Emerson Network Power Publishes a White Paper on Selective Coordination

The paper addresses code requirements and the importance of selective coordination to a power distribution system. Ultimately, the white paper explains how to determine and achieve effective selective coordination and properly describes the roles of power transfer switches and specifying cycle times. Also watch a video on selective coordination.

Design Engineers need to balance the importance of business continuity, equipment protection, personnel safety and cost management rather than assume 30-cycle transfer switches are the best solution for every selective coordination application.

Typically, a 3- or 18-cycle transfer switch will perform just as well as the 30-cycle switch at a fraction of the cost. A survey of engineers showed more than 88 percent of respondents believe that effective selective coordination requires the proper cycle time, not a cookie cutter approach for all applications.

Specifying Cycle Times
The approach used by Bard, Rao + Athanas Consulting Engineers for specifying transfer switch cycle times recognizes that projects are custom designed and built because one size doesn’t necessarily fit all applications..

The majority of transfer switches that BR+A engineers specify are 3-cycle switches. They have specified 30-cycle switches where one or more of the following conditions exist:

1. Larger projects, where the fault current on the emergency system is high.
2. Where the instantaneous is defeated in order to selectively coordinate (note that this may change with the acceptance of the new NFPA 99).
3. Where transfer switches are served by ANSI Switchgear, which also has 30-cycle withstand ratings.

“Selective coordination requirements for emergency and legally required standby systems are challenging electrical engineers to consider how systems respond to short circuits,” Jim Degnan, principal of Sparling, a consulting engineering firm based in Seattle, said.