Electric spark is a strong current column in the process of breakdown discharge between electrodes. A large number of electric sparks converge into an arc. The temperature of electric spark is as high as several thousand degrees, which can not only directly cause combustible materials to burn, but also make metal melt and splash, forming a secondary fire source. Electrical appliances such as knives, circuit breakers, contactors and relays will generate electric sparks during normal operation or normal work; The sliding contact between the brush of DC motor and commutator and the sliding contact between the brush of wound asynchronous motor and slip ring will also produce electric spark; Sparks or arcs may occur when the insulation of electrical equipment or electrical lines is over-voltage, short-circuited, grounded, disconnected or loose. When the fuse melts, it will also produce dangerous sparks or arcs; Lightning discharge, electrostatic discharge and electromagnetic induction discharge can also produce electric sparks.

When the inductance circuit is cut off, there will be a strong electric spark or arc at the fracture. Its spark energy is:

Where the unit of spark energy WL is J, the unit of inductance L is H, and the unit of current I is A. ..

Lightning discharge will produce a strong arc. Direct lightning discharge can produce an arc of 20 000℃, which is extremely dangerous for ignition. The ignition risk of lightning secondary discharge is also great. Lightning impulse overvoltage breaks through the insulation of electrical equipment to form a short circuit, which also has great fire risk.

The voltage of static electricity generated in the process of technology or human action can be as high as thousands to hundreds of thousands of volts, which is easy to discharge. The spark energy of electrostatic discharge is:

Where the unit of spark energy We is j, the unit of capacitance c between discharge conductors is f, and the unit of voltage u between discharge conductors is v. ..

Tall metal structures located near the radio transmitting station, such as tower cranes, piping systems, frames, lightning rods, etc., can receive electromagnetic induction, receive radiant energy and be charged, and can also generate spark discharge.