Oil circuit breakers are the oldest type of circuit breakers. The separating contacts of the oil circuit breakers are made to separate within the insulating oil which has better insulating properties than air. On the occurrence of the fault as the breaker contacts open under the oil, an arc is struck between the breaker contacts and the heat of the arc evaporates the surrounding oil and dissociates into a substantial volume of gaseous hydrogen (hydrogen gas with a small percentage of methane, ethylene and acetylene) at high pressure.
Oil Circuit breakers have the virtues of reliability, simplicity and relative cheapness. In order to determine the advantages of oil circuit breakers, advantages and disadvantages of oil as a arc quenching medium should be understood
Advantages:
- Arc energy is absorbed in decomposing of oil
- The gas formed which is mainly hydrogen, has high diffusion rate and high heat absorption in changing from diatomic to mono-atomic and thus provides good cooling properties
- The oil has high dielectric strength and provides insulation between the contacts after the arc has been finally extinguished and there has been time for the oil to flow into the gap between contacts
- Cooling oil presents the cooling surface in close proximity to the arc
- The oil used (such as transformer oil) is a very good insulator and allows smaller clearance between live conductors and earth components
Disadvantages:
- Oil may be flammable and can cause fire hazards, if a defective oil circuit breaker should fail under pressure and cause an explosion
- There is a risk of formation of explosive mixture with the air
- Due to the decomposition of the oil in the arc, the oil becomes polluted by carbon particles, which reduces its dielectric strength. Hence periodical maintenance and replacements are required