UFC 3 -520-01
June 10, 2002
Figure 13-1. Open Transition Transfer
Utility
Emergency
Initial
Generator
Conditions
Closed
Normal
Load
Utility
Emergency
Condition
Both
Generator
During
Open
Transfer
Load
Utility
Emergency
Condition
Generator
After
Transfer
Closed
Emergency
Load
13-5.2.1.2 Open transition transfer is usually used for transfers from normal commercial
power to an emergency generator, and back. In this case, open transition transfer
avoids issues arising with possibly paralleling, even momentarily, an emergency
generator with commercial power.
13-5.2.2 Closed Transitio n Transfer (Make Before Break).
13-5.2.2.1 Closed transition transfer is a make before break transfer in which the normal
source is momentarily paralleled with the alternate source (refer to Figure 13-2). Closed
transition transfer is often used on UPS s ystems to transfer to and from its bypass, and
it is also a standard feature on high end rotary UPS systems. Closed transition can be
used between an energized normal source and an energized alternate source, including
two emergency generators. Note that closed transition transfer is only possible when
both the normal and alternate sources are available. Upon loss of the normal source,
the transfer switch must operate in open transition transfer mode to the alternate
source.
13-14
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