What is Short Circuit Study?
Short circuit is an accidental or intentional conductive path between two or more conductive parts forcing the electric potential differences between these parts to be equal or close to zero. Definition of short circuit study is an examination or analysis of an electrical system to determine the magnitude of currents that can flow during an electrical fault then compare those values to the ratings of installed equipment and setting of installed short circuit protection devices. Short-circuit studies are related to in-depth analysis of short circuits, taken into consideration when making new designs, analytical activities for operational validation of existing power systems. Short-circuit fault and protection study is very important in power systems. Without short-circuit simulation, we cannot conduct protection coordination and the arc flash study and others. – Omazaki Engineering is a consultant serving short circuit fault study and analysis using software such as ETAP and DigSilent, etc. for commercial and industrial facilities. If you are looking for an electric short circuit fault analysis and study consultants for your project in Indonesia and South East Asia, please contact us by sending an email to cs@omazaki.co.id or by filling in the form in contact.
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Basic Knowledge
Definition of Short Circuit
Definition of short circuit its self, according to IEC 60909 standard, is accidental or intentional conductive path between two or more conductive parts (e.g. three- phase short circuit) forcing the electric potential differences between these conductive parts to be equal or close to zero.
A short circuit is a fault. A short circuit is an abnormal connection between two nodes of an electric circuit intended to be at different voltages. This results in an excessive electric current.
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General Causes of Short Circuits
Here are some general causes of short-circuit fault:
- Direct contact connection with live conductors
- Excess temperature due to over-current or overload
- Faulty wire insulation ‒ Wiring insulation could deteriorate and wear off over time, exposing the metal leads and copper wires
- Poor appliance wiring ‒ Short circuit could occur in outlet plugs, power cords, and the internal circuit board of appliances and devices
- Loose electrical connections ‒ Electrical fixtures loosen up over time, which increases the likelihood of metal components to come in contact with each other
- The release of destructive electrons due to excess voltage
- Arc due to condensation together with air, especially in insulators
- etc.
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Dangerous of Short-Circuit
Within milliseconds, a short circuit fault current can be thousands of times larger than the normal operating current of the systems.
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. Overload protection must be chosen according to the current rating of the circuit. Circuits for large home appliances require protective devices set or rated for higher currents than lighting circuits. Wire gauges specified in building and electrical codes are chosen to ensure safe operation in conjunction with the overload protection. An overcurrent protection device must be rated to safely interrupt the maximum prospective short-circuit current.
In an improper installation, the overcurrent from a short circuit may cause ohmic heating of the circuit parts with poor conductivity (faulty joints in wiring, faulty contacts in power sockets, or even the site of the short circuit itself). Such overheating is a common cause of fires. An electric arc, if it forms during the short circuit, produces high amount of heat and can cause ignition of combustible substances as well.
In industrial and utility distribution systems, dynamic forces generated by high short-circuit currents cause conductors to spread apart. Busbars, cables, and apparatus can be damaged by the forces generated in a short circuit.
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Types of Short Circuit
There are two types of short circuit faults. They are:
- Symmetrical short circuit
This short circuit occurs on 3 phase systems only. This short circuit occurs in all three conductors with short currents simultaneously. Symmetry type short circuit only for 3 phase short circuit with or without ground. Only 5% of the total short circuit fault events are 3 phase short circuit. - Asymmetrical short circuit
This short circuit occurs in 1 and 3 phase systems. This short circuit occurs between current conductors with or without ground. This asymmetric short circuit is divided into
– Line-to-line fault —Any two phases shorted without ground
– Double line-to-ground fault —Any two phases connected together to ground
– Single line-to-ground fault —Only one phase shorted to ground
Statistics also show that about 70-80 percent of the short-circuit faults in the power systems are asymmetrical. Therefore, maintaining electrical system installations from all sorts of electrical fault is essential.
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Why We Should Conduct Short Circuit Study?
Since short-circuit fault cannot always be prevented, we can only attempt to reduce it and withstand the potentially damaging effects to some extent. In the early stages of development, a short-circuit study is needed to design an electrical system so that the possibility of a short circuit becomes small.
However, if a short circuit occurs then something needs to be done to reduce its effect by a) managing the undesirable magnitude of the fault current, and b) isolating the smallest possible part of the system around the fault area to maintain service to the rest of the system.
The Main Reasons for Performing Short-circuit Studies
- Verification of the adequacy of existing interrupting equipment. The same type of studies will form the basis for the selection of the interrupting equipment for system planning purposes.
- Determination of the system protective device settings, which is done primarily by quantities characterizing the system under fault conditions. These quantities also referred to as “protection handles,” typically include phase and sequence currents or voltages and rates of changes of system currents or
- Determination of the effects of the fault currents on various system components such as cables, lines, busways, transformers, and reactors during the time the fault Thermal and mechanical stresses from the resulting fault currents should always be compared with the corresponding short-term, usually first-cycle, withstand capabili- ties of the system equipment.
- Assessment of the effect that different kinds of short circuits of varying severity may have on the overall system voltage profile. These studies will identify areas in the system for which faults can result in unacceptably widespread voltage
- Conceptualization, design and refinement of system layout, neutral grounding, and substation grounding.
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When do We Require a Short Circuit Study and How Often?
Short-circuit studies are as necessary for any power system as other fundamental system studies such as power flow studies, transient stability studies, harmonic analysis studies, etc. Short-circuit studies can be performed at the planning stage in order to help finalize the system layout, determine voltage levels, and size cables, transformers, and conductors. For existing systems, fault studies are necessary in the cases of added generation, installation of extra rotating loads, system layout modifications, rearrangement of protection equipment, verification of the adequacy of existing breakers, relocation of already acquired switch-gear in order to avoid unnecessary capital expenditures, etc. “Post-mortem” analysis may also involve short-circuit studies in order to duplicate the reasons and system conditions that led to the system’s failure.
NFPA 70E 2018 (Standard for Electrical Safety in a workplace) also recommends to perform short-circuit study at least every 5 years or whenever a major modification takes place within the facility.
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Objective and Purposes of Short Circuit Studies
Objectives and purposes of short circuit analysis and studies are:
- Short circuit fault study is used to determine the available fault current or short circuit current at each point in the system
- Calculate maximum available symmetrical fault duties to compare to Low Voltage equipment short circuit ratings and Medium or High Voltage equipment to interrupting ratings
- Calculate maximum available peak fault duties to compare to Low Voltage equipment unpublished peak ratings and Medium or High Voltage closing and latching ratings
- Identify deficiencies in system protection
- Provide recommended solutions to help correct problem areas
Some Important Questions
The study results may be used for recommending changes to existing plants or for proposing an initial design for a system in its planning and/or expansion stage. Some important questions for which fault studies may help provide answers are as follows:
- Is circuit interrupting equipment adequate for the system interrupting requirements at all voltage levels? Can the medium- and high-voltage switchgear withstand the momentary and interrupting duties imposed by the system? Is this switchgear adequate for line to ground faults? If not, should new equipment be purchased or can some changes to the system be effected to avoid the extra capital expenditure?
- Is there any reserve in the interrupting capability of the circuit breakers for accommodating future system expansion? If not, is it necessary to have a safety margin for future expansion? If so, how can the system be changed to accommodate these concerns?
- Is non-interrupting equipment, i.e., reactors, cables, transformers, bus ducts, adequately rated to withstand short-circuit currents until cleared by the interrupting equipment?
- Do load circuit breakers or disconnecting switches have sufficient momentary bracing and/or close-and-latch capabilities?
- What will be the effect on the calculated short-circuit currents in the plant system if there is an increase in the power company’s short-circuit level? Economically, what can be done to anticipate such an eventuality?
- Is special protective equipment or circuitry necessary to provide protective device selectivity for both maximum and minimum value of short-circuit currents?
- During faults, do the voltages on unfaulted buses in the system drop to levels that can cause motor-starter contactors to drop out or undervoltage relays to operate?
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How to Conduct Short Circuit Study
Short Circuit Calculation and Study Standards
IEC 60909 – International Standard
Standard IEC 60909 series distinguishes four duty types resulting in four different calculated short circuit fault currents:
- The initial short-circuit current I”k
- The peak short-circuit current Ip
- The breaking short-circuit current Ib
- The steady-state fault current Ik
ANSI Standards – North American standard
IEEE standards covering short circuit current fault calculations for low voltage electrical systems (below 1000 V), are:
- IEEE Standard 242-1986
- IEEE Standard 241-1990
- IEEE Standard C37.13-1990
- IEEE Standard 141-1993
IEEE standards dealing with short circuit current calculations for medium and high voltage electrical networks are:
- IEEE Standard 141-1993
- IEEE Standard C37.5-1979
- IEEE Standard 241-1990
- IEEE Standard 242-1986.
- IEEE Standard C37.010-1979
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Factors Affecting the Accuracy of Short-Circuit Fault Studies
IEEE Std.399 stated that are many some factors affecting the accuracy in a short-circuit study. They are:
- System configuration
- System impedance
- Neutral grounding
- Pre fault system loads and shunts
- Mutual coupling in zero sequence
- Phase shifts in delta wye transformer banks
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Software for Short Circuit Study
Various internationally recognized software are available worldwide which can be used to perform short circuit study for commercial and industrial facilities. The topmost, widely used ones are the following:
- ETAP
- SKM
- EasyPower
- PSS
- Etc.
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Steps in Short-Circuit Study
General steps in a short circuit study are as follow:
- Data collection
Information on all the components is obtained during a field visit and then tabulated. Additional data is obtained from the electric utility, manufacturers, or calculated from field data.
- Single-line diagram
A power system diagram shows how all components electrically connected is created, or, if one already exists, updated. Additional data needed for the study, such as cable impedance, can be obtained with information from this one-line diagram. - Software analysis
Using one of the many short-circuit analysis softwares available, the system data is input and the short-circuit currents at various points in the system are calculated as output. - Tabulate results
The output of the power system software is typically put into a table for comparison with the ratings of equipment in the system. If the calculated short-circuit current at a given point exceeds the short-circuit rating of the equipment installed at that point, then that location is flagged as being inadequately. protected. - Final report
A detailed report will describe the scope of the study, all assumptions, the origin of the data, the methods used for calculating the currents, the tabulated results, and recommendations for corrective actions are published at the conclusion of the study.
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The Deliverables
The typical short circuit study reports are expected to include the following deliverables provided by consultant:
- Comprehensive model of the facility in modern power systems software
- Evaluation of the system under short circuit conditions for normal and emergency scenarios
- Design parameters of the protective devices that are required for optimal protection
- Comparisons between the existing protective device ratings and the available fault current
- Recommendations for the protection of equipment and personnel
- Recommendations for the corrective action of the protective devices for protection against high fault currents
- Providing the necessary information compliant with IEC, NEC, NFPA 70E and IEEE 1584 standards.
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Contact Omazaki Engineering if you are looking for a short circuit fault analysis and study consultant who delivers consulting services to help with your project or for other purposes related to your electrical system in Indonesia and South East Asia.
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Related Articles
- Power System Study & Analysis
- Load Flow Study & Analysis
- Protection Coordination Study
- Motor Starting Study and Analysis
- Power System Transient Stability Study
- Harmonic Study and Analysis
- Arc Flash Study & Assessment
- Power Quality Study & Assessment
- Voltage Drop Study & Analysis
- Voltage Imbalance (Unbalance) Study
References
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