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The main difference between a PC class automatic transfer switch (ATS) and a CB class ATS is how short-circuit protection is handled. A conventional PC class ATS transfers power and relies on coordinated external protective devices to clear short circuits. A CB class ATS uses circuit breakers that provide short-circuit interruption as part of the transfer arrangement.
Both can be used for mains-to-generator or mains-to-mains transfer. Choosing between them means deciding how switching and protection should work together—not simply choosing the faster or more reliable class.
This guide compares their protection functions, transfer performance and installation requirements, with AISIKAI product examples for panel builders, generator manufacturers and electrical distributors.
A PC class ATS is primarily a power-switching device. Its main contacts carry the load and transfer it between sources. In a conventional switch-based design, these contacts are not intended to interrupt short-circuit current; a coordinated circuit breaker or fuse performs that task.
PC classification does not mean the device has no fault-current capability. The switch must have suitable declared short-circuit ratings for its application. The distinction is between withstanding a fault until protection operates and interrupting that fault.
PC class switches can use different operating mechanisms. For example, AISIKAI SKT1 uses a motor-operated mechanism, while ASKQ6 uses solenoid operation. Therefore, “PC class” and “solenoid-operated” are not interchangeable descriptions.
AISIKAI SKT1 motorized automatic transfer switch is a PC class product without integrated overload or short-circuit protection. Protection must be arranged elsewhere in the circuit and coordinated with the selected switch.
A CB class ATS uses circuit breakers as its switching devices. Depending on the design and current rating, these may be miniature, molded case or air circuit breakers. Interlocks and control logic coordinate the source-switching sequence.
The breakers provide short-circuit interruption. Overload protection and other protective functions depend on the fitted breakers and trip units; these functions should be checked for the exact configuration.
AISIKAI SKQ1 is a CB class series available in MCB-based and MCCB-based configurations. It is an option when a project specifies circuit-breaker-based transfer with protection incorporated into the assembly.
However, integrated breakers do not automatically eliminate other protective devices. Incoming cables, upstream equipment and outgoing circuits still require appropriate protection within the overall distribution design.
| Selection point | PC class ATS | CB class ATS |
|---|---|---|
| Main function | Power-source transfer | Power-source transfer with circuit-breaker protection |
| Short-circuit interruption | Conventional switch contacts do not clear the fault; coordinated external protection does | Circuit breakers interrupt faults within their declared ratings |
| Overload protection | Normally provided separately | Depends on the installed breakers and trip units |
| Operating mechanism | May be motorized or solenoid-operated | Circuit-breaker operating mechanisms; arrangement varies |
| Transfer speed | Depends on the model, mechanism and control settings | Depends on the model, mechanism and control settings |
| Protection coordination | Coordinate the switch with the external protective devices | Coordinate the incorporated breakers with upstream and downstream protection |
| Cabinet space | Assess the switch, protection, terminals and wiring together | Assess the breakers, operators, interlocks, terminals and wiring together |
| Typical selection reason | Dedicated switching with protection arranged separately | A specification requiring breaker-based transfer and local interruption |
| AISIKAI examples | SKT1, ASKQ6, ASKQ8 and ASKQ1 | SKQ1 |
Neither class alone establishes transfer speed, cabinet size, service life or suitability for a particular building.
The IEC 60947-6-1 standard covers transfer switching equipment. PC and CB are important classifications within this subject, but they are not the only classifications: CC is also relevant to certain contactor-based equipment. Do not classify an assembly solely by its appearance.
For a PC class ATS, review the manufacturer's applicable short-time withstand current (Icw), associated duration, making capacity (Icm), and conditional short-circuit current (Iq), where declared. A conditional rating depends on the specified short-circuit protective device and coordination conditions. A high current figure without its conditions is not enough for selection.
For a CB class ATS, check the breaking and making ratings declared for the complete transfer equipment. Breaker values such as Icu and Ics are relevant supporting information, but the ratings of an individual breaker should not automatically be treated as those of the complete ATS assembly.
Compare the ratings at the required operating voltage against the available fault current at the installation point. For generator applications, review protection under both utility and generator supply conditions.
Practical example: If a distribution board already has correctly coordinated incoming protection, a PC class ATS may fit the intended architecture. If the specification calls for protective circuit breakers within the transfer assembly, a CB class solution may be appropriate. In both cases, the coordination study and product documentation determine suitability.
Some dedicated transfer-switch designs offer rapid contact transfer, but PC classification does not guarantee a transfer time below 50 milliseconds. Motorized and solenoid-operated products can have different operating times even within the same class.
For a generator transfer switch, distinguish the mechanical switching interval from the complete interruption experienced by the load. That interruption may include:
Detection and confirmation of a source failure.
Generator starting and stabilization.
Any configured transfer delay.
The interval between disconnection from one source and connection to the other.
A fast mechanism cannot supply generator power before the generator is ready. Where equipment cannot tolerate the interruption, the system may need a UPS or another continuity solution.
During product comparison, request the manufacturer's definition and test conditions for each quoted time. Also check whether the intended transfer sequence includes a deliberate OFF interval. Normal breaker-based transfer should not be described as requiring a protective trip and fault reset on every operation.
Start with the single-line diagram. Identify which devices protect each incoming feeder, the transfer equipment and the outgoing circuits.
Consider PC class when the design separates switching from protection. Consider CB class when the design calls for circuit breakers within the transfer arrangement. Neither choice removes the need to coordinate protection across the system.
Rated current alone is insufficient. Review operating voltage, frequency, starting current, load composition and expected switching frequency.
Check the declared utilization category for the exact ATS. For example, the AISIKAI SKT1 is specified as AC-33A. The utilization category describes switching duty; it should not be confused with the PC/CB classification or treated as a substitute for load assessment.
Specify whether the application needs two-position or three-position operation, a controlled OFF interval, automatic return to the normal source, and external generator-start control.
For an open-transition arrangement, the operating sequence and interlocks must prevent simultaneous connection of the two sources. This requirement applies to both PC and CB designs.
Avoid assuming that PC is always smaller or CB always saves space. Compare actual installation drawings, including protective devices, operating mechanisms, terminal access, cable bending space and maintenance clearances.
Existing incoming breakers may already provide the protection needed for a PC arrangement. Conversely, a CB assembly may suit a design that deliberately incorporates the incoming breakers into the transfer equipment. Assess the complete cabinet cost and layout.
A hospital, data center or industrial plant is not automatically a PC-only or CB-only application. Its power architecture, permitted interruption, fault levels, protection requirements and maintenance strategy determine the appropriate solution.
For fire pumps and other life-safety circuits, follow the applicable project specification and local requirements. Avoid selecting a class from a generic claim that one is universally mandatory.
For either design, plan access for inspection and functional testing. Where service continuity during maintenance is essential, assess a suitable bypass/isolation arrangement as part of the system design.
AISIKAI offers both approaches, allowing the transfer equipment to match the project rather than forcing every application into one architecture.
SKT1: Motorized PC class ATS for projects requiring a motor-operated transfer mechanism.
ASKQ6: Solenoid-operated PC class ATS, with configurations selected according to the required switching positions.
ASKQ8: PC class option for higher-current transfer requirements.
ASKQ1: PC class option for lower-current distribution circuits.
SKQ1: CB class ATS using MCB-based or MCCB-based configurations.
Use the AISIKAI ATS Selection Guide to compare product families, or explore the CB class automatic transfer switch range for breaker-based options.
For a quotation, send AISIKAI your rated current, voltage, frequency, pole requirement, source arrangement, load type and available fault-current information. Include the existing protection details and required transfer sequence so the proposed configuration can be checked against the project.
A conventional PC class switch does not interrupt short-circuit current through its switching contacts. A coordinated protective device clears the fault. The switch must still have suitable fault-current ratings for the installation
Check the installed breakers and trip units. Many CB class products provide both overload and short-circuit protection, but the exact functions and settings must be confirmed for the selected configuration.
Yes. Both can be used for mains-to-generator transfer when their ratings, control functions, interlocks and protection arrangements suit the system. Generator starting and stabilization also affect the total interruption.
No. “Motorized” describes the operating mechanism. “PC” and “CB” describe equipment classifications. AISIKAI SKT1 is an example of a motorized PC class ATS.
Neither class is universally better. Select the complete system around the load's continuity requirements, protection coordination and maintenance needs. Class alone does not establish reliability or uninterrupted operatio
No. Adding upstream protection does not change the classification of the PC switch itself. A complete transfer assembly must be assessed according to its declared design, testing and ratings.
I am Eric, Electrical Engineer in AISIKAI Team. I will share technical articles on Switches, Circuit Breakers and other electrical devices. With 10 years of electric project experience, I am commited to provide professional electrical solutions.
