Introduction
When the main power supply fails, every second of downtime costs money — and in critical facilities like hospitals, data centers, mines, and manufacturing plants, it can cost lives. An Automatic Transfer Switch (ATS) is the device that eliminates that risk by detecting a power failure and switching to a backup source within milliseconds, automatically, without human intervention.
This guide covers everything industrial buyers need to know about ATS: how it works, the key types, critical selection parameters, IEC standards, and how to evaluate a manufacturer.
What Is an Automatic Transfer Switch (ATS)?
An Automatic Transfer Switch is an electrical switching device that monitors the primary power source and automatically transfers the load to a secondary (backup) source when the primary fails — and transfers back when the primary is restored. The backup source is typically a diesel generator, UPS system, or a second utility feed.
The core function of an ATS is to ensure continuity of power supply to critical loads. Unlike a manual changeover switch, an ATS operates without operator intervention, making it essential for unmanned facilities and 24/7 operations.
How Does an ATS Work?
A modern intelligent ATS continuously monitors the incoming voltage and frequency of the primary supply. The monitoring circuit checks for:
- Undervoltage (typically below 85% of nominal)
- Overvoltage (typically above 110% of nominal)
- Phase loss or phase imbalance (for 3-phase systems)
- Frequency deviation
When any of these conditions is detected for longer than the set delay time (typically 0.5–3 seconds to avoid nuisance switching), the ATS controller sends a start signal to the generator and simultaneously begins the transfer sequence:
- Detection: Primary supply fault confirmed after delay timer expires
- Generator start: Start signal sent; generator reaches stable voltage and frequency (typically 10–30 seconds)
- Transfer: Load disconnected from primary, connected to generator
- Monitoring: Primary supply continuously monitored for restoration
- Retransfer: When primary is stable for the set retransfer delay (typically 1–5 minutes), load transfers back
- Generator cooldown: Generator runs unloaded for cooldown period before shutdown
Open Transition vs Closed Transition ATS
Open Transition (Break-Before-Make)
The load is briefly disconnected from both sources during transfer — typically for 100–200 milliseconds. This is the standard type used in most industrial and commercial applications. It is simpler, less expensive, and suitable for most loads including motors, lighting, and general equipment.
Limitation: The brief interruption can cause issues for sensitive loads like computers, PLCs, and variable frequency drives (VFDs) that cannot tolerate even a momentary power loss.
Closed Transition (Make-Before-Break)
Both sources are briefly paralleled during transfer — the new source is connected before the old one is disconnected, resulting in a seamless, zero-interruption transfer. This requires both sources to be synchronized in voltage, frequency, and phase angle.
Application: Data centers, hospitals, semiconductor fabs, and any facility with sensitive electronic loads or processes that cannot tolerate interruption.
Key Types of ATS by Pole Configuration
2-Pole ATS (1-Phase Systems)
Used in single-phase applications: residential backup power, small commercial buildings, single-phase industrial equipment. Switches both the live and neutral conductors.
3-Pole ATS (3-Phase, 3-Wire)
Standard for 3-phase industrial systems where the neutral is not switched. Common in motor control centers, industrial machinery, and power distribution panels where the neutral is solidly grounded.
4-Pole ATS (3-Phase, 4-Wire)
Switches all three phases plus the neutral. Required when the backup source has a separately derived neutral (e.g., a generator with its own neutral-ground bond). This is the correct choice for most generator-backed systems to prevent neutral current issues and ensure proper ground fault protection.
Critical Selection Parameters
1. Rated Current (In)
The ATS must be rated for the full load current of the circuit it protects, with a safety margin. Never select an ATS at exactly the calculated load current — allow at least 125% for continuous loads per IEC standards.
Common ratings: 63A, 100A, 160A, 250A, 400A, 630A, 800A, 1000A, 1250A, 1600A, 2000A, 3200A
2. Rated Voltage (Ue)
Must match the system voltage. Standard ratings: 230V (single-phase), 400V (3-phase, IEC), 480V (3-phase, North America).
3. Transfer Time
- Standard ATS: 0.1–3 seconds (adjustable)
- Fast transfer ATS: <100ms
- Static transfer switch (STS): <4ms (for critical loads)
4. Utilization Category (IEC 60947-6-1)
- AC-31A/B: Non-inductive or slightly inductive loads
- AC-33A/B: Motor loads and mixed loads — specify this for most industrial applications
5. Controller Intelligence
- Programmable transfer and retransfer delays
- Voltage and frequency monitoring thresholds (adjustable)
- Generator exercise scheduling
- RS485 / Modbus RTU communication for SCADA integration
- LCD display with real-time voltage, frequency, and status
IEC Standards for ATS
The primary standard is IEC 60947-6-1. Key requirements include dielectric withstand testing, temperature rise limits, short-circuit withstand (Icw), and mechanical/electrical endurance. The Chinese equivalent is GB/T 14048.11. CE marking is required for European projects.
ATS Applications by Industry
Mining & Heavy Industry
Requires IP54+ enclosures, AC-33 utilization category for large motor loads (crushers, conveyors, ventilation fans), and high Icw ratings.
Data Centers & Telecommunications
Requires closed-transition or static transfer switches (<4ms), Modbus/BACnet integration with BMS and UPS systems.
Hospitals & Healthcare
Subject to IEC 60364-7-710. Requires fast transfer, redundant monitoring, and detailed event logging for compliance audits.
Water Treatment & Utilities
Requires remote monitoring capability, IP65 outdoor enclosures, and reliable pump motor load switching.
How to Evaluate an ATS Manufacturer
- Certifications: IEC 60947-6-1 test reports from CNAS/TÜV/SGS, CE declaration, CCC certificate
- Icw rating: Must match or exceed the prospective fault current at the installation point
- Mechanical endurance: Class M1 = 1,000 cycles minimum; Class M2 = 5,000 cycles; quality units test to 10,000+
- Modbus documentation: Full register maps must be available for SCADA integration
- Spare parts availability: Confirm 10-year parts support for motor drive mechanism and controller board
Wanlong Electric ATS — Dual Power Changeover Solution
Wanlong Electric's Automatic Transfer Switch (ATS) is designed for industrial and commercial dual-power applications:
- Rated current: 63A–3200A
- Rated voltage: AC 400V / 690V
- Poles: 2P, 3P, 4P configurations available
- Transfer time: ≤0.1 seconds (mechanical), adjustable delay
- Controller: Intelligent microprocessor with LCD display, RS485/Modbus RTU
- Standards: IEC 60947-6-1, GB/T 14048.11 | Certifications: CE, CCC, ISO 9001
- OEM/ODM available — custom ratings, enclosures, and branding
For technical specifications or a quotation, contact our engineering team. We respond within 24 hours with English-language support.
Related: Low Voltage Switchgear | Air Circuit Breaker (ACB) | Molded Case Circuit Breaker (MCCB)
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