Achieving effective selective coordination design

The concept of full selective coordination has changed the way engineers must think when designing electrical distribution systems.

03/29/2013


Figure 1: Designing electrical systems requires knowledge of how overcurrent protective devices interact throughout the entire distribution system—from the available fault current supplied by the utility to the individual loads within the facility. CourteSelective coordination requires integrating different components, technologies, manufacturers, and standards. There is no standard, cookie-cutter approach that can be applied effectively across system designs. Although selective coordination is about to enter its third National Electrical Code (NEC) cycle as a mandated requirement—not left to engineering judgment since the 2005 cycle—issues continue to circulate about the necessity of mandating it, what constitutes compliance, and how other aspects of power system design might be compromised (see “Selective coordination issues”).

The concept of full selective coordination has changed the way engineers must think when designing electrical distribution systems (see Figure 1). For example, when selectively coordinating emergency and legally required standby power systems, overcurrent protective device specifications must accommodate a range of demands. All overcurrent protective devices must be fully selective with all upstream devices for all levels of overcurrent from all sources. Each overcurrent protective device must remain closed long enough for every device below it to clear for all levels of overcurrent, which include:

  • Soft, low-current sources
  • Stiff, high-current sources
  • Low-impedance (bolted) faults
  • High-impedance (arcing) faults
  • Overloads

Defining selective coordination

The goal of selective coordination is to isolate a faulted circuit while maintaining power to the rest of the electrical distribution system. Although selective coordination will not prevent problems from occurring, it will help retain system reliability by decreasing the potential for a smaller scale problem to become a larger scale problem. Depending on the location, a fault could still cause a large-scale outage.

According to NEC Article 100, selective coordination is the localization of an overcurrent condition to restrict outages to the affected circuit or equipment. It is accomplished by the choice of overcurrent protective devices and their ratings or settings.

The overcurrent condition may be due to an overload, short circuit, or ground fault. In a selectively coordinated system, only the overcurrent protective device protecting that circuit in which a fault occurs opens. Upstream overcurrent protective devices will remain closed. In other words, they do not open, which averts cutting power to the complete panel. 

Low-voltage circuit breakers: When selecting circuit breakers as overcurrent protective devices, tables can help determine proper upstream and downstream circuit breakers. Each manufacturer provides tables only for the overcurrent protective devices it produces (see the online version of this article for an example of a manufacturer’s coordination table). Tables and time-current curves should be used in tandem to meet selective-coordination requirements. 

Emergency, legally required, and critical operations power: Emergency, legally required, and critical operations power systems require selective coordination, except when selectively coordinating a system could create safety hazards such as disconnecting fire pumps.

Selective coordination involves trade-offs between personnel safety due to the threat of arc flash, and maintaining power to critical systems while preventing damage to electrical wiring and equipment. 

NEC requirements: Selective coordination is mandatory for emergency electrical systems for healthcare facilities, emergency systems, legally required standby systems and critical operations power systems. NEC requirements help ensure electrical circuit and system designs that provide reliable power for life safety and critical loads to help protect life, public safety, national security, and business continuity.

Fault types: Types of faults include bolted, arcing, and ground. Bolted faults are rare. A bolted fault occurs when energized conductors are rigidly connected. The maximum available fault current flows until the overcurrent protective device clears the fault, which protects the circuit.

Arcing faults occur when energized conductors come into proximity. While bolted faults and arcing faults are both short circuits, an arcing fault has significantly higher impedance than a bolted fault, resulting in lower current flow. Because the current flowing through an arcing fault is lower than current flowing through a bolted fault, the overcurrent protective device takes a longer amount of time to clear the fault condition. This is why arcing faults can present significant challenges to selective coordination.

According to IEEE, the most common type of fault is a ground fault. A ground fault occurs when one or more electrical phase conductors come in contact with a grounded conductor, as opposed to a phase-to-phase fault. However, the same principles apply: the lower the impedance, the quicker the overcurrent protective device clears the fault. Bolted faults are comparatively simple to selectively coordinate; arcing faults, not so much.

Protecting both personnel and equipment is vital. Every facility needs protection. Often, selective coordination is only one element in the overall protection scheme.


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