Introduction to Short Circuit Analysis

A. Bhatia, B.E.

Course Outline

The protection for an electrical system should not only be safe under all service conditions but, to insure continuity of service, it should be selectively coordinated as well. Even the most sophisticated and well-designed facilities have experienced the effects of electrical system failure or misoperation. Since unplanned outages can cost millions of dollars in lost production, information, and customers, it pays to explore how outages typically occur and to better understand how you can prevent them.

The purpose of this course 4-hour course is to gain a basic understanding of short circuit capacity.

The course includes a glossary and multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

Learning Objective

• What is short circuit and what scenarios can lead to short circuits?
• Why short circuit is dangerous and what are potential hazards of short circuits?
• Why do you need short circuit study and what codes mandate the short circuit analysis?
• What is bolted, arcing and ground faults?
• What is co-ordination of protective devices?
• What areas are critical for short circuit calculations?
• What is symmetrical and asymmetrical current in short circuit analysis?
• What is interrupt and withstand rating of protective devices?
• What is the relationship between the power factor and X/R ratio?
• What is making and breaking capacity of overcurrent protective devices?
• What is impedance method for calculating the short circuit currents?
• How to perform detailed short circuit calculations?

Intended Audience

This course is aimed at students, electrical engineers, facility managers, architects, H &S professionals, O&M personnel, energy auditors, technical and sales representatives.

Course Introduction

A short circuit is an accidental path of low resistance which passes an abnormally high amount of current. At an instant short circuit occurs, a very low impedance path is created through which the full system voltage is applied. The resultant current will be many times greater than the normal circuit current and if the circuit is not opened and the current interrupted, extensive damage can occur. To protect from adverse affects of short circuits, it is important to estimate or calculate the value of prospective current likely to occur under short circuit conditions and ensure that the protective devices provided to interrupt that current are rated to withstand and interrupt it.

This course discusses some of these aspects followed by an example on how to calculate short circuit currents.

Course Content

The course content is in a PDF file Introduction to Short Circuit Analysis. You need to open or download this document to study this course.

Course Summary

A short circuit results in excessive current flow in the power source through the 'short,' and may even cause the power source to be destroyed. In electrical devices, unintentional short circuits are usually caused when a wire's insulation breaks down, or when another conducting material (such as water) is introduced, allowing charge to flow along a different path than the one intended. A short circuit may be in a direct- or alternating-current (DC or AC) circuit.

Damage from short circuits can be reduced or prevented by employing fuses, circuit breakers, or other overload protection, which disconnect the power in reaction to excessive current. If a fuse is in the supply circuit, it will do its job and blow out, thus opening the circuit and stopping the current flow. If a circuit breaker is in the supply circuit, it will trip and open the circuit.

Overload protection must be chosen according to the maximum prospective short circuit current in a circuit. For example, large home appliances (such as clothes dryers) typically draw 10 to 20 amperes, so it is common for them to be protected by 20 - 30 ampere circuit breakers, whereas lighting circuits typically draw less than 10 amperes and are protected by 10 - 15 ampere breakers. Wire sizes are specified in building and electrical codes, and must be carefully chosen for their specific application to ensure safe operation in conjunction with the overload protection.

Electrical switchgear may be rated on a symmetrical current basis or an asymmetrical basis. It may be necessary therefore to determine other short circuit values such as the "average asymmetrical value", the "maximum RMS asymmetrical value" or even the "peak value" of the short circuit current.

Short circuit analysis of facility electrical systems should be performed on 3-phase bolted condition since this will cause highest fault currents. Periodically, the short- circuit analysis should be reviewed to ensure that modifications to the facility electrical system and changes in equipment over time do not result in potential fault currents which exceed device interrupting ratings. Literature containing a detailed discussion of short circuit capacity calculations is available within the electrical power transmission industry. Considering the fact that the cost of a short circuit and coordination study is typically a small fraction of the electrical system cost, it's a wise investment that can pay dividends in the form of increased safety and availability.

Quiz

Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.