<P> In mains circuits, short circuits may occur between two 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 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 . </P> <P> A short circuit may lead to formation of an electric arc . 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 Fahrenheit), 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 . </P> <P> 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 breakers, 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 home appliances require protective devices set or rated for higher currents than lighting circuits . Wire gauges 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 . </P> <P> 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 fires . An electric arc, if it forms during the short circuit, produces high amount of heat and can cause ignition of combustible substances as well . </P>

What happens when you short a voltage source