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What is short-circuit?

A short circuit is an electrical circuit that allows a current to travel along an unintended path with no or a very low electrical impedance. Short circuit is normally caused by faulty insulation, loose connection or unintentionally wrong connection.

As a consequence, short circuit incidents are dangerous events in which it is harmful to people safety and equipment operation. Therefore, electrical systems commonly utilize fuses or circuit breakers to protect electrical equipment such as conductors, transformers, motors and other related components. However, a large percentage of electrical power distribution systems are designed with insufficient protective device coordination. The effectiveness of the protection system is always in doubt without proper selection and coordination of protective devices.

Objectives of Protective Coordination Study

Protective coordination study is the process of selecting and arranging protective devices to limit the effects of an overcurrent (short-circuit) situation to the smallest area. This study can be used to determine fuse sizes, settings for breakers and relays, or as an aid to selection of new equipment. Besides, the study helps to determine the fault clearing times and coordination of upstream electrical protective devices. Proper coordination and fault clearing times can help reduce damage to electrical equipment and protect people from hazards.

Requirement of Protective Coordinate Study

Protective coordination study is 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.

“The objectives of electrical system protection and coordination are to prevent injury to personnel, to minimize damage to the system components and to limit the extent and duration of service interruption whenever equipment failure, human error or adverse natural events occur in any portion of the system.” -IEEE/ANSI std. 242.

Short-circuit and protective coordination study

In order to perform short circuit and protective coordination study, it is necessary to be able to establish both phase to phase and phase to earth faults at any point in the electrical distribution system. In order to do this, Nhat Ha Electric use Etap software.

Etap software allows us to build a mimic model of entire distribution system from different electrical equipment and load characteristics to various protective devices. The short circuit current and voltage throughout the network during fault condition can be calculated quickly and effectively with Etap simulation capabilities. The protective functions are selected from comprehensive library of different manufacturer. The coordination among different devices will be analyzed by our experienced engineer to develop table of protective device rating and setting to minimize impact of the fault.

Finally, a triggering sequence of protective device will be simulated to ensure expected result of the coordination will happen in an actual incident.

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