Short circuit current Electrical Arc
The subject of the electrical arc is of interest from the theoretical viewpoint and also
of considerable practical importance. The design of circuit breakers is mainly based
on experience rather than precise science. Empirical formulae can be used to
determine dimensions of certain general types and the breaking capacity rating. There
has been a noticeable lack of co-ordination between theoretical and practical work.
There has been no lack of experimental work on this subject, but the bulk of this
work has referred to problems of a scientific rather than a practical nature.
In circuit breakers, the arc exists in a mixture of air, nitrogen, oxygen and metallic
vapor. Interruption is due to elongation of the arc, which results in cooling, and deionization
by diffusion. Owing to the high temperature of the arc relative to the
surrounding air, the arc is subjected to strong convection currents, which coupled
with the electromagnetic effect of the current loop, causing the arc to move.
An overcurrent is a current flow more than the rated of current of the equipment. This
may result from equipment overload or the failure of a component. This could cause
insulators to fail. In a short circuit current, there is a very high magnitude of
overcurrent from a fault of negligible impedance between conductors having a
difference in potential under normal operating conditions. The conductors and
insulators could melt and vaporise immediately. Additionally, the magnetic forces
from high short circuit current can damage both circuits and circuit breakers [2].
When a short circuit occurs, the current flow through the circuit rises up rapidly and
continues to the peak current of the AC. A natural current zero occurs every 10 ms
for a 50 Hz cycle, if there is no protection circuit. This peak current is called the
Prospective Peak Short circuit current (Ippscc) as shown in Figure 1.1.
