UFC 3 -520-01
June 10, 2002
loads with high inrush currents. UL 1008, Standard for Automatic Transfer Switches ,
requires that an ATS be capable of withstanding inrush currents of 20 times the
continuous current rating. Verify the selected ATS has contacts rated for heavy duty
13-220.127.116.11 An ATS is rated for different types of loads, such as the following UL 1008
Resistive load consisting of heating and other noninductive loads.
Electric discharge lamp load consisting entirely of electric discharge lamps, including
Incandescent lamp load consisting entirely of incandescent lamps.
Total system load consisting of any combination of motors, electric discharge lamps,
resistive heating, and incandescent lamp loads. The incandescent lamp loads
cannot exceed 30 percent of the ATS conti nuous current rating.
13-5.3.5 Simultaneous Closure of Both ATS Power Sources. An ATS must be
designed to prevent the normal source and the alternate source from being
inadvertently connected to the load at the same time. The operating mechanism should
be simple, yet ensure that the two sources are interlocked from ever being
13-5.3.6 ATS Power Source. An ATS should receive its power to transfer from the
source to which it is transferring the loads. For example, upon loss of normal power,
the power to the ATS must come from the emergency power source to which it
transfers. The controls used for ATS operation should ensure reliable switching
operation and not allow the transfers to cross through an off position.
13-5.3.7 Maintainable Design. In order to accomplish periodic maintenance, the ATS
design must be maintainable. Most components should be accessible from the front of
the ATS. Also, the ATS must be in a location that allows interior access. A parallel
bypass feature s hould be provided if power to the load cannot be interrupted or if
outages cannot be easily scheduled.
Static Transfer Switches. Static transfer switches provide digital switching
without the use of electromechanical components. This allows very fast transfer times
compared to conventional ATS designs. Static transfer switches are commonly
provided with UPS systems and are usually provided with an independent certification
based on UL 1008. The ATS design criteria apply also to static transfer switches. In
particular, inrush and overload capability must be confirmed during the design process.