MCC Panels

Air Circuit Breakers (ACB) in Automatic Transfer Switch (ATS) Panel

Air Circuit Breakers (ACB) selection, integration, and best practices for Automatic Transfer Switch (ATS) Panel assemblies compliant with IEC 61439.

Air Circuit Breakers (ACB) in Automatic Transfer Switch (ATS) Panel

Overview

Air Circuit Breakers (ACB) are the preferred main switching and protection devices in Automatic Transfer Switch (ATS) Panel assemblies where continuity of supply, high fault levels, and monitored source changeover are critical. In IEC 61439-2 low-voltage switchgear assemblies, the ACB must be selected not only for its nominal current range, typically from 630 A up to 6300 A, but also for its utilization category, short-time withstand current (Icw), peak withstand current (Ipk), and service short-circuit breaking capacity to match the incoming network and downstream distribution scheme. For ATS duties, ACBs are commonly used as incomers, bus couplers, or main outgoing breakers with motor operators, spring-charged mechanisms, and programmable electronic trip units that support undervoltage, overload, short-circuit, earth fault, and selective coordination functions. In a true ATS configuration, the ACB can be integrated with an automatic transfer controller or PLC-based logic so that source priority, dead-bus transfer, open transition, delayed transfer, or closed transition strategies are executed with defined interlocking. Typical systems use two incoming ACBs with mechanical and electrical interlocks to prevent paralleling unless the design explicitly permits synchronised transfer. In critical facilities such as hospitals, data centers, wastewater plants, airports, and industrial process lines, the ACB-based ATS panel ensures that the preferred source and standby source are transferred within a controlled time window while protecting the busbar system and downstream MCCs, VFD panels, or distribution feeders. Compliance is governed by IEC 61439-1 and IEC 61439-2 for assembly design verification, temperature-rise limits, dielectric properties, clearances, creepage distances, and internal arc considerations where applicable. The ACB itself is generally evaluated to IEC 60947-2, and the switching and auxiliary devices that support ATS logic must align with IEC 60947-1 and IEC 60947-3 as applicable. When the panel is installed in hazardous locations or adjacent to classified areas, enclosure selection and component placement may also require consideration of IEC 60079. In high-energy utility or industrial environments, internal arc mitigation measures may be validated against IEC 61641 for assemblies where fault containment is required. A well-engineered ATS panel with ACBs should consider form of internal separation, commonly Form 2b, Form 3b, or Form 4, depending on the required segregation between incomers, busbars, and outgoing functional units. This improves maintainability and reduces the risk of cascading failures. Thermal performance is especially important because ACBs with electronic trip units, shunt releases, undervoltage releases, motor operators, communication modules, and metering CTs add heat within the enclosure. The panel designer must verify derating, ventilation, and cable termination temperatures under full load and fault conditions. Modern ACBs support Modbus, Profibus, Ethernet gateways, and dry-contact status outputs for SCADA and BMS integration, allowing remote position indication, trip diagnostics, load trending, and maintenance scheduling. Common configurations include fixed or withdrawable ACBs, metered incomers, bus-section couplers, and dual-source ATS schemes with source availability monitoring. Patrion, based in Turkey, designs and manufactures IEC-compliant ATS panel assemblies for EPC and industrial projects, integrating ACBs from leading platforms into compact, maintainable, and fully coordinated low-voltage systems tailored to site-specific fault levels and operational requirements.

Key Features

  • Air Circuit Breakers (ACB) rated for Automatic Transfer Switch (ATS) Panel operating conditions
  • IEC 61439 compliant integration and coordination
  • Thermal management within panel enclosure limits
  • Communication-ready for SCADA/BMS integration
  • Coordination with upstream and downstream protection devices

Specifications

PropertyValue
Panel TypeAutomatic Transfer Switch (ATS) Panel
ComponentAir Circuit Breakers (ACB)
StandardIEC 61439-2
IntegrationType-tested coordination

Other Components for Automatic Transfer Switch (ATS) Panel

Moulded Case Circuit Breakers (MCCB)

Branch protection 16A–1600A, thermal-magnetic or electronic trip

Contactors & Motor Starters

DOL/star-delta/reversing starters, overload relays, Type 2 coordination

Protection Relays

Overcurrent, earth fault, differential, generator protection relays

Metering & Power Analyzers

Energy meters, power quality analyzers, CT/VT, communication gateways

Other Panels Using Air Circuit Breakers (ACB)

Main Distribution Board (MDB)

Primary power distribution from transformer to sub-circuits. Rated up to 6300A. Houses main incoming breaker, bus-section, and outgoing feeders.

Power Control Center (PCC)

High-capacity power distribution for industrial facilities. Controls and distributes incoming power to MCC, APFC, and downstream loads.

Generator Control Panel

Genset start/stop sequencing, synchronization, load sharing, and paralleling controls.

Custom Engineered Panel

Bespoke panel assemblies for non-standard requirements — special ratings, unusual form factors, multi-function combinations.

Frequently Asked Questions

For a 2500 A ATS panel incomer, the ACB should be selected with a continuous current rating equal to or above the design load, typically 2500 A or 3200 A depending on ambient temperature, enclosure ventilation, and future expansion margin. The critical parameters are not just In, but also Icw and Icu/Ics, which must exceed the prospective fault level at the point of installation. Under IEC 61439-2, the assembly must be verified for temperature-rise and short-circuit withstand with the chosen breaker and busbar system. In practice, engineers often use electronic-trip ACBs with adjustable LSI or LSIG settings to coordinate with downstream MCCBs and final circuits while maintaining selectivity.
Most automatic transfer panels use two ACBs, one for the normal source and one for the standby source, because this allows interlocked changeover with defined source priority and protection on both incomers. A single ACB with a mechanically switched bypass arrangement is less common in utility-grade systems. Dual-ACB ATS panels are typically designed with electrical and mechanical interlocking to prevent unintended paralleling unless synchronised closed-transition transfer is explicitly required. IEC 61439-2 governs the assembly design verification, while the ACBs themselves must comply with IEC 60947-2. This arrangement is preferred in hospitals, data centers, and process plants where source changeover must be reliable and maintainable.
ACB coordination with generator transfer logic is achieved through the ATS controller or PLC, which monitors source voltage, frequency, phase sequence, and recovery timers. When utility supply fails, the controller issues an open command to the utility ACB, starts the generator, verifies generator stability, and then closes the generator ACB after the programmed delay. In open-transition schemes, the ACBs never parallel; in closed-transition schemes, synchronization equipment is required and the design becomes more complex. Protection settings must coordinate with upstream and downstream devices to avoid nuisance trips. The assembly must still satisfy IEC 61439-1/2 requirements for temperature rise, dielectric performance, and short-circuit withstand.
The required short-circuit rating depends on the prospective fault current available at the ATS point of connection, not on the load current alone. The selected ACB must have sufficient Icu and Ics to interrupt the maximum fault level, and its Icw must match the duration needed for selective tripping and downstream coordination. In large LV systems, this may range from 36 kA to 100 kA or higher, depending on the transformer size and impedance. Under IEC 60947-2, the breaker’s ratings are defined at specific voltage levels and duty cycles, while IEC 61439-2 requires the complete assembly to be verified for short-circuit withstand with the busbars, supports, and internal separation arrangement.
Yes, withdrawable ACBs are commonly used in ATS panels where maintainability and uptime are priorities. They allow safe isolation, inspection, testing, and replacement without major disturbance to the assembly. This is particularly useful in hospitals, critical manufacturing lines, and utility buildings. The withdrawable mechanism must be mechanically robust and interlocked with the ATS logic to prevent unsafe racking operations under load. IEC 61439-2 applies to the assembled panel, and the ACB itself remains subject to IEC 60947-2. Engineers should also verify internal separation form, cable compartment accessibility, and the impact of the withdrawable mechanism on thermal performance and busbar clearances.
Communication-ready ACBs integrate through built-in or accessory communication modules that provide breaker status, trip cause, current measurement, energy data, and remote open/close commands. Common interfaces include Modbus RTU, Modbus TCP, and gateway-supported Ethernet protocols, depending on the breaker platform. This data is valuable for SCADA, BMS, and power monitoring systems in airports, commercial buildings, and industrial plants. The engineer must ensure auxiliary power availability, wiring segregation, EMC control, and correct mapping of status contacts and trip alarms. These functions do not replace the core compliance requirements of IEC 61439-1/2, but they enhance diagnostics, preventive maintenance, and operational visibility.
The preferred internal separation depends on maintainability, cable density, and criticality. For ACB-based ATS panels, Form 3b or Form 4 is often selected when separate compartments are needed for busbars, functional units, and outgoing terminals, especially in high-availability facilities. Form 2b may be sufficient for simpler installations with lower maintenance demands. The choice must be validated as part of the IEC 61439-2 assembly design, including access control, temperature rise, and short-circuit behavior. Higher separation forms improve safety and serviceability but increase panel size and cost, so the design should align with the project’s operational requirements.
Typical ACB accessories in ATS panels include motor operators for remote charging, shunt trip and undervoltage releases, auxiliary contacts, position indication, trip alarms, communication modules, and sometimes zone-selective interlocking. Electronic trip units are usually specified with adjustable long-time, short-time, instantaneous, and ground-fault protection. For ATS service, these accessories support automatic changeover, remote supervision, and selective coordination with upstream transformers, feeder MCCBs, and downstream distribution boards. The final selection must be coordinated with the ACB manufacturer’s application data and verified in the panel assembly under IEC 61439-2, with the breaker itself compliant to IEC 60947-2.

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