Principles of electrical safety:Arc and flashover burns

Arc and flashover burns

Arc burns, also known as flashover burns, are commonly associated with the failure of insulation in electrical equipment, leading to an arc developing in the air between adjacent conductors.

A common cause is metal objects, such as screwdrivers, spanners and other foreign objects forming a short circuit between a phase conductor and earth, or across conduc- tors at different voltages. More often than not the initial fault is between two adjacent conductors, such as a phase conductor and earthed metalwork, but the ionized gases created by the fault allows arcs to develop between other conductors. In three-phase systems, the result is often described as a full three-phase flashover. The typical conse- quence is the expulsion from the short circuit of a highly energetic arc and hot gases, with temperatures in the plasma typically exceeding 1000°C. A person in the immediate vicinity of the arc will suffer burn injuries which are often severe and life-threatening.

The amount of power that can be supplied into the fault is determined by the volt- age and impedance in the fault circuit, and is quoted in megavoltampere or kiloampere (strictly, the second of these parameters is a measure of short-circuit current rather than power). Modern systems, even at low voltage, often have very high fault levels.

A typical fault level in a low-voltage domestic installation is 6 kA or 2.4 MVA and in an 11 kV high voltage installation, it can be as high as 13 kA or 250 MVA.

Fire injuries

Electrical systems that are poorly designed, or that have certain fault conditions, may overheat due to excess current flowing to such an extent that adjacent flammable materials may be ignited. Fires may also be started by arcs and sparks evolved from short-circuit faults, most frequently resulting from a breakdown in insulation. Hot spots in circuits can develop, for example, when poorly made connections have sufficiently high resistance to cause localized heating, which may lead to fire.

Most people caught in fire situations who are killed or seriously harmed are affected by the smoke and toxic fumes emitted from the burning substances, including electrical cables and components, rather than by being burned.

Explosion injuries

If standard electrical equipment is installed in places, called hazardous areas, where a flammable or explosive atmosphere exists, the arcs, sparks, electrostatic discharges or hot surfaces created during normal operation or under fault conditions may have enough energy for them to act as an ignition source, leading to an explosion. Anybody in proximity to the explosion may suffer burns and physical injuries, which may be serious enough to be fatal.

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