2 What is an fault arc - Die BG ETEM

10
3 Hazards of fault arcs
3.1 Physical and technical effects
Depending on power and duration of an electric arc quite different physical effects can
arise which result from the extremely high temperature in the arc column. Temperatures
of more than 5.000° C are possible within an electric arc. In the arc development metal
of the arc electrodes is vaporized and ionized. It is formed into a conductive connection
between the electrodes. Due to the intensified current flow the temperature rises and a
plasma develops between the electrodes. Radiation is emitted by the arc plasma.
A plasma is distinguished by the fact that all the chemical compounds in it have been
broken up and are ionized. Thus this plasma cloud has got a very high chemical
aggression. Due to the vaporization of metal and the following immense heating
up, a mass and gas expansion takes place which rapidly transports metallic vapor
and splash away from the arc roots. As a result of cooling down and reacting with
atmospheric oxygen, metal oxides can be found which, in the course of further
cooling, appear as black or grey smoke. As long as vapor and smoke have got sufficient
temperature they deposit a quite sticky sort of contamination (see Figure 3.1).
An immediately physical reaction during the development of the arc is the huge
pressure rise which in 5–15 ms can reach its first peak of up to 0,3 MPa. This corresponds
to a pressure of 20–30 t/m 2 . If an unhindered pressure wave spreading
can not take place, one runs the risk of destroying the electrical installation and its
surroundings mechanically. Thus doors and coverings can be blown up, casings,
compartments and partitions can burst and break down.
The optical radiation and a convective heat transfer of the flowing hot plasma and
gas, and plasma jets occurring on the arc roots result in thermal exposures and damages.
Dependent on the intensity of the electric arc, the heat flux can ignite nearby
flammable materials. The molten metallic splashes which originate from the electric
arc increase the fire hazard.
3.2 Effects on the human body
Due to the physical arc consequences described, according risks result for persons
working at or in the vicinity of live parts, a direct exposure is likely because the equipment
is opened for these working activities.
The main hazards of personal injuries result from:
• Pressure effects, forces on the body and shrapnel due to the rapid heating-up
of the arc surrounding gas