IB140036EN
For more information visit: www.eaton.com
Instruction Booklet
Effective: October 2014
Page
3
O&M Manual for 40-1200A (480/600 Vac) ATC-900 3-Position,
Open/Closed Transition Contactor Based Transfer Switch
1.2 General Information
Transfer switches are used to protect critical electrical loads
against loss of power. The load’s Source 1 power source is
backed up by a Source 2 power source. A transfer switch is
connected to both the Source 1 and Source 2 power sources and
supplies the load with power from one of the two sources. In the
event that power is lost from Source 1, the transfer switch trans-
fers the load to the Source 2 power source. This transfer is auto-
matic. Once Source 1 power is restored, the load is
automatically transferred back to the Source 1 power source
(Figure 2).
S
ource 1
S
ource 2
Lo
a
d
Figure 2. Typical Load Transfer Switch Schematic (Contactor
Type).
In ATS equipment, the switch’s intelligence system initiates the
transfer when the Source 1 power fails, falls below, or rises
above a preset voltage. If the Source 2 power source is a
standby generator, the ATS initiates generator startup and trans-
fers to the Source 2 power source when sufficient generator
voltage is available. When Source 1 power is restored, the ATS
automatically transfers back and initiates generator shutdown.
In the event the Source 1 power source fails and the Source 2
power source does not appear, the ATS remains connected to
the Source 1 power source until the Source 2 power source does
appear. Conversely, if connected to the Source 2 power source
and the Source 2 power source fails while the Source 1 power
source is still unavailable, the ATS remains connected to the
Source 2 power source.
ATSs automatically perform the transfer function and include
three basic elements:
1. A power contactor to connect and disconnect the load to
and from either power source.
2. Solenoids to make the transfer of the main contacts from
source to source.
3. Intelligence/supervisory circuits to constantly monitor the
condition of the power sources and thus provide the intelli-
gence necessary for the switch and related circuit operation.
1.2.1 Design Configuration
The Eaton contactor based ATS is a compact design that uses a
power contactor to transfer essential loads from one power
source to another (Figure 3
[
1200A
])
.
LOGIC
PANEL
POWER
PANEL
VOLTAGE SELECTION
& TRANSFORMER PANEL
Figure 3. Typical 1200A Open Transition ATS.
The Eaton contactor based ATS is designed with easy installation
and simplified maintenance in mind. Three main panels comprise
the contactor based ATS design:
1. Power panel;
2. Voltage selection and transformer panel; and
3. Microprocessor-based logic panel.
