Brochures and Data Sheets

Three Source Priority System
ASCO three-source priority systems are similar to the ASCO three-source systems except they protect two critical
loads rather than one critical load. Upon the loss of the utility power source, the system provides all necessary
controls to start both alternative power sources. The most critical load is transferred to the first alternate power
source that achieves acceptable voltage and frequency. The second most critical load is then transferred to the
second alternate power source when it achieves acceptable voltage and frequency. If the alternate power source
feeding the most critical load should fail, the most critical load would be transferred to the second alternate power
source and the second most critical load would be shed. When the utility power source is restored, the system
controls automatically retransfer the loads to the utility and both alternate power sources are run for a cooldown
period and then shutdown.
Figure 11 shows a three source priority system containing a priority selector transfer switch (ATS-2) and seven
distribution circuit breakers. ATS-1 and ATS-2 are wall mounted automatic transfer switches and are not shown in
this line-up.
Product Features
• Priority selector transfer switch.
• Control logic for selecting between alternate 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 through 4000 amperes utilizing ASCO 940, 962,
434, 436 and ASCO's new 7000 series automatic transfer switches.
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Fig. 12 One line diagram of the three
source priority system shown in
figure 11.
Engine 1 Engine 2
NORMAL
SOURCE
No.1
Critical
Load
No. 2
Critical
Load
ATS - 1
ATS - 3
ATS - 2
ATS = Automatic Transfer Switch
Fig. 11 Three source priority system.