What Is The Difference between A PC-class And A CB-class Automatic Transfer Switch (ATS)?

According to the national standard GB/T 14048.11, Automatic Transfer Switching Equipment (ATSE) is classified into PC class (Power Circuit) and CB class (Circuit Breaker) (the standard also defines CC class, but here we will mainly discuss PC and CB).

PC Class (Power Circuit switching device)

Based on a switching device, it is responsible for transferring between power sources.
PC class devices do not have short-circuit breaking capability and must be used in coordination with an upstream short-circuit protective device (SCPD).

CB Class (Circuit Breaker switching device)

Based on a circuit breaker, it can not only switch between power sources but also provide short-circuit and overload protection, with independent breaking and protection capability.

Typical Applications

PC Class Applications

• Final distribution boards: e.g., lighting, fire shutters, smoke exhaust windows within fire compartments. In these cases, an upstream breaker/fuse already provides protection, while the PC class device only performs switching.

• Cost-sensitive projects: simpler structure, smaller footprint, lower cost.

• When selectivity is important: protection is provided by upstream devices, avoiding nuisance tripping due to overlapping protection.

CB Class Applications

• Main incoming or critical busbars: e.g., main incomer of a fire pump room, transfer between substation and generator, where local short-circuit breaking capability is required.

• Critical loads: hospital operating theaters, data center server rooms, etc., where relying solely on upstream protection is not acceptable.

• High reliability requirements: switching and protection are performed independently, minimizing dependency on upstream devices.

Summary

• PC Class: Performs switching only, relies on upstream SCPD for protection, suitable for final distribution and sectional applications, emphasizes selectivity and cost-effectiveness.

• CB Class: Provides both switching and protection, suitable for main incomers and critical loads, emphasizes local breaking capability and reliability.

There is no absolute “better” or “worse” between the two; the choice depends on application requirements. Other factors to consider include: permissible outage duration, transfer time, installation location and space, wiring arrangements, etc.

Differences in Standard Requirements

Short-circuit test method

• PC Class: Conditional short-circuit current test, must be tested in coordination with a specified upstream protective device.

• CB Class: Full short-circuit making/breaking tests, independently rated with Icu / Ics / Icm values.

Overload protection

• PC Class: No overload protection function. Some controllers may provide current monitoring, but it is generally not used as a protection basis due to coordination difficulties.

• CB Class: Equipped with thermal-magnetic or electronic trip units, with overload characteristics tested and certified.

Implementation

• PC Class: Cannot achieve protection on its own; must specify the model and rating of the SCPD used in coordination.

• CB Class: Can operate independently to provide both switching and protection, or coordinate with upstream devices.