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Increasing Electrical Safety and Ensuring Compliance by Protective Device Coordination

Last updated: Sep 19, 2020

ā€‹ā€‹Overcurrent protection is critical to personal safety and protection from a number of hazardous conditions that can result from materials igniting. Short-circuits can result in arc-flashes, which can cause severe burns and sometimes prove fatal.

In addition, over-heating from the flow of high currents can result in premature equipment failure, like motors and transformers.

HP Motor protected by an LSI Circuit Breaker
Figure 1. A 132-HP Motor protected by an LSI Circuit Breaker (AHU-1-01S FDR)

ā€‹Fuses, circuit breakers, and relays are used to provide protection against over-currents (and hence over-heating) by opening (and hence interrupting) the paths of abnormal current flow.


The Need for Coordination of Protective Devices

ā€‹Electrical power systems serve many loads, of varying importance, in many facilities. Some of those loads, particularly those related to life safety like fire-pumps, elevators, and illuminated signs for panic control are regarded as more important than others like general illumination of buildings. A short circuit in one critical location should not affect the function of another emergency circuit; for example, a short circuit leading to fire in a panic-control illumination sign should not render the fire pump at that location non-functional. Such cases may occur if protective devices are not selectively coordinated with each other.

Figure 2. Outage due to a short circuit fault in a coordinated system vs. an uncoordinated one. Image Courtesy Electrical¬ Engineering Portal

Importance of Over Current Protection and Coordination as dictated by NFPA 70E

ā€‹NFPA 70E has dedicated Article 240 to Overcurrent Protection and has specified in great detail which equipment is required to be protected and how.

NFPA 70E has also described the situations in which Selective Coordination is mandatory, the list of concerned articles is as follows:

  1. 620.62: Elevators, Dumbwaiters, Escalators, Moving Walks, Platform Lifts and Stairway Chairlifts
  2. 645.27: IT Equipment
  3. 695.3 (C)(3): Fire Pumps
  4. 700.32: Emergency Systems
  5. 701.27: Legally Required Standby Systems
  6. 708.54: Critical Operation Power Systems
By requiring Selective Coordination of Protective Devices NEC ensures that outages remain limited to only the minimum number of concerning equipment in critical areas of operation.

A Caveat

Selective Coordination is a term explicitly used in certain NEC articles, as noted above. Selective Coordination requirement is a stricter clause in the code that asks the engineers to coordinate protective devices throughout the range of their operation time, which in some cases can start at 0.01 seconds. Adhering to the code often means sacrificing low clearing times, leading to higher Arc Flash levels. Therefore, a good Protection & Coordination study is a precursor to a good Arc Flash study, leading to increased safety and economic operation of a facility.

Figure 3. Selective Coordination, beyond 0.1 s, between 2 LSI Circuit Breakers

Why AllumiaX?

ā€‹AllumiaX, LLC specializes in conducting Short Circuit, Protection & Coordination, and Arc Flash studies and complies with the global standards set up by OSHA, IEEE, and NFPA. We conduct and provide comprehensive Protection & Coordination study reports, often alongside Arc Flash studies. Moreover, we also provide training on Protection & Coordination, educating the employees on how to set up and maintain equipment.

Not only that, we believe in the ideology "Knowledge for All" and this has led us to partner with GeneralPAC to create a series of webisodes. GeneralPAC is an educational website that aims at helping people learn new concepts and polish existing ones. Visit Training & Education and watch our video series on Power Systems Selective Coordination Study.

AllumiaX, LLC is a licensed Engineering company headquartered in Seattle, Washington USA. We work with sub-contractors to deliver Power System Engineering Studies for industrial customers, hospitals, multi-residential housing buildings, heavy concrete facilities, commercial buildings, power plants, and more.

You can inquire for a quote at any time and get a response promptly within 12 hours. Our team will coordinate with you regarding any queries and haveyour required studies completed as soon as possible.

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