Backup power promotes operational continuity by mitigating the risks of power issues such as utility outages or faults. One approach to protecting commercial buildings from power interruptions is practicing selective coordination to localize overcurrent conditions. Specifying transfer switches properly rated to withstand fault and short circuit currents helps make selective coordination possible. This article lists considerations when selecting transfer switches for selectively coordinated systems.
Selective Coordination is the practice of setting overcurrent protection device trip times to minimize the amount of equipment deenergized when faults are cleared from electrical circuits. Where the same trip times are applied to multiple circuit levels, the opening of a single breaker to clear a fault current could depower an unnecessarily large amount of load equipment, reducing the impact to facility operations. Additional information is available in the ASCO Power Technologies Technical Brief entitled Selective Coordination Basics.
Figure 1 compares systems without and with a selective coordination strategy. The diagram at left shows an uncoordinated system where breakers at each level use the trip time. The diagram at right shows a selectively coordinated system that tiers trip times, causing only the breaker closest to a fault to open. This clears the fault while depowering the least amount of load equipment.