According to the international standard IEC 60947-6-1, Automatic Transfer Switching Equipment (ATSE) is classified into PC class (Power Circuit) and CB class (Circuit Breaker) .
A PC Class ATS is designed primarily for power transfer only, not for protection.
PC class devices do not have short-circuit breaking capability and must be used in coordination with an upstream short-circuit protective 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.
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.
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.
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.
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.
