The plain-break oil circuit breaker is the earliest type
from which all other circuit breakers have developed.
It has a very simple construction. It consists of fixed
and moving contacts enclosed in a strong weathertight
earthed tank containing oil up toa certain level
and an air cushion above the oil level.
The air cushion provides sufficient room to allow for
the reception of the arc gases without the generation
of unsafe pressure in the dome of the circuit breaker.