Handbook of Electrical Installation Practice - BeKnowledge

Switchgear 365
Fig. 14.4 Contacts and arc control device of an oil circuit-breaker.
During the arcing period the presence of the arc tends to prevent the exhaust of
gases from the arc control device through its side vents, but as the current reaches
current zero these gases are released and sideways displacement of the ionisation
products occurs due to the high pressure gas which surrounds the arcing zone. The
replacement of the ionisation in the contact gap by clean hydrogen ensures a dielectric
strength sufficient to withstand the rapidly rising transient recovery voltage. In
an oil circuit-breaker of this type the effort required to extinguish the arc increases
as the current rises, but so does the energy injected into the electric arc. Consequently,
the extinguishing effort rises to match the increasing fault current.
The arc voltage (see Fig. 14.1) is important for two basic reasons. First, the arc
voltage controls the amount of energy being generated in the arc and this has an
important effect on the mechanical design of the enclosure. Second, the arc voltage
plays a part in modifying some of the electrical parameters concerned with the