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.


<< First < Previous Page 1 Page 2 Page 3 Next > Last >>

Top Plant
The Top Plant program honors outstanding manufacturing facilities in North America.
Product of the Year
The Product of the Year program recognizes products newly released in the manufacturing industries.
System Integrator of the Year
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
June 2018
2018 Lubrication Guide, Motor and maintenance management, Control system migration
May 2018
Electrical standards, robots and Lean manufacturing, and how an aluminum packaging plant is helping community growth.
April 2018
2017 Product of the Year winners, retrofitting a press, IMTS and Hannover Messe preview, natural refrigerants, testing steam traps
August 2018
SCADA standardization, capital expenditures, data-driven drilling and execution
June 2018
Machine learning, produced water benefits, programming cavity pumps
April 2018
ROVs, rigs, and the real time; wellsite valve manifolds; AI on a chip; analytics use for pipelines
Spring 2018
Burners for heat-treating furnaces, CHP, dryers, gas humidification, and more
August 2018
Choosing an automation controller, Lean manufacturing
February 2018
Setting internal automation standards

Annual Salary Survey

After two years of economic concerns, manufacturing leaders once again have homed in on the single biggest issue facing their operations:

It's the workers—or more specifically, the lack of workers.

The 2017 Plant Engineering Salary Survey looks at not just what plant managers make, but what they think. As they look across their plants today, plant managers say they don’t have the operational depth to take on the new technologies and new challenges of global manufacturing.

Read more: 2017 Salary Survey

The Maintenance and Reliability Coach's blog
Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
One Voice for Manufacturing
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Maintenance and Reliability Professionals Blog
The Society for Maintenance and Reliability Professionals an organization devoted...
Machine Safety
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
Research Analyst Blog
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Marshall on Maintenance
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
Lachance on CMMS
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
Material Handling
This digital report explains how everything from conveyors and robots to automatic picking systems and digital orders have evolved to keep pace with the speed of change in the supply chain.
Electrical Safety Update
This digital report explains how plant engineers need to take greater care when it comes to electrical safety incidents on the plant floor.
IIoT: Machines, Equipment, & Asset Management
Articles in this digital report highlight technologies that enable Industrial Internet of Things, IIoT-related products and strategies.
Randy Steele
Maintenance Manager; California Oils Corp.
Matthew J. Woo, PE, RCDD, LEED AP BD+C
Associate, Electrical Engineering; Wood Harbinger
Randy Oliver
Control Systems Engineer; Robert Bosch Corp.
Data Centers: Impacts of Climate and Cooling Technology
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
Safety First: Arc Flash 101
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
Critical Power: Hospital Electrical Systems
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
click me