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Coordination Studies

Why do I need a protective device coordination study?

Electrical systems commonly use fuses and circuit breakers to protect electrical equipment such as conductors, transformers, motors, and other components. If a failure occurs within this equipment, usually a short circuit results. It would be desirable that this short circuit would affect only that portion of the system where the failure occurs.

In a properly coordinated system the protective devices are selected and adjusted to minimize the impact of equipment failures within a system. A coordination study analyzes the characteristic curves of the fuses and breakers and compares them against one another on log-log plots similar to that shown in the following figure. Any areas of miscoordination will be apparent by overlapping of curves from the various devices.

Most electrical power distribution systems are not planned with protective device coordination in mind. A distribution system can be designed for minimum losses and minimum upfront investment and yet fail in the proper coordination of the protective devices. As a result, equipment failures within the system can easily result in the shutdown of the entire facility. There have been many instances where the failure of a single motor or branch circuit within a building has resulted in the shutdown of most of the facility.

What does a Coordination Study do for me?

A protective device coordination study analyzes the impacts of short circuits and equipment failures within a facility and determines the effects on the facility operation. This study is combined with the  Arc Flash Hazard Analysis, which is required by present electrical safety codes and enforcement agencies. Informed decisions can then be made as to the changes necessary for the system.

Objective of Coordination Studies

The goal of a Protective Device Coordination Study is to achieve an optimum balance between equipment protection and selective isolation that is consistent with the operating requirements of the overall power system.

In addition, the interrupting ratings of all protective equipment, conductors, and switches are checked for adequacy in coordination studies. Inadequate equipment ratings can result in either extensive damage to the equipment during faults and system operation and may introduce hazards to plant operating personnel.

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