Short circuit

A short circuit (sometimes abbreviated to short) is an accidental low-resistance connection between two nodes of an electrical circuit that are meant to be at different voltages. This results in an excessive electric current limited only by the internal impedance of the current source and 'potentially' causes circuit damage, overheating, fire or explosion. Although usually the result of a fault, there are cases where short circuits are caused intentionally, for example, for the purpose of circuit bending and in voltage-sensing crowbar circuit protectors.

Explanation

In circuit analysis, the term short circuit is used by analogy to designate a zero-impedence connection between two nodes. This forces the two nodes to be at the same voltage. In an ideal short circuit, this means there is no resistance and no voltage drop across the short. In simple circuit analysis, wires are considered to be shorts. In real circuits, the result is a connection of nearly zero impedance, and almost no resistance. In such a case, the current drawn is limited by the rest of the circuit.

The electrical opposite of a short circuit is an open circuit, which is infinite resistance between two nodes.

It is common to misuse "short circuit" to describe any electrical malfunction, regardless of the actual problem.

Example

A short circuit is to connect the positive and negative terminals of a battery together with a low-resistance conductor, like a wire. With low resistance in the connection, a high current flows, causing the cell to deliver a large amount of energy in a short time. (See also: Ohm's law, power).

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 large current through a battery (also called a cell) can cause the rapid buildup of heat, potentially resulting in an explosion or the release of hydrogen gas and electrolyte, which can burn tissue and may be either an acid or a base. Overloaded wires can also overheat, sometimes causing damage to the wire's insulation, or a fire. High current conditions may also occur with electric motor loads under stalled conditions, such as when the impeller of an electrically driven pump is jammed by debris.

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 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.

In mains circuits, phases, between a phase and neutral or between a phase and earth (ground). Such short circuits are likely to result in a very high current flowing 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 flowing and are therefore less likely to be detected. Possible effects include unexpected energisation of a circuit presumed to be isolated.

Virtual short

In circuit design, a common cause of a short circuit is a virtual short.

This is when a node is e through a resistance (typically an output impedance) that is larger than usually seen in a direct short, but still small enough to draw larger than normal current. For example, outputting a voltage on a pin that is connected to ground will cause a virtual short. In such an example, a common output impedance of 50 ohms with 5 volts across it will draw 100 mA, much more than a typical draw of a few milliamperes. Another cause is when a highly capacitive load is connected to a supply rail, usually via a FET switch. When switched, the supply rail is directly connected to a load that is at ground, and the amount of current necessary to charge the capacitive load is more than can be supplied by the power source. Voltage droop is experienced in cases where the power supply reaches its maximum current output before the load is charged.

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Explanation

Example

Virtual short