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Selective Coordination

 
Selective Coordination is an approach for minimizing equipment outages by ensuring that Over-Current Protection (OCP) devices nearest to faults trip before upstream devices. Although selective coordination is achieved using fuses and circuit breakers, ATSs placed in these systems must support the selective coordination strategy. This brief summarizes the issue.

Uncoordinated Power Distribution Systems

If all the OCP devices in a power distribution system were set to trip at the same time, the amount of equipment impacted would depend on which device opened first. In the following figure, all devices trip in 0.1 seconds. Because a protector opened in a secondary circuit, loads supplied by multiple downstream circuit panels were deenergized, disrupting the operation of all associated loads.
 
Coordinated Power Distribution Systems

To limit the impact of faults, OCP with longer trip times can be used upstream and OCP with shorter times can be used downstream. This assures that the breaker closest to the fault trips first to minimize the number of impacted loads, as shown in the following sketch.
 
In systems with Automatic Transfer Switches (ATSs), it’s important to remember two things:
  1. The primary function of an OCP device is to disconnect power sources from downstream circuits and loads to avoid unsafe conditions.
  2. The primary function of an ATS is to ensure loads are always connected to power to maximize availability.
Consequently, designers can minimize the extent of disrupted operations by ensuring that transfer switches will hold overcurrents for a longer time than OCP devices. National Electrical Code® provisions regarding selective coordination include those identified in the following table.
 
Selecting Appropriate Transfer Switches

In order to select the most appropriate ATS, one needs to know (1) the nominal voltage and amperage of the load, (2) the maximum fault current that will be available to the ATS location, and (3) the duration for which the ATS must hold the current. By comparing that information to ATS Withstand and Close-On Ratings and Short Time Ratings, an appropriate model can be selected. The WCR Ratings and Short Time Ratings of the ATS must equal or exceed the three referenced values for the application. The following table shows values for select ASCO transfer switches.
 

Summary

Selective coordination is accomplished by using OCP devices with longer trip times near the power source and shorter times near load equipment. Automatic transfer switches are not used to selectively coordinate power protection. Instead, transfer switches must support selective coordination schemes by closing on and holding greater fault currents than nearby OCP devices and must remain closed until OCP devices clear the circuit. For more detailed information review the following resources:

ASCO White Paper: Selective Coordination Basics

ASCO White Paper: UL1008 Transfer Switch Withstand and Closing Ratings

ASCO Publication 1128 R26: Engineering Application Information: Withstand and Closing Ratings For Transfer Switch Equipment

National Fire Protection Association: NFPA 70 - National Electrical Code