A short circuit (sometimes abbreviated to short or s/c) is an

electrical circuit An electrical network is an interconnection of electrical components (e.g., batteries, resistors, inductors, capacitors, switches, transistors) or a model of such an interconnection, consisting of electrical elements (e.g., voltage source ...

that allows a current to travel along an unintended path with no or very low

electrical impedance In electrical engineering, impedance is the opposition to alternating current presented by the combined effect of resistance and reactance in a circuit. Quantitatively, the impedance of a two-terminal circuit element is the ratio of the c ...

. This results in an excessive current flowing through the circuit. The opposite of a short circuit is an " open circuit", which is an infinite resistance between two nodes.

Definition

A short circuit is an abnormal connection between two nodes of an electric circuit intended to be at different voltages. This results in an

electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The movi ...

limited only by the Thévenin equivalent resistance of the rest of the network which can cause circuit damage, overheating,

fire Fire is the rapid oxidation of a material (the fuel) in the exothermic chemical process of combustion, releasing heat, light, and various reaction products. At a certain point in the combustion reaction, called the ignition point, flames ...

explosion An explosion is a rapid expansion in volume associated with an extreme outward release of energy, usually with the generation of high temperatures and release of high-pressure gases. Supersonic explosions created by high explosives are known ...

. Although usually the result of a fault, there are cases where short circuits are caused intentionally, for example, for the purpose of voltage-sensing crowbar circuit protectors. In

circuit analysis A network, in the context of electrical engineering and electronics, is a collection of interconnected components. Network analysis is the process of finding the voltages across, and the currents through, all network components. There are many ...

, a ''short circuit'' is defined as a connection between two nodes that forces them to be at the same voltage. In an 'ideal' short circuit, this means there is no resistance and thus no voltage drop across the connection. In real circuits, the result is a connection with almost no resistance. In such a case, the current is limited only by the resistance of the rest of the circuit.

Examples

A common type of short circuit occurs when the positive and negative terminals of a

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are connected with a low- resistance conductor, like a

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. With a low resistance in the connection, a high current will flow, causing the delivery of a large amount of

energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of ...

in a short period of time. A high current flowing through a battery can cause a rapid increase of temperature, potentially resulting in an explosion with the release of

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...

gas and

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(an

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or a base), which can burn tissue and cause blindness or even death. Overloaded wires will also overheat causing damage to the wire's insulation, or starting a fire. In electrical devices, unintentional short circuits are usually caused when a wire's insulation breaks down, or when another conducting material is introduced, allowing charge to flow along a different path than the one intended. In mains circuits, short circuits may occur between two phases, between a phase and neutral or between a phase and

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(ground). Such short circuits are likely to result in a very high current and therefore quickly trigger an overcurrent protection device. However, it is possible for short circuits to arise between neutral and earth conductors and between two conductors of the same phase. Such short circuits can be dangerous, particularly as they may not immediately result in a large current and are therefore less likely to be detected. Possible effects include unexpected energisation of a circuit presumed to be isolated. To help reduce the negative effects of short circuits, power distribution transformers are deliberately designed to have a certain amount of leakage reactance. The leakage reactance (usually about 5 to 10% of the full load impedance) helps limit both the magnitude and rate of rise of the fault current. A short circuit may lead to formation of an

electric arc An electric arc, or arc discharge, is an electrical breakdown of a gas that produces a prolonged electrical discharge. The current through a normally nonconductive medium such as air produces a plasma; the plasma may produce visible light. ...

. The arc, a channel of hot ionized plasma, is highly conductive and can persist even after significant amounts of original material from the conductors has evaporated. Surface erosion is a typical sign of electric arc damage. Even short arcs can remove significant amounts of material from the electrodes. The temperature of the resulting electrical arc is very high (tens of thousands of degrees), causing the metal on the contact surfaces to melt, pool and migrate with the current, as well as to escape into the air as fine particulate matter.

Damage

A short circuit fault current can, within milliseconds, be thousands of times larger than the normal operating current of the system. * Damage from short circuits can be reduced or prevented by employing fuses,

circuit breaker A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by an overcurrent or short circuit. Its basic function is to interrupt current flow to protect equipment and to prevent the ris ...

s, 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

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s require protective devices set or rated for higher currents than lighting circuits.

Wire gauge Wire gauge is a measurement of wire diameter. This determines the amount of electric current the wire can safely carry, as well as its electrical resistance and weight. Types of wire gauge Wire gauges may be broadly divided into two groups ...

s 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

s. 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.

Related concepts

electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons using electronic devices. Electronics uses active devices to control electron flow by amplification ...

, the ideal model (infinite

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) of an operational amplifier is said to produce a ''virtual short circuit'' between its input terminals because no matter what the output voltage is, the difference of potential between its input terminals is zero. If one of the input terminals is connected to the ground, then the other one is said to provide a

virtual ground In electronics, a virtual ground (or virtual earth) is a node of a circuit that is maintained at a steady reference potential, without being connected directly to the reference potential. In some cases the reference potential is considered to be th ...

because its potential is (ideally) identical to that of the ground. An ideal operational amplifier also has infinite input impedance, so unlike a real short circuit, no current flows between the terminals of the virtual short.

References

External links