Critical Power: Selective Coordination
Selective coordination is a means of localizing an overcurrent condition to restrict electrical outages to the affected equipment, circuit, or feeder. In a properly coordinated system, a fault induces operation of the nearest device on the line side of the fault and limits the outage to only the faulted portion of the system.
Selective coordination is a means of localizing an overcurrent condition to restrict electrical outages to the affected equipment, circuit, or feeder. In a properly coordinated system, a fault induces operation of the nearest device on the line side of the fault and limits the outage to only the faulted portion of the system. If overcurrent devices are not selectively coordinated, the fault has the potential to impact one or several devices upstream resulting in a much larger scale outage than necessary for system protection. In extreme cases faults can open the main overcurrent protective device and cause an outage for the entire facility.
Both emergency and legally required systems are required to be selectively coordinated to ensure only the closest upstream device from an electrical fault operates. Optional standby loads should be selectively coordinated to ensure faults do not result in business lost revenue. Selective coordination requires analysis of time-current curves of overcurrent protective devices—both fuses, circuit breakers, and relays. For circuit breakers both fixed and adjustable parameters must be clearly understood to ensure selective coordination can be achieved.
Ground fault protection settings must also be scrutinized as they are a very typical source of nuisance trips often at the building service level. Some documentation, specifically with British Standards, refers to selective coordination as "discrimination."
- The audience will understand the applicable codes and standards: NFPA 70: National Electrical Code (NEC); and NFPA 110: Standard for Emergency and Standby Power Systems.
- Viewers will learn which types are mandated to have selectively coordinated electrical systems.
- The audience will learn how to design selectively coordinated electrical systems using circuit breakers, fuses, relays, and/or a combination thereof.
- Viewers will understand the differences between partial and full coordination and why the distinction is important.
Rick Reyburn, PE, Director of Electrical, jba consulting engineers, Las Vegas, Nev.
Syed Peeran, PE, PhD, Senior Engineer, CDM Smith, Cambridge, Mass.
Moderator: Jack Smith, Consulting-Specifying Engineer, Pure Power, and CFE Media, LLC