IB140042EN For more information visit: www.eaton.com
Instructional Booklet
Effective: May 2015 Page 3
ATC-300/900 Breaker 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 con-
nected to both the Source 1 and Source 2 power sources and sup-
plies 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 can be
automatic or manual, depending upon the type of transfer switch
equipment being used. Once Source 1 power is restored, the load
is automatically or manually transferred back to the Source 1
power source, again depending upon the type of transfer equip-
ment being used (Figure 2).
Figure 2. Typical Load Transfer Switch (Circuit Breaker Type)
Schematic.
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 gener-
ator, the ATS initiates generator startup and transfers to the
Source 2 power source when sufficient generator voltage is avail-
able. 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. Main contacts to connect and disconnect the load to and from
the power source.
2. A mechanism to transfer the main contacts from source to
source.
3. Intelligence/supervisory circuits to constantly monitor the con-
dition of the power sources and thus provide the intelligence
necessary for the switch and related circuit operation.
1.2.1 Design Configuration
The Eaton ATS is a rugged, compact design that uses molded
case switches and/or circuit breakers to transfer essential loads
from one power source to another (Figures 3 [225-1200 A] and 4
[30-150 A]). Molded case switches are mechanically and electri-
cally interlocked to prevent both switching devices from being
closed at the same time.
Figure 3. Typical Power Panel for 225-1200 A Models (Deadfront
Covers Removed).
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