do not show whether the power devices are intended to interrupt a short circuit.
Verify controller compatibility with the selected switching devices, position feedback, interlocks and required operating modes. Define how the arrangement responds to a protective trip and prevents inappropriate transfer or re-energization of a faulted load.
Open, delayed and closed transition are separate choices from PC or CB class. Bypass/isolation and neutral requirements also need their own review; they are not implied by the class label.
Socomec’s C25/C35 brochure lists model-dependent PC, CB and CC compatibility and specific tested combinations. It demonstrates why a controller name alone is not full-system evidence.
A six-stage selection workflow
Decide the fault role before releasing the order
Resolve the design basis first, then check the offered equipment against it.
Define the function
Specify manual, remote or automatic operation; transfer behavior; bypass/isolation; controller scope; and acceptable load interruption.
Freeze the electrical basis
Confirm sources, voltage, frequency, load duty, poles, neutral/earthing, fault level and protective-device locations on the current one-line.
Assign fault interruption
Evaluate PC where associated protection clears faults. Evaluate CB where the TSE must include the breaker fault-interruption role.
Verify the complete TSE
Match devices, releases, controller, interlocks, enclosure, accessories and sequence to the documented manufacturer configuration.
Review coordination
Check applicable withstand/conditional ratings for PC, or breaker capability, assigned withstand duty and settings for CB. Review selectivity.
Plan acceptance
Assign functional tests, interlocking proof, settings records, alarms, documentation and operator handover before procurement closes.
Stop at an evidence gap. A matching ampere rating is not an approval. The qualified designer and manufacturer should resolve missing fault-duty, protection or configuration evidence before order release. This guide is not a wiring, switching or energized-work procedure.
Illustrative scenario / not a site design
The same 400 V load can lead to different architectures
Consider a motor and mixed auxiliary-load panel that needs automatic transfer. The load voltage alone cannot select the class.
Option A / PC-Class
Coordinated external protection
A purpose-built transfer device is paired with the documented protective arrangement. The review checks declared short-circuit conditions, exact protective-device references and the utilization category for the load.
Prove the TSE + protection combinationOption B / CB-Class
Breaker-based source changeover
A complete CB-Class TSE includes the selected breaker paths and releases. The review checks fault-interruption duty, any assigned withstand rating, interlocking, controller behavior and responsibility for settings.
Prove the complete TSE + settingsBoth may be candidates only if their respective arrangements are verified. This example gives no model selection, current rating, universal fault threshold or cost ranking.
Common selection errors
Catch the shortcuts before award
“Automatic” is the class
It describes control. Request the IEC class marking and supporting TSE documentation separately.
A non-fused PC switch will clear the fault
The switch is not the fault interrupter. Check the associated protection and all declared coordination conditions; do not generalize this to fuse-combination units.
The breaker frame is enough
For CB, obtain release details, breaking capability, any assigned withstand duty, settings, interlocks and complete TSE evidence.
Separate certificates prove the assembly
They do not establish the complete source-changeover claim. Match the actual assembled configuration to manufacturer evidence.
Class replaces load-duty checks
It does not replace utilization category, load behavior, neutral design or an approved transfer narrative.
The latest IEC edition is adopted everywhere
State the required edition and applicable national adoption. A newer publication does not automatically invalidate every earlier project specification.
Buyer’s evidence pack
A practical PC / CB RFQ checklist
Ask for a documented arrangement with clear responsibilities, not a loosely defined set of components.
| Include in the RFQ | What the supplier and project team must resolve |
|---|---|
| 01 Required standard edition | IEC 60947-6-1:2026 or the formally adopted project edition, plus applicable local requirements. |
| 02 Operating designation | MTSE, RTSE or ATSE; controller scope, sensing, alarms, communications and transfer/retransfer logic. |
| 03 Class and design rationale | PC or CB requirement, where faults are cleared and the short-circuit data supporting the choice. |
| 04 Electrical and load data | Voltage, frequency, current, poles, neutral/earthing, load duty, fault level and utilization category. |
| 05 Protection evidence | Exact associated protection and coordination for PC; breaker/release, ratings, settings and coordination for CB. |
| 06 Complete assembly evidence | Type-test declaration, class marking, wiring, interlocks, enclosure/IP needs, accessories and configuration limits. |
| 07 Acceptance and handover | Agreed tests, settings records, as-builts, manuals, training, warranty responsibilities and maintenance information. |
Clarify the submission boundary. Ask whether a document applies to the complete TSE, an individual breaker or switch, an ATS controller, or the surrounding panel. A declaration for one layer should not be presented as evidence for every layer.
Verification and handover
Leave a traceable protection-and-transfer record
Commissioning verifies the specified functions in the installation. It does not substitute for missing manufacturer type-test evidence.
Assign qualified personnel to the approved acceptance plan. Record who owns wiring and interlocking checks, source sensing, transfer/retransfer behavior, protection settings, alarms and communications. Use the exact manufacturer instructions and project criteria.
- Identity and configurationMarked TSE data, exact device references and the approved configuration.
- Protection recordsCoordination evidence, releases, settings and controlled changes.
- Functional evidenceInterlock, sensing and transfer results, plus resolved deficiencies.
- Operational handoverOne-lines, as-builts, manuals, training and maintenance ownership.
Do not infer safety, isolation or compliance from any photograph on this page. Installation, testing and maintenance require the applicable qualified-person procedures.
Frequently asked questions
Eight answers for a clearer class decision
Keep the class, protection scheme and evidence requirements connected.
What is the main difference between PC-Class and CB-Class ATS equipment?
In the non-fused, switch-based PC arrangements compared here, the transfer switch makes and withstands the declared fault duty while coordinated protective devices interrupt the fault. CB-Class TSE includes the fault-interruption function. Class PC also includes fuse-combination units with a fault-breaking role, so the non-fused comparison is not a universal definition. Verify the complete arrangement, applicable edition and declared ratings; CB short-time withstand depends on an assigned manufacturer rating.
Is PC-Class the same as a switch-disconnector ATS?
Often, but do not use the device label as the proof. Manufacturers commonly describe switch-disconnector-based source-changeover equipment as PC-Class. The correct decision still requires the product's IEC 60947-6-1 class marking, utilization category, short-circuit coordination data, and the exact associated protective-device arrangement.
Does ATSE mean the equipment can only be PC-Class or CB-Class?
No. ATSE describes automatic transfer operation, not an exhaustive list of short-circuit classes. Class CC contactor-based equipment also exists. This guide compares PC-Class and CB-Class; neither the automatic label nor controller compatibility proves the class of the complete transfer assembly. Confirm the marking and documentation for the exact configuration.
Can a PC-Class ATS be used when fault current is high?
Possibly, but only within the manufacturer's declared short-circuit ratings and stated protective-device coordination. High prospective fault current is not an automatic rejection or approval. Obtain the exact TSE coordination data, verify the protective devices and settings, and have the qualified project team confirm applicability. A conditional rating is not a standalone fault-interruption rating.
Does CB-Class eliminate the need for a protection coordination study?
No. CB-Class equipment includes the fault-interruption and overcurrent-release function, but breaker ratings, release settings, source contributions, discrimination/selectivity, interlocking, and the complete system still need review. The equipment class is one input to a coordinated protection design, not a substitute for it.
What standard edition should be cited in a new RFQ?
As of this guide's publication, IEC 60947-6-1:2026 is the fourth edition and replaced the 2021 edition on 2 April 2026. The RFQ should state the edition required by the project and any national adoption or authority requirement. Do not assume a standard is legally adopted everywhere simply because it is current internationally.
Can a third-party panel builder combine breakers and a controller to make CB-Class TSE?
Only if the resulting configuration is documented, marked, and type-tested as complete transfer switching equipment to the applicable requirements. The IEC 2026 scope excludes configurations not fully manufacturer type-tested or marked as a complete transfer switch. Request evidence for the exact arrangement, not only component certificates. Applicable component tests may support derived-TSE verification where the standard allows; complete-configuration requirements still apply.
What documents should be delivered at handover?
Request the marked TSE data, one-line and wiring diagrams, protective-device and controller settings, coordination evidence, test results, interlocking/transfer test records, manuals, as-builts, training records, and maintenance information. These records let the operator understand both the transfer function and the protective architecture after commissioning.
Technical references
Verify the standard and the exact product
The standard edition, application literature and configuration-specific records serve different purposes. Check which one supports each claim.
- IEC · IEC 60947-6-1:2026Official fourth-edition status, scope and complete-TSE exclusions.
- IEC · IEC 60947-6-1:2021Previous-edition status and context for CB-type performance requirements.
- ABB · IEC classification training (2022)PC fuse-combination and CB assigned-withstand nuances; not a substitute for the project-adopted edition.
- IEC · Published 2026 samplePrimary IEC text, hosted by iTeh: terminology and derived-TSE component-test context.
- Schneider Electric · Icw explanationManufacturer-assigned short-time withstand current and stated conditions.
- Schneider Electric · Source changeoverExamples of breaker-based CB and switch-disconnector-based PC architectures.
- Schneider Electric · EasyPact WATSNModel-family example separating PC class from automatic or remote operation.
- Socomec · ATyS a MProduct-specific PC class and utilization-category declarations.
- Socomec · C25/C35 controller brochureModel-dependent controller compatibility and documented TSE combinations.
- Socomec · ATyS a M CB Scheme certificateExample of a certification-scheme document that explicitly declares equipment class PC.
A defensible specification starts with the system
Bring the one-line—not just the ampere rating
Share source data, fault-current information, load duty, transfer sequence and the required standard edition. Ask the qualified designer and supplier to confirm the complete PC-Class or CB-Class arrangement before procurement.
