UFC 3 -520-01
June 10, 2002
13-5.2.2.5 Regularly scheduled testing of a system with closed transition transfer will
usually only transfer the load from an energized source to another energized source.
This configuration does not represent the actual system configuration when normal
power is lost. When normal power fails, the transfer switch must transfer the
deenergized load to the energized emergency side, and all equipment must be capable
of handling the subsequent inrush currents. This capability cannot be tested without
actually deenergizing the loads and requiring the emergency system to then pick up the
loads.
13-5.2.2.6 NEC Article 700.6 (2002 Edition) requires that transfer equipment be
designed and installed to prevent the inadvertent interconnection of normal and
emergency sources of supply in any operation of the transfer equipment. For closed
transition transfer, a switch failure could cause both sources to remain connected to the
load.
13-5.2.2.7 The above cautions have been provided because closed transition transfer
requires careful design and still might not be accepted by the local utility if commercial
power is involved. Instead, a UPS system should be used for those loads that require
uninterrupted power. In the case of a UPS, closed transition transfer to and from the
bypass is appropriate because the ac power source to the UPS is the same in each
case.
13-5.2.3 Fast Transfer (Break Before Make). Fast transfer schemes accomplish a
transfer within a few cycles. Typically, an in-phase monitor is used to keep the transfer
within some degree of synchronization. This approach has been used as one method
of transferring motor loads. Under loss of norma l power conditions, even a fast transfer
scheme can cause significant stress on electrical equipment; an in-phase monitor
cannot fully anticipate or compensate for the complex rate of change of frequency. Fast
transfer schemes require careful evaluation.
13-5.3
Design Considerations.
13-5.3.1 Continuous Current Rating.
13-5.3.1.1 An ATS is usually installed upstream of branch circuits because it is
supplying power to a variety of loads. Thus, the ATS must be capable of maintaining
power to continuous loads and withstanding different types of abnormal loading or short
circuit conditions. An ATS is continuously exposed to system full-load current. During
normal operation, current flows through the ATS to supply the operating loads. Upon
loss of power, the ATS transfers to a backup source of power, such as an engine
generator. This load requirement is different than for other backup power supplies that
only have to provide power during the period that normal power is lost.
13-5.3.1.2 The ATS has to do more than provide power 24 hours a day for its expected
service life. During this period, load switching, periodic short circuits, or abnormal
environmental conditions must not degrade its performance below acceptable levels.
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