Brochures and Data Sheets

Three Source
Automatic Transfer System
ASCO three-source systems are similar to ASCO two-
source systems except that a second alternate power
source is added to back up the first if that power source
fails. Upon the loss of the utility power source, the
system provides all necessary controls to start both
alternate power sources. The critical loads are
automatically transferred to the first alternate power
source that achieves acceptable voltage and frequency.
The second alternate power source is then automatical-
ly shutdown after a time delay and cooldown period. If
the first alternate power source fails, the second
alternative power source will be automatically
re-started and the load will be transferred from the first
alternative power source to the second alternative power
source. When the normal power is restored, the
controls automatically retransfer the load to the utility
power source.
Figure 9 shows a three source automatic transfer system
with two 940 automatic transfer switches and three
distribution circuit breakers for feeding critical loads.
Product Features
• Control logic for selecting between the alternative
power sources.
• Normal, emergency, and/or load circuit breakers.
• Protective relaying (as required).
• Optional instrument meters for normal, emergency, or
load for local monitoring.
• Designs are based on NEMA and UL 891 standards.
• Optional engine start control logic and/or load control
logic.
• Designs available up to 4000 amperes utilizing ASCO
940, 962, 434, 436 and ASCO's new 7000 series
automatic transfer switches.
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ATS = Automatic Transfer Switch
Fig. 10 One line diagram of the three
source automatic transfer
system shown in figure 9.
Engine 1
Engine 2
ATS1
ATS2
NORMAL
SOURCE
Load
Circuit
Breakers
Fig. 9 Three source automatic transfer system.