In the hierarchy of electrical distribution, the Automatic Transfer Switch (ATS) acts as the gatekeeper of continuity. When the main utility fails, the ATS ensures that critical loads—whether in a hospital, a data center, or an industrial plant—are transitioned to an emergency power source.
Switch-based ATS
Breaker-based ATS
What is a CB Type ATS? (Circuit Breaker Based)
A Class CB ATS is essentially an assembly consisting of two circuit breakers (usually MCCBs—Molded Case Circuit Breakers) equipped with a mechanical interlock and an electrical operating mechanism (motorized drive).
Technical Construction
In a CB Type ATS, the switching elements are standard circuit breakers. A controller monitors the voltage and frequency of both sources. When a transfer is required, the controller signals the motor mechanism to open one breaker and close the other. The mechanical interlock is a critical safety feature that prevents both breakers from being closed simultaneously, which would cause a catastrophic short circuit between sources.
Key Characteristics
Standardization: The breakers used must comply with IEC 60947-2, while the overall ATS assembly complies with IEC 60947-6-1.
- Integrated Protection: Since the switching elements are circuit breakers, they contain overcurrent releases (thermal-magnetic or electronic). This means the ATS itself provides short-circuit and overload protection for the downstream load.
- Complexity: These units involve more moving parts, including the motorized linkages and the internal mechanisms of the breakers themselves.
- Standardization: The breakers used must comply with IEC 60947-2, while the overall ATS assembly complies with IEC 60947-6-1.
What is a PC Type ATS? (Switch Based)
A Class PC ATS is a dedicated, purpose-built switching device. Unlike the CB type, it does not use circuit breakers as its primary switching contacts. Instead, it utilizes a simplified, rugged contact system—often driven by a high-speed solenoid or a heavy-duty motor.
Technical Construction
Class PC units are designed exclusively for switching and withstanding current; they are not intended to break short-circuit currents. They feature a “double-throw” design where a single mechanism moves the contacts between Source A and Source B.
Key Characteristics
No Overcurrent Protection: A PC Type ATS does not have built-in trip units. It relies entirely on upstream circuit breakers or fuses to clear faults.
High Reliability: With fewer mechanical parts and a dedicated design, PC types often have a much higher MTBF (Mean Time Between Failures) and a higher mechanical endurance rating.
High Withstand Capacity: Because they don’t have to trip, their contacts are often built larger and more robustly to “ride out” a short circuit until an upstream device clears the fault.
Direct Technical Comparison: CB vs. PC
For an industrial buyer or systems integrator, the following technical parameters define the selection process:
Short-Circuit Withstand and Breaking Capacity
CB Type: Has a rated Breaking Capacity (). It can safely interrupt a fault. However, the mechanical stress of tripping can sometimes affect the alignment of the transfer mechanism over time.
PC Type: Has a rated Short-Time Withstand Current (). It is designed to remain closed during a fault without contact welding, allowing the upstream protection to handle the interruption.
Transfer Speed
PC Type: Generally faster. Solenoid-operated PC switches can transfer in as little as 30–50ms.
CB Type: Typically slower (usually 100ms to several seconds) because the motor mechanism must physically “crank” the breakers open and closed.
Space and Footprint
CB Type: Often more compact for lower amperages because it combines “switch” and “protection” in one unit.
PC Type: Requires upstream protection, which might increase the overall footprint of the electrical switchboard.
Comparison Table
| Feature | Class CB (Breaker-based) | Class PC (Switch-based) |
|---|---|---|
| Overcurrent Protection | Integrated (Thermal/Magnetic) | None (Requires Upstream Protection) |
| Contact Material | Optimized for breaking arcs | Optimized for carrying high current |
| Transfer Speed | Slower (Motor Driven) | Faster (Solenoid or High-speed Motor) |
| Endurance | Lower (6,000 - 10,000 cycles) | Higher (10,000 - 50,000+ cycles) |
| Fault Response | Trips and disconnects load | Remains closed (Withstands fault) |
| Maintenance | Higher (due to complex linkages) | Minimal |
Industry Applications: Which to Choose?
Selection is rarely about “which is better,” but rather “which is appropriate for the application.”
When to specify CB Type ATS:
Budget-Sensitive Projects: When you need to save costs by eliminating the need for separate upstream breakers.
Non-Critical Loads: For residential or small commercial buildings where transfer speed is not a life-safety issue.
Space Constraints: When the electrical room cannot accommodate additional protective equipment.
When to specify PC Type ATS:
Critical Infrastructure: Data centers, healthcare facilities (Operating Rooms), and telecommunications hubs where “Seconds Matter.”
Industrial Heavy Loads: High-power applications (above 800A) where the ruggedness of a dedicated switch is required.
High-Fault Environments: Near large transformers where the prospective short-circuit current is very high. A PC-class switch with a high rating is safer here.
Selection Guidance: Design Considerations
When integrating an ATS into your power distribution project, follow these engineering steps:
Step 1: Analyze the Coordination Study
If you choose a PC Type, you must ensure that the upstream protective device will trip before the ATS exceeds its withstand limit. If you choose a CB Type, you must ensure it coordinates with both upstream and downstream breakers to prevent “nuisance tripping” of the entire building for a minor branch fault.
Step 2: Evaluate the Load Type
Inductive Loads (Motors): Require an ATS with high “Inrush” capability. PC types are generally superior for large motors.
Electronic Loads (Servers): Require fast transfer. If the transfer is too slow, the UPS batteries may deplete, or the equipment may reboot.
Step 3: Check Environmental Ratings
Ensure the enclosure matches the site conditions. In Zhejiang’s industrial zones or Sharjah’s desert heat, the IP (Ingress Protection) rating and the operating temperature range of the controller are as important as the switch type.
Frequently Asked Questions (FAQs)
1. Does a PC Type ATS require a fuse?
Not necessarily a fuse, but it must have an upstream overcurrent protective device (breaker or fuse). Using a PC type without upstream protection is a violation of IEC 60947 standards and a major fire hazard.
2. Can I use a CB Type ATS as a service entrance?
Yes. Because CB Type ATS units have built-in overcurrent protection, they are often used as the “Service Entrance” equipment where the utility power first enters the building.
3. Which type is easier to maintain?
PC Type units are generally easier to maintain because they are simpler. CB Type units require checking the motor drive, the mechanical interlock alignment, and the trip settings of the internal breakers.
4. What is Class CC?
While less common than CB and PC, Class CC refers to an ATS based on contactors. It is similar to a PC type in that it has no overcurrent protection but is typically used for smaller, lighter-duty applications.
Conclusion
The choice between a CB Type and a PC Type ATS defines the reliability profile of your power system. For integrated protection and cost-efficiency in smaller systems, the CB Type remains a popular choice. However, for mission-critical applications where speed, endurance, and high-fault withstand are non-negotiable, the PC Type is the industry benchmark.
By adhering to IEC 60947-6-1 standards and performing a thorough load analysis, engineers can ensure that their power transfer strategy is both safe and efficient.
Need help selecting the right ATS class for your project? Our technical team provides customized solutions for low-voltage power distribution. Whether you require the protection of a CB-type or the speed of a PC-type, we have the engineering expertise to support your infrastructure.
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