The making current flow does not start as the contacts touch but several milliseconds earlier because the insulating medium between the contacts suffers a
dielectric breakdown as shown in Fig. This called a pre-strike.
The electrical field in the gap between the contacts increases as the contacts approach each other. When the field exceeds the dielectric strength of the insulating medium, the gap breaks down and a switching arc is initiated.
If the energy is large, excessive melting of the contact surfaces occurs. Mating contacts with molten surfaces may cause welding. This may prevent the switching device to appropriately respond to the next opening command, if its operating mechanism does not provide an opening force sufficient to break the welded points.
The short circuit current contains a DC component and therefore its peak value can be much higher than for a pure AC short circuit current. The arc voltage is very dependent on the interrupting medium, even in case of very short arc lengths, there are significant voltage drops near the electrodes.
Making under short circuit may occur in both CB & LBS.
Fig show different stages during making operation of a switchgear
