UFC 3 -520-01
June 10, 2002
controls to start the backup power source, such as an engine generator. When the
from the normal source to the generator. An ATS often includes controls to retransfer
power back to the normal source after it has been restored. Thus, the ATS can
coordinate the complete sequence of operation, including engine starting , transfer of
power to the generator, retransfer back to normal after power is restored, and
subsequent engine shutdown.
13-5.1.2 Considering that the ATS can control the entire process of power transfer to
the backup source and eventual retransfer to the normal source, the ATS is an
important part of a backup power system. If the ATS fails for any reason, backup power
cannot be transferred even if it is available, unless a manual option is included with the
ATS or if a manual bypass is installed in parallel with the ATS.
13-5.1.3 Some of the design criteria provided in this section relate to ATS applications
in which the choice of power is either the utility (commercial power) or an onsite
emergency source. In many ATS applications, the potential for cross-connecting
emergency power to the utility supply will not be a design concern because of the
location of the ATS in the electrical distribution system.
13-5.2
Transfer Methods.
13-5.2.1. Open Transition Transfer (Break Before Make) .
13-5.2.1.1 Open transition transfer is a break before make transfer in which the loads
are momentarily deenergized during the transfer; the original connection to the source is
opened followed by a connection to the alternate source (refer to Figure 13-1). The
length of time that the loads are deenergized can vary from a few milliseconds to
several seconds or more, depending on the transfer switch design and operating logic.
13-13
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