Electrical overcurrent protective system Coordination can be complex and daunting. It is best approached with knowledge of the basics and proper system design. Understanding these basics requires determination of key concepts of the types of overcurrent protective devices, the amount of fault current available within the electrical system, the reaction time of the individual fuse or circuit breaker and the type of fault that may occur.


Once the basics are understood, the electrical system design can be accomplished, incorporating the appropriate devices to selectively open and protect the circuit, while still ensuring an orderly shutdown with a minimal hazard to the equipment and personnel.


Article 240 in the 2014 National Electrical Code (NEC) covers electrical system coordination as a part of overcurrent protection general requirements. Section 240.12 states, “Where an orderly shutdown is required to minimize the hazard(s) to personnel and equipment, a system of coordination based on two conditions shall be permitted: (1) coordinated short-circuit protection and (2) overload indication based on monitoring systems or devices.”


Item 1 describes a condition where the overcurrent device that is closest upstream from the fault will open, clearing the fault without opening other devices and causing power loss to other critical loads. Where an overload may occur, Item 2 permits a monitor to provide an alarm indicating the overload and permitting maintenance personnel to respond and clear the overload.


Most electrical designs are focused on clearing a short circuit or a ground fault; in most cases, they treat overload monitoring as less important than the fault condition. However, an overload lasting for a certain time could cause a short circuit or a ground fault. A short circuit occurs between two ungrounded conductors or an ungrounded conductor and a grounded conductor, while a ground-fault current path occurs from an ungrounded conductor to a normally noncurrent-carrying conductor, to the equipment enclosure or to the earth.


The NEC defines selective coordination as the “localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the selection and installation of overcurrent devices and their ratings or settings for the full range of available overcurrents, from overload to the maximum available fault current, and for the full range of overcurrent device opening times associated with those overcurrents.” The definition of “selective coordination” relates perfectly to “electrical system coordination” in 240.12. 


Five locations within the 2014 NEC mention selective coordination. For example, 620.62, which covers elevators, ensures proper operation where drive machines are supplied by a single feeder, the overcurrent protective devices will be selectively coordinated with supply-side overcurrent devices. In such a case, only the overcurrent device immediately ahead of the fault will open.


Selective coordination is extremely important for any critical circuits, such as emergency-power circuits in 700.28, legally required standby circuits in 701.27, critical operation power system circuits in 708.54, and the life safety branch in hospitals in accordance with 517.26. The critical branch provides power for task illumination and selected receptacles in critical-care areas, acute nursing areas, coronary care areas, emergency rooms, intensive care areas, as well as similar locations within the hospital in accordance with 517.33.


The life safety branch in a hospital provides power for illumination for means of egress; exit signs; alarm and alerting systems; communications systems; generator set battery chargers and other generator accessories; elevator cab lighting, control, communications and signaling; and automatically operated doors for building egress. Providing hospital egress lighting and similar critical power is necessary, and, ultimately, the same must apply to hospital critical power circuits.


To accomplish system coordination for the critical branch in a hospital facility, Section 517.30(G) requires overcurrent protective devices, serving the essential electrical system, to be coordinated for the period of time that the fault duration extends beyond 0.1 second. Since the critical branch and the life safety branch are both part of the essential electrical system, and Section 517.26 requires the life safety branch to comply with Article 700’s requirements—except where amended by Article 517—both the critical branch and the life safety branch are not required to comply with 700.28 for total selective coordination.


In this case, selective coordination for hospital systems applies only to a fault that extends longer than 0.1 second, not as close to zero seconds as possible as required for other critical power systems within the NEC. The NFPA 99 Electrical Committee has determined the level of selective coordination for hospital life-safety and critical branch-overcurrent devices.