What is a Short Circuit Study and why do it?
A short circuit study is necessary to ensure that equipment is rated for the available fault current of the system.
Breakers, cables, and bus work all are rated for a maximum electrical current. Current above this value can cause failure of the equipment, often with disastrous results. As current flows through a conductor, magnetic forces also act on the conductor. These forces can pull or push on the conductor and surrounding magnetic materials. When the forces are strong enough, they may pull apart support structures, or bend the conductors. This can create a short circuit in the system.
A short circuit study is also used as part of a proper breaker and relay coordination study. The maximum short circuit current expected at a location is used to determine the maximum value for an instantaneous setting on a breaker or relay. If the setting is above this point, it will never operate.
Because the maximum short circuit current is different from one location to the next, this value is also used to study coordination between devices. If a downstream device operates faster than one upstream, it may not matter if at current levels above the downstream maximum short circuit current. Because a large number of faults are short circuits, this distinction can be critical for device coordination for expected high fault current levels.
Short circuit data in a substation helps to determine step and tough potential. This is important in phase to ground faults. By determining the maximum expected phase to ground short circuit, if the ground impedance is known, the step and touch potential at a substation can be determined. This determination is used to ensure personnel safety in a substation during a fault. When high currents flow to and through the ground, the resistance between points just a couple of feet apart can cause a large voltage difference. By limiting this voltage difference through proper grounding and bonding, personnel safety can be assured.
Of course, all of this ignores the other reason why short circuit studies are performed: they are required. NFPA 70, the National Electrical Code (NEC), has several articles which mention the available short circuit current. In most of these instances, it is necessary to determine the available fault current in order to determine the minimum rating of the equipment. Articles 110.9 and 110.10:
110.9 Interrupting Rating. Equipment intended to interrupt current at fault levels shall have an interrupting rating at nominal circuit voltage sufficient for the current that is available at the line terminals of the equipment. Equipment intended to interrupt current at other than fault levels shall have an interrupting rating at nominal circuit voltage sufficient for the current that must be interrupted.
110.10 Circuit Impedance, Short-Circuit Current Ratings, and Other Characteristics. The overcurrent protective devices, the total impedance, the equipment short circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit protective devices used to clear a fault to do so without extensive damage to the electrical equipment of the circuit. This fault shall be assumed to be either between two or more of the circuit conductors or between any circuit conductor and the equipment grounding conductor(s) permitted in 250.118. Listed equipment applied in accordance with their listing shall be considered to meet the requirements of this section.
The short circuit study is necessary to determine the minimum short current rating of the electrical equipment. While it could be assumed that the fault levels are extremely high, and order high rated equipment, this can quickly increase a project cost to many times the cost of a study.
110.24 Available Fault Current.
(A) Field Marking. Service equipment in other than dwelling units shall be legibly marked in the field with the maximum available fault current. The field marking(s) shall include the date the fault-current calculation was performed and be of sufficient durability to withstand the environment involved.
Informational Note: The available fault-current marking(s) addressed in 110.24 is related to required short circuit current ratings of equipment. NFPA 70E-2012, Standard for Electrical Safety in the Workplace, provides assistance in determining the severity of potential exposure, planning safe work practices, and selecting personal protective equipment.
(B) Modifications. When modifications to the electrical installation occur that affect the maximum available fault current at the service, the maximum available fault current shall be verified or recalculated as necessary to ensure the service equipment ratings are sufficient for the maximum available fault current at the line terminals of the equipment. The required field marking(s) in 110.24(A) shall be adjusted to reflect the new level of maximum available fault current.
Exception: The field marking requirements in 110.24(A) and 110.24(B) shall not be required in industrial installations where conditions of maintenance and supervision ensure that only qualified persons service the equipment.
This article in the NEC explicitly states that equipment must be marked with the available fault current. This fault current is determined by a short circuit study performed by a qualified engineer. The exceptions apply to dwelling units, and industrial installations with qualified electricians. It still applies to commercial units and some industrial locations.
The Institute of Electrical and Electronic Engineers (IEEE) has many standards, guides, and recommended practices that outline short circuit studies and how to perform them. The most complete would be the Violet Book, IEEE 551, Recommended Practice for Calculating AC Short-Circuit Currents in Industrial and Commercial Power Systems. This book is being replaced in an effort by the IEEE by IEEE 3002.3, IEEE Recommended Practice for Conducting Short-Circuit Studies and Analysis of Industrial and Commercial Power Systems. These documents address short circuit analysis in new and existing systems. They talk about the methods and assumptions which may be made in the calculations, including motor contributions and other data requirements.
A short circuit study should be performed on any electrical system for the safety of the site occupants and the people who will have to operate or maintain the equipment. A fault with current levels higher than expected can have disastrous effects, such as fires and explosions. Equipment can easily be destroyed if it is not properly rated. By performing a study before the equipment is installed, proper ratings can be determined. This will help keep equipment costs down and safety up.