Selectively coordinated overcurrent protection for power systems

When designing a safe and reliable power distribution system, it is imperative to consider life safety and equipment protection.


Learning Objectives

  • Know the industry best practices for electrical circuits overcurrent protection and selective coordination of protective devices.
  • Understand how these best practices can be applied on projects to minimize power disruption.
  • Understand overcurrent-based protection of electrical systems and how to achieve selective coordination.

Even the most robustly designed and well-maintained electrical systems can experience faults. Faults can be the result of natural events including lightning; environmental factors, such as aging and deterioration of electrical equipment; or human error, like the shorting of bus bars with a metallic tool. If left unchecked, faults in the electrical system can lead to equipment damage, arc blasts, and building fires. This is due to the mechanical forces and thermal energy that result from the huge fault currents that flow in the system. Protection, therefore, is vital for the electrical power system and its components.

The most basic function of a protection system is to recognize abnormal or faulty circuit operation and to then remove the faulted circuit from the electrical system to minimize damage to equipment and safeguard personnel and property. Protection can be defined based on the electrical system component being protected-generator protection, transformer protection, transmission-line protection, bus protection, feeder protection, and motor protection (which is the most common type).

An alternative way to define protection is based on the principle used in the protection scheme. This includes overcurrent protection, differential protection, distance protection, over/under voltage protection, and over/under frequency protection.

The following discussion will be limited to coordinated and selectively coordinated overcurrent protection-the most common form of circuit protection. Coordinated protective devices provide an optimal balance between fault localization and circuit protection based on the responsible engineer's judgment. Selectively coordinated protection is required for a few select power systems, such as emergency systems, critical operations power systems, and fire pumps. The objective of selective coordination is to ensure coordination in the full range; there is no engineering judgment as to what level of coordination is acceptable.

Key electrical codes, standards, and definitions

There are multiple electrical codes and standards that apply when designing and constructing projects, and they all require diligent consideration to ensure life safety, reliable power, and equipment protection. NFPA 70: National Electrical Code (NEC), is the industrywide standard in the United States regarding safely installing electrical wiring and equipment. It is typically adopted by state and local agencies to standardize the enforcement of safe electrical practices in their jurisdictions. An overview of key terms from 2017 Edition of Figure 1: An electromechanical overcurrent relay installed in a switchgear at a federal facility. All graphics courtesy: CDM Smith NEC Article 100 includes the following:

  • Overcurrent: Any current in excess of the rated current of equipment or the ampacity of the conductor. This may result from overload, short circuit, or ground fault.
  • Overload: The operation of equipment in excess of the normal, full-load rating or in excess of rated ampacity (conductors) when it persists for a sufficient length of time to cause overheating. A fault, such as a short circuit or ground fault, is not considered overload.
  • Short-circuit current: An overcurrent resulting from a fault of negligible impedance between live conductors having a difference of potential under normal operating conditions.
  • Ground fault: An unintentional electrically conducting connection between an ungrounded conductor of an electrical circuit and the normally non-current-carrying conductors, metallic enclosures, metallic raceways, metallic equipment, or earth.
  • Selective coordination: Refers to the localization of an overcurrent condition to restrict outages to the circuit or equipment affected. This is accomplished by the selection and installation of overcurrent protective devices (OCPDs) with 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 OCPD opening times associated with those overcurrents.

NEC's selective coordination requirements specifically define selective coordination in Article 100 and mandate the proper selection and coordination in Article 110.10. Selective coordination shall be selected by a licensed professional engineer or another qualified person primarily in the design, installation, or maintenance of electrical systems. Selective coordination is not typically required between overcurrent devices connected in series if no loads are connected in parallel with downstream devices. Additional key selective coordination requirements defined in NEC include:

  • Article 240.12 (Electrical System Coordination) defines electrical system coordination based on two conditions: coordinated short-circuit protection and overload indication based on monitoring systems.
  • Article 517.17 (Ground-Fault Protection) requires that the ground-fault protection of the feeder and disconnecting means to be fully selective for hospitals and other buildings with life-support equipment, and for buildings that provide essential services for the operation of critical care equipment. Also, article 517.17 (G) states that essential electrical systems are required to be coordinated for the period that a fault's duration extends beyond 0.1 second.
  • Article 620.62 (Selective Coordination) requires selective coordination for elevators in the cases where more than one driving machine's disconnecting means is supplied by a single feeder.
  • Article 645.27 (Selective Coordination) requires that the overcurrent devices associated with the critical operation data systems to be selectively coordinated with all supply-side OCPDs.
  • Article 695.3 (C)(3) (Multibuilding Campus-Style Complexes [Fire Pumps]) requires selectivity between each disconnecting means and all supply-side OCPDs.
  • Article 700.32 (Emergency Systems Selective Coordination) requires that the emergency system overcurrent devices be selectively coordinated with all supply-side OCPDs.
  • Article 701.27 (Selective Coordination) requires that the standby system overcurrent devices be selectively coordinated with all supply-side OCPDs for Legally Required Standby Systems.
  • Article 708.54 (Selective Coordination) has similar requirements as shown above in Article 701.27 but for Critical Operations Power Systems [COPS].

The 2018 edition of NFPA 99: Health Care Facilities Code also includes requirements regarding selective coordination in Article It states coordination is required for essential systems when the fault duration exceeds 0.1 second. The coordination will be revised every 3 years (if changes are made to the power system).

Figure 2: Power fuses installed inside a switchboard in a federal facility.Additionally, there are professional association standards related to circuit protection that require consideration, such as the Institute of Electrical and Electronics Engineers (IEEE) and the National Electrical Manufacturers Association (NEMA). Key standards and practices include:

  • IEEE 141: Recommended Practice for Electric Power Distribution for Industrial Plants (former IEEE Red Book), which provides best practices for electrical power distribution design of industrial facilities (manufacturing, industrial production, research and development).
  • IEEE 242: Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems (former IEEE Buff Book), which provides techniques involved in the protection and coordination of industrial and commercial power systems.
  • IEEE 399: Recommended Practice for Industrial and Commercial Power Systems Analysis, which provides techniques involved in the power system studies prior to designing a new system or expanding an existing power system.
  • IEEE 551: Recommended Practice for Calculating AC Short-Circuit Currents in Industrial and Commercial Power Systems, which provides short-circuit current information including calculated short-circuit current duties for the application in industrial plants and commercial buildings, at all power system voltages.
  • IEEE 1015: Recommended Practice for Applying Low-Voltage Circuit Breakers Used in Industrial and Commercial Power Systems (former IEEE Blue Book), which provides information for selecting proper circuit breakers including selective coordination.
  • NEMA ABP 1: Selective Coordination of Low-Voltage Circuit Breakers, which provides guidance to engineers regarding NEC requirements for selective coordination.

The formerly IEEE Color Book set that includes 13 standards is now part of the IEEE 3000 Standards Collection for Industrial & Commercial Power Systems. This collection is organized into approximately 70 IEEE "dot" standards that cover specific technical topics. 

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