When utility power fails, a diesel generator must do more than start successfully. The electrical load must also be disconnected safely from the failed mains supply and transferred to generator power. This process is managed by an Automatic Transfer Switch, commonly known as an ATS panel.
A correctly designed ATS system monitors both power sources, starts the generator when required, transfers the load only after generator voltage and frequency become acceptable, and returns the load to utility power after the mains supply stabilizes.
Powerpack Generator supplies project-based diesel generator systems with automatic transfer solutions for commercial buildings, industrial facilities, hospitals, data centers and other standby power applications.
An ATS panel is an electrical switching system that transfers a connected load between two power sources. In a typical standby generator installation, these sources are the utility supply and the diesel generator.
The ATS does not produce electricity. Its purpose is to monitor source availability and connect the load to the correct power source without allowing an unsafe connection between utility and generator power.
A complete ATS panel may include:
The exact operating sequence depends on the controller settings and project requirements, but a standard mains-to-generator system generally follows these steps:
Detection, starting, transfer, return and cooling delays should be configured according to the application. There is no single transfer time suitable for every generator installation.
An ATS panel and an Automatic Mains Failure controller perform related but different functions.
The AMF controller monitors the mains supply, manages the generator starting sequence and provides engine and generator protection. The ATS panel contains the power-switching equipment that physically transfers the electrical load between sources.
In some systems, the ATS controller is installed inside the transfer panel. In others, the generator controller provides the AMF logic and sends commands to contactors, motorized circuit breakers or a dedicated transfer switch.
The control architecture must be coordinated so that source monitoring, generator starting, switching and protection functions operate as one complete system.
A conventional ATS does not provide truly uninterrupted power. Most generator installations use open-transition switching, which disconnects the load from one source before connecting it to the other. A short interruption therefore occurs while the generator starts and the transfer is completed.
Facilities with loads that cannot tolerate this interruption may require a UPS or battery system to support critical equipment during generator starting and transfer.
An ATS should not be confused with a UPS. The ATS selects the available power source, while the UPS provides immediate ride-through energy for compatible critical loads.
Open transition is also called break-before-make transfer. The first source is disconnected before the second source is connected. This prevents the utility and generator from being connected together during normal transfer.
It is the most common arrangement for standard standby diesel generator applications.
Delayed transition includes an intentional period during which the load is disconnected from both sources. This delay can allow residual voltage from motors and transformers to decay before the load is connected to the alternative source.
Closed transition briefly connects both acceptable sources during transfer to reduce interruption. This requires synchronization, additional protection and approval from the utility or relevant authority.
Closed-transition operation should not be specified unless the complete electrical system has been engineered for parallel connection with the mains supply.
A three-pole ATS switches the three phase conductors while the neutral remains continuously connected. A four-pole ATS switches the neutral together with all three phases.
The correct choice depends on the earthing arrangement, neutral bonding, residual-current protection, generator configuration and local electrical regulations.
Four-pole switching may be required where the generator is treated as a separately derived source or where the neutral must be isolated during transfer. However, changing from three-pole to four-pole switching without reviewing the complete grounding design can create protection and safety problems.
Powerpack ATS panels can be configured with three- or four-pole switching according to the approved project design.
For a three-phase generator, the approximate rated current can be calculated using the following formula:
Current (A) = kVA × 1000 / (√3 × Voltage)
For example, the approximate current of a 100 kVA generator at 400 V is:
100 × 1000 / (1.732 × 400) = approximately 144 A
This calculation is only the starting point. The final ATS rating must also consider:
An ATS should not be selected only by choosing the nearest current rating above the calculated generator current. The complete distribution and protection design must be reviewed.
ATS panels can use different switching technologies depending on current rating, application and project requirements.
Mechanically and electrically interlocked contactors may be suitable for certain low-voltage applications. Their utilization category and short-circuit coordination must match the connected load.
Motorized circuit breakers can provide switching and overcurrent protection when correctly selected and coordinated. Interlocking must prevent both source breakers from closing simultaneously unless the system is specifically designed for synchronized operation.
Purpose-built transfer switching equipment combines source switching and mechanical interlocking in a dedicated assembly. The selected equipment should have ratings and certifications appropriate for the installation.
Facilities containing pumps, compressors, elevators, HVAC equipment and other motor loads require additional attention. Motor starting current can be several times higher than normal running current and may cause a significant voltage drop when the load is transferred to the generator.
Large loads may need to be connected in stages rather than transferred simultaneously. Load sequencing, time delays and load shedding can reduce the initial step load and help the generator recover more reliably.
A conventional ATS is normally designed to transfer a load between two available sources. It does not automatically synchronize several generators or share load between them.
Projects using multiple generators require additional synchronizing controllers, generator breakers, load-sharing controls and protection systems. Powerpack Generator provides project-based generator synchronization systems for applications requiring parallel operation, redundancy or expandable power capacity.
An ATS panel should be tested as part of the complete emergency power system. A functional test should confirm mains-failure detection, generator starting, source acceptance, load transfer, utility return and generator cooling operation.
Routine inspections may include:
Manual operation should only be performed according to the manufacturer’s instructions and after the required sources have been safely isolated. ATS servicing must be carried out by qualified electrical personnel.
The following generator models can be evaluated for standby power projects requiring an automatic transfer system. Final selection must be based on the calculated load, starting currents and installation requirements.
Powerpack Generator can prepare generator and automatic transfer solutions according to the required power, voltage, pole configuration, switching method and project operating conditions.
