Electrical Studies / Analysis

Protective Device Coordination Study

Protective Device Coordination studies are required to ensure the safety including personnel and general public and to ensure that equipment is not damaged under fault conditions per the requirements of NEC 110-10.

Analysis of Time Current Characteristics (TCC) data for the protective devices and equipment (such as transformers and cable) is used to permit the circuit-protective devices to clear a fault without extensive damage to the electrical components of the circuit.

There is a second type of coordination, known as Selective Coordination which uses the TCC curves to ensure that faults (short circuits) affect the least possible portion of the electrical distribution system. Improper selective coordination or no selective coordination can result in faults on branch circuits shutting down complete operations for parts or possibly a whole building, manufacturing facilities. Such undesirable events result in loss of production, loss of business income, inconvenience to customers and risk of injury. These losses can be minimized or completely eliminated by proper Selective Coordination, the second benefit of performing a coordination study.

objectives

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. The objective of any protection scheme in a power system is to minimize hazards to personnel and equipment while causing the least disruption of power service. The purpose of the Protective Device Coordination Study is to select protective device settings which provide equipment and personnel protection while assuring that the minimum portion of the power system is affected by a fault. This permits and maintains continuity of service to as large a portion of the system as possible.

Correct setting values can only be determined by a study based on all the pertinent electrical characteristics of the equipment in the system.

typical protective device coordination study

A typical Protective Device Coordination Study will consist of the following:

• • Protective device coordination / evaluation

• Description, purpose, basis and scope of the study plus a simplified single-line diagram of the portion of
the power system which is included within the scope of study.
• Tabulations of circuit breaker, fuse and other equipment ratings versus calculated short-circuit duties and commentary regarding same.
• Protective device time versus current coordination curves, tabulations of relay and circuit breaker trip settings, fuse selection and commentary regarding same.
• Fault-current tabulations including a definition of terms and a guide for interpretation.
• Tabulation of appropriate tap settings for relay seal-in units.
• Relay Settings.
• Determine the correct protective device settings so that your system will be 100% protected

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