"Electrocution is usually the first thing that comes to mind when someone is killed or injured by electricity. The fact is that the blast and thermal energy from an arc flash is equally relevant and is a significant source of long-term injuries and sometimes a slow and painful death. Many
hundreds of people in the UK and many thousands in the US receive severe and debilitating burns each year.
There is evidence to suggest that, although burns can be caused by electric current passing through the body from direct contact with energised conductors, most burns from electrical accidents actually come from arc flash. Many electrical staff routinely work on live high power equipment, carrying out tasks such as fault finding and diagnostic testing without fully understanding the consequences of what will happen to them should an
electrical flashover occur." Mike Frain – Electrical Safety (UK) Ltd.
A typical arc flash incident can produce more explosive energy than a pound of TNT. Therefore, heat from an electric arc is an important hazard that workers need to be protected from. Most burns from electrical accidents actually come from an arc flash and can cause second and third degree burns.
IEEE Fellow, Ralph H. Lee a DuPontTM pioneer of electrical safety published, "The Other Electrical Hazard: Electric Arc Burns” in the 1981 IEEE Industry Applications Society Annual paper which argued that electric arc burns posed as great a risk for burns as electric shocks.
Lee was the first to scientifically study the problem of electrical arc burns and said "there is another hazard which few appreciate, which we do not even need to touch to incur injury." In his paper, Lee developed a formula for the distance required for various degrees of burns as related to the various amounts of power and time of exposure
There are three basic electrical hazards that cause injury and death:
- Arc-flash (electrical flashover)
Current can pass through the human body’s nervous or vascular systems, and across the surface of the body. Even a small amount of current flow for example, the current required to light a 7½ watt, 120 V lamp, passing through the chest, can cause death. Where death has not occurred this is due simply to the fortuitous nature of the current path in missing vital organs. What is therefore required in this situation to survive an electric shock from mains voltage is luck!
Of those killed while working on voltages below 600 V, half were intentionally working on "live" energized equipment. Most electrocutions can be avoided with proper training, planning, job preparation, procedures, and equipment.
Arc-flash (electrical flashover)
This is extremely high temperature conductive plasma and gases resulting from an arc fault incident. Arc temperatures can reach up to 35,000oF, four times hotter than the sun’s surface.
The material at the end points of the arc will consequently be vaporized, which will cause the formation of a continuous conductive path between the electrodes. The increasing current will cause a further increase of the temperature and cause the transformation of the heated air between the electrodes into plasma.
A plasma is characterized by the fact that all chemical bonds are broken up and the single atoms completely ionised. This cloud of plasma is chemically very aggressive. The metal-oxides are formed by cooling down and the reaction of the vapours with the oxygen of the air.
Further cooling down will make them visible as black or grey smoke. As long as the vapours and the smoke remain sufficiently hot, they will cause strongly adhering contaminations, when depositing themselves onto surrounding surfaces.
This simultaneously occurs with the electrical flashover and is a pressure wave and explosion-like bang caused by the rapid expansion of gases and conducting material with high flying molten materials and shrapnel.
An arc blast may result in a violent explosion of circuit components and thrown shrapnel. The blast can destroy structures, and knock workers from ladders or across a room. The blast can rupture eardrums and collapse lungs. With the type of 11kv electrical distribution system found in rural overhead cables or in larger factories and chemical plants, a short circuit or fault can release 200-250 MVA in a fraction of a second. In layman’s terms, the electrical power released in one electric arc could potentially approximate to that released by 11 rockets at launch. If a person were standing 1m away from the arc, they would experience a severe blast together with an instantaneous temperature rise of up to 1500oc.