Short-circuit current is an electric current in a given short-circuit [this term is defined in the IEC 60050-195-2021]. The same definition of the term is given in another part of the International Electrotechnical Vocabulary (IEV), IEC 60050-826.
British Standard BS 7671 defined the term “short-circuit current” similarly to how it was defined in IEC 60050-826:1982: An overcurrent resulting from a fault of negligible impedance between live conductors having a difference in potential under normal operating conditions.
Short-Circuit Current Meaning
Short-circuit current is a type of overcurrent. In contrast to overload current short-circuit current usually occurs under fault conditions when the insulation of any conductive parts under different electrical potentials is faulted and electrical contact with negligible impedance occurs between them.
Under fault conditions it is also possible to short-circuit energized parts to exposed and extraneous-conductive-parts, which in electrical installations of buildings with TN-S, TN-C-S and TN-C system earthing types have an electrical connection to the earthed neutral of the power supply.
Earth-fault currents in TN systems flowing through phase conductors and protective or PEN conductors will be comparable to single-phase short-circuit currents flowing through phase conductors and neutral or PEN conductors.
Short-circuit current can also occur under normal conditions, when there is no fault, through the incorrect connection of conductive parts with different electrical potentials during the installation and operation of the electrical installation of a building. If the wrong electrical connection is made, for example between the phase and neutral conductors of a circuit, a single-phase short-circuit current will flow through both conductors when the circuit is switched on.
Short-Circuit Current Value
The value of short-circuit current can many times (by several orders of magnitude) exceed the value of overload current and even more so the value of rated current.
Even its short-term effect on some elements of the electrical installation of buildings can cause their mechanical destruction, overheating, fire and, as a consequence, cause a fire in the building. Therefore, electrical equipment in electrical installations of buildings, first of all the conductors of electrical circuits, must be reliably protected against short-circuit currents by means of overcurrent protection devices – circuit-breakers and melting fuses.
Short-circuit currents are determined when designing the electrical installations of buildings and are taken into account when selecting the characteristics of electrical equipment. The maximum short-circuit currents are always related to the overcurrent limits, which can disconnect switching devices and overcurrent protection devices and can also pass through some types of electrical equipment. Minimum short-circuit currents are used to test the ability of overcurrent protection devices to disconnect within the rated or preferred time period.
About the method of calculating short-circuit currents.
Methodologies for calculating short-circuit currents are outlined in IEC 60909 standards and technical reports. The IEC 60909 complex is used to calculate short-circuit currents in low-voltage and high-voltage AC power systems with a frequency of 50 or 60 Hz. However, as specified in IEC 60909-0, electrical systems with voltages of 550 kV or more with long distribution lines require special consideration.
List of References
- IEC 60050-195-2021
- BS 7671
- IEC 60909