CB Type vs PC Type Automatic Transfer Switch Explained

Introduction

In modern dual-power systems, Automatic Transfer Switches (ATS) are essential for ensuring uninterrupted power supply. Whether in hospitals, commercial buildings, data centers, or industrial plants, an ATS automatically transfers load from the main power source to a backup source when abnormalities occur.

Key question: Should you choose a CB Type ATS or a PC Type ATS?

Although both types comply with IEC 60947-6-1, their internal structure, protection philosophy, and system design logic are fundamentally different. This guide explains what each type means and how to choose correctly from an engineering perspective.

What Is a CB Type Automatic Transfer Switch?

A CB Type ATS (Class CB Automatic Transfer Switch) is a transfer device built around circuit breaker technology. In essence, it integrates switching and protection into one device.

1.1 Structural Principle

A CB Type ATS is typically composed of:

• Two molded case circuit breakers (MCCB)
• Mechanical interlocking mechanism (prevents two sources closing simultaneously)
• Motorized operating mechanism
• Intelligent controller (monitoring + decision logic)

When the main source becomes abnormal (e.g., undervoltage, phase loss), the controller trips the normal source breaker, ensures isolation, then closes the standby source breaker. This approach prioritizes safety by preventing source paralleling.

1.2 Protection Philosophy of CB Type ATS

The defining feature of CB Type ATS is:

It can interrupt fault current directly.

Because it is breaker-based, CB Type ATS can provide overcurrent/short-circuit protection as part of the transfer assembly, often simplifying panel architecture and reducing the number of separate devices required.

1.3 Advantages of CB Type ATS

• Integrated protection and transfer
• Reduced component count
• Simplified wiring and installation
• Generally more cost-effective for integrated designs

What Is a PC Type Automatic Transfer Switch?

A PC Type ATS (Power Switching Class) is designed primarily for power transfer, not for protection. Its philosophy is to separate switching from protection so the ATS focuses on transfer reliability while upstream devices handle fault interruption.

2.1 Structural Principle

A PC Type ATS typically includes:

• Dedicated switching contacts
• Transfer mechanism and mechanical interlock
• Controller for monitoring and transfer logic

Unlike CB Type, a PC Type ATS generally does not contain an overcurrent trip unit or a short-circuit interrupting mechanism. Upstream MCCB/ACB devices provide protection and coordinate fault clearance.

2.2 Protection Philosophy of PC Type ATS

Switching and protection are intentionally separated for better coordination.

In large or critical systems, separating switching from protection improves selectivity and makes it easier to maintain protection devices without impacting the transfer mechanism.

2.3 Why PC Type Is Preferred in High-End Systems

PC Type ATS is often selected where protection coordination is critical, such as:

• Data centers
• Critical infrastructure
• Petrochemical and heavy industry
• Large commercial complexes

CB Type vs PC Type – Core Differences

The real difference is not simply “current rating.” It is a system design philosophy: integration vs separation.

Summary: CB Type = Protection + Transfer in one device. PC Type = Transfer only, protection handled elsewhere.

Engineering Selection Guide

Instead of asking “Which is better?”, ask: What kind of power system am I designing?

4.1 Choose CB Type ATS When:

• You prefer integrated protection and simpler architecture
• You want fewer devices and simpler wiring
• Cost efficiency is important
• Space in the cabinet is limited

4.2 Choose PC Type ATS When:

• Protection coordination/selectivity is critical
• You are designing large or layered protection systems (e.g., ACB-based)
• High-end critical loads demand highly engineered discrimination
• Independent maintenance of upstream protection is required

Short-Circuit and Coordination Perspective

From a fault analysis standpoint, the difference becomes clearer:

• CB Type ATS: breaker-based design can clear downstream faults directly.
• PC Type ATS: relies on upstream protection; ATS focuses on switching reliability and coordination.

If your project requires selectivity studies, multi-level discrimination, and strict protection coordination, PC Type often integrates better into layered protection systems.

Conclusion

CB Type and PC Type Automatic Transfer Switches are designed for two different engineering philosophies: integration vs separation.

CB Type ATS is often chosen for simplified designs with integrated protection. PC Type ATS is often preferred for large, critical systems where protection coordination and selectivity are key.