Automatic Transfer Switching Equipment (ATSE) is a critical apparatus in ensuring uninterrupted power supply for both industrial and civil applications. The national standard GB/T 14048.11 divides ATSE into three classifications: PC-class (Power Circuit switching devices), CB-class (Circuit Breaker switching devices), and CC-class. While CC-class has its own niche, this discussion concentrates on PC and CB, as they are the most widely used in engineering practice.

PC-Class (Power Circuit Switching Device)

PC-class ATSE is designed primarily as a switching mechanism. Its responsibility is confined to transferring loads between two power sources. Unlike circuit breakers, a PC-class switch does not possess short-circuit breaking capacity. For this reason, it must be coordinated with an upstream short-circuit protective device (SCPD) such as a fuse or circuit breaker.

In essence, a PC-class unit is lightweight, cost-effective, and highly specialized for switching. Its internal design focuses on mechanical and electrical endurance rather than protective capabilities.

CB-Class (Circuit Breaker Switching Device)

CB-class ATSE, on the other hand, is founded on the structure of a circuit breaker. It is inherently capable of not only switching between power sources but also providing short-circuit and overload protection. With integrated trip units—thermal-magnetic or electronic—CB-class devices can operate independently without reliance on external protection.

This dual functionality makes CB-class switches robust solutions where reliability and self-sufficiency are paramount. They are subjected to rigorous short-circuit testing and must demonstrate capabilities such as Icu (ultimate breaking capacity), Ics (service breaking capacity), and Icm (making capacity).

Differences in Standards Requirements

Short-Circuit Testing

• PC-class: Tested under conditional short-circuit conditions. Compliance requires operation in conjunction with a specified upstream protective device.

• CB-class: Tested for complete short-circuit making and breaking performance, proving its ability to independently handle fault currents.

Overload Protection

• PC-class: Does not provide overload protection. While some controllers integrate current monitoring, this is usually for supervision only and not intended for protection.

• CB-class: Equipped with overload protection through thermal-magnetic or electronic trip units. These must pass standardized overload testing.

Method of Implementation

• PC-class: Relies entirely on coordination with an upstream SCPD. The manufacturer must specify compatible devices and rated parameters.

• CB-class: Provides protection autonomously, while still allowing coordination with upstream protective devices for selectivity.

Common Application Scenarios

PC-Class Applications

• Terminal distribution boards: Frequently used in fire-protection zones for lighting, rolling shutters, and smoke exhaust systems. These circuits already have upstream fuses or breakers.

• Cost-sensitive projects: Offers compact structure, reduced footprint, and lower cost.

• Selective protection schemes: Since protection is handled by upstream devices, PC-class ensures cleaner selectivity and reduces nuisance tripping.

CB-Class Applications

• Main incomers or critical busbars: Applied in generator transfer schemes or main incoming lines for fire pump rooms and substations. Local fault interruption is essential here.

• Critical loads: Hospitals, surgical theatres, and data centers where depending solely on upstream protection is unacceptable.

• High-reliability systems: Self-contained switching and protection minimize dependence on external coordination.

Conclusion

PC-class and CB-class automatic transfer switches are engineered with distinct philosophies.

• PC-class: Dedicated to switching. It depends on upstream SCPD for fault protection, making it suitable for distribution endpoints, sub-zones, and cost-conscious designs where selectivity and compactness are emphasized.

• CB-class: Combines switching and protection. It is best suited for main feeders, critical facilities, and high-reliability installations where independent fault clearing and local protection are non-negotiable.

There is no absolute superiority between the two. The selection should hinge upon the application environment, reliability requirements, protection coordination, permissible outage duration, transfer timing, installation space, and wiring arrangements.