Automatic Transfer Switch (ATS)
Concepts & Applications
BuildingsOne | March 8, 2016
The purpose of an Automatic Transfer Switch (ATS) is to
transfer electrical power between two sources, typically a
building’s primary utility source and secondary backup
power source. The ATS is also utilized in other electricity
switching applications including “utility to utility” (two
independent public utility power sources), “generator to
generator” (two or more standby electrical power sources),
and “three-source systems” (two independent utility power
sources + a standby electrical power source).
The typical commercial building’s ATS configuration transfers power between a primary electric utility
source and a secondary standby-emergency generator electrical source (ATS Illustration - Page 2 & 3).
In this configuration, primary electricity flows through the “normal power” feed within the ATS. Upon a
loss of primary (normal) power, the ATS sends a “start signal” to the standby generator. After the
standby generator starts, and is providing a stable electrical power source (estimated 15-30 seconds),
the ATS transfers the electrical load from the primary power source to the standby generator.
With the ATS arrangement outlined above, electrical power can be distributed, to critical systems and
equipment, e.g., life safety and fire systems, during a loss of power. This standard ATS configuration
results in a short delay, in electric power availability, until the emergency generator auto-starts and the
ATS switch transfers power through the “emergency generator feed”. There are many variations of ATS
electrical power applications, particularly in “business critical environments”, such as data centers and
critical health care facilities. In these types of environments, the use of Uninterruptable Power Systems
(UPS) and “make before break” ATS configurations ensures continuous electrical power, to critical
systems and equipment, at all times.
Automatic Transfer Switch (ATS)
Automatic Transfer Switch (ATS) - Configuration & Operation
Secondary Standby Power
Line Side
Voltage Sensing Line
Signal to Start Generator
Primary Utility Power
Automatic Transfer
Switch
(ATS)
Primary Utility
Operation
Load Side
Critical Systems & Equipment
 Elevators
 Emergency
Lighting
 Fire Panel & Fire Alarm System
 Emergency Fire Pump
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Automatic Transfer Switch (ATS)
Automatic Transfer Switch (ATS) - Configuration & Operation
Secondary Standby Power
Line Side
Voltage Sensing Line
Signal to Start Generator
Primary Utility Power
Automatic Transfer
Switch
(ATS)
Standby Generator
Operation
Load Side
Critical Systems & Equipment
 Elevators
 Emergency
Lighting
 Fire Panel & Fire Alarm System
 Emergency Fire Pump
3
Automatic Transfer Switch (ATS)
Modern ATS systems incorporate enhanced inspection and monitoring diagnostic capabilities such
as LED panel indicators and web-based networking options. These enhancements facilitate
improved operator monitoring, inspection and early warning component failure. In addition to the
building operator’s monitoring and visual inspections, an effective service and preventive
maintenance (PM) program would include periodic testing and inspection of the following elements:
1. Periodic testing of the ATS “switching” operation
2. Periodic infra-red or ultrasound testing as part of the building’s non-destructive electrical system
preventive maintenance program
3. Periodic inspection of all ATS internal components and controls, in particular the “setting and
testing” of the line side voltage sensing device
4. Inventory of critical spare components subject to failure, in particular fuses for ATS’s having fuse
protection for over current and short-circuit conditions.
INSPECTION | TESTING | SERVICE
With advancements in technology,
additional ATS service options include
maintenance and testing of the transfer
switch without disrupting power to
critical electric loads. With certain
ATS models, bypass features are
available in order to maintain service,
to critical electric loads, when the ATS
is isolated due to maintenance or
component failure.
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