A short circuit is an unintended or otherwise undesired electrical connection between two points that have a significant potential difference, creating a path with much lower impedance than the normal intended circuit path. Because current is strongly influenced by the impedance of the available path, a short circuit can cause a very large current to flow.
In an elementary DC model, Ohm's law gives I = V divided by R, so if resistance becomes extremely small while voltage remains substantial, the resulting current can become extremely large. In a real electrical system, the current is limited by the resistance and impedance of conductors, the source's internal impedance, transformers, generators, batteries, connections and protective devices. A short circuit therefore does not literally require resistance to become exactly zero. A sufficiently low impedance relative to the normal circuit can constitute a short circuit and create dangerous operating conditions.
Short circuits occur when conductors that should remain electrically separated become connected. Causes include damaged insulation, incorrectly connected wiring, loose conductive objects, manufacturing defects, component failure, moisture contamination, metal tools bridging terminals, mechanical damage, cable deterioration and insulation breakdown. In electronic equipment, a failed semiconductor can internally create a low resistance path between supply rails. In wiring systems, a damaged cable can allow a live conductor to contact another conductor or an earthed conductive structure.
A short circuit can also occur intentionally in specialised circuits when the connection is controlled and designed for a particular purpose, so the term describes the electrical condition rather than automatically implying an accidental event. The important characteristic is the presence of an abnormally low impedance path between electrically separated points.
The large current associated with a short circuit can produce substantial heating because resistive power is P = I squared R. Even though the resistance of the fault path may be very small, the enormous current can result in significant energy release throughout conductors, contacts and source components. The high current can melt conductors, weld contacts together, damage semiconductor devices and ignite combustible materials. Short circuits can also create magnetic forces because current flowing through conductors generates magnetic fields.
In high current power systems, the mechanical forces associated with fault currents can be large enough to deform or damage busbars, cables and equipment. Electrical arcs can also form when conductors separate or when the fault path involves air or another medium capable of ionisation. Arc faults can generate intense heat, light, pressure and electromagnetic effects and therefore require specialised protective and engineering measures.
The current produced by a short circuit depends on the electrical source and the impedance of the complete fault loop. A low voltage battery with significant internal resistance may produce a high but relatively limited current when its terminals are shorted. A high capacity battery or a large power distribution system can provide much greater fault current. In an AC power network, fault current depends on source impedance, transformer impedance, conductor impedance, fault type and system configuration.
Different faults include line to line, line to neutral, line to earth and three phase faults. Their currents can differ substantially. Engineers calculate prospective fault current or short circuit current when designing protective equipment because fuses, circuit breakers, switchgear and conductors must be capable of interrupting or surviving expected fault conditions safely.
Protective devices are designed to detect and interrupt excessive current before a fault causes unacceptable damage. A fuse contains a calibrated element that melts when sufficient current flows for an appropriate time. A circuit breaker uses a mechanism that opens the circuit when current exceeds a defined threshold or when another monitored fault condition occurs. Different protective devices have different time current characteristics because some faults require extremely rapid interruption while ordinary temporary current surges may be acceptable.
Circuit protection must be coordinated with conductor ratings, equipment characteristics and expected fault currents. If protection is incorrectly selected, it may fail to interrupt the fault safely, interrupt unnecessarily during normal operation or allow dangerous heating to continue. Proper protection therefore requires consideration of both magnitude and duration of fault current.
A short circuit is distinct from an overload, although both can cause excessive current. An overload occurs when a circuit carries more current than its intended continuous rating because the connected load is too large or operating conditions have changed, but the circuit may still have its normal resistance and topology. A short circuit instead creates an abnormally low impedance path that bypasses some or all of the intended load.
For example, connecting too many high power appliances to a circuit can create an overload, while directly connecting a supply's positive and negative conductors creates a short circuit. An open circuit is the opposite type of fault in terms of continuity: an open circuit creates a broken path and generally prevents normal current flow through that branch. Distinguishing these conditions is important when diagnosing electrical faults and selecting appropriate protective measures.
Short circuits can occur in both DC and AC systems and have different physical characteristics depending on voltage, current, frequency and environment. In low voltage electronic circuits, a short can damage integrated circuits, printed circuit board traces and power supplies. In automotive electrical systems, accidental shorts can generate very high currents from batteries and may cause wiring to overheat rapidly. In domestic and industrial AC systems, protective devices are designed to interrupt faults according to the electrical installation's characteristics.
High voltage systems introduce additional issues involving insulation breakdown, electric arcs, transient recovery voltage and electromagnetic forces. At very high fault currents, the interruption process itself can be challenging because an arc may attempt to continue conducting after contacts begin separating. Specialised switchgear and protection systems are therefore used in power networks.
Preventing short circuits involves maintaining adequate insulation, physical separation, secure connections, correct component selection and appropriate protective devices. Electrical cables are insulated and mechanically protected so that conductors do not contact each other or accessible conductive structures. Circuit boards maintain clearances and creepage distances appropriate to their voltage and environmental conditions. Enclosures prevent foreign conductive objects from contacting hazardous parts. Fuses, circuit breakers and electronic current limiting systems reduce the duration and consequences of faults.
Fault diagnosis often involves visual inspection, resistance or continuity measurements with the circuit safely de-energised, insulation testing and controlled investigation of affected components. Short circuits are therefore both a fundamental circuit concept and a major engineering concern because a low impedance fault can rapidly convert stored or supplied electrical energy into heat, magnetic forces, mechanical stress, light and potentially destructive electrical arcing.
