Such circuit breakers
utilize dielectric oil (transformer oil) for arc extinction. In bulk oil
circuit breakers, the contacts are separated inside a steel filled with
dielectric oil. In minimum oil circuit breakers, the contacts are
separated in an insulation housing (interrupter)filled with dielectric
oil.
The oil in oil circuit breakers( OCBs) serves two purpose. It insulates
between the phases and between the phases and the ground, and it
provides the medium for the extinguishing of the arc. When electric arc
is drawn under oil, the arc vaporizes the oil and creates a large bubble
that surround the arc. The gas inside the bubble is around 80%
hydrogen, which impairs ionization. The decomposition of oil into gas
requires energy that comes from the heat generated by the arc. The oil
surrounding the bubble conducts the heat away from the arc and thus also
contributes to deionization of the arc.
Main disadvantage of the oil circuit breakers is the flammability of
the oil, and the maintenance necessary to keep the oil in good condition
)i.e. changing and purifying the oil).
Bulk oil circuit breaker (tank type circuit breakers) are becoming obsolete and have been described here in brief.
Minimum oil circuit breaker have the following demerits:
- Short contact life.
- frequent maintenance.
- Possibility of explosion.
- Larger arcing time for small current.
- Prone to restrike
They are being superseded by SF6 circuit breakers in all ranges.
