Bulk Oil and Minimum Oil Circuit Breaker

Bulk Oil Circuit Breaker: A breaker which uses a large quantity of oil for arc extinction is called a bulk oil circuit breaker. Such type of circuit breaker is also known as dead tank-type circuit breaker because their tank is held at ground potential. The quantity of oil requires in bulk oil circuit breaker depends on the system voltage. If the output rating of the voltage is 110 KV, then it requires 8 to 10 thousand kg of oil, and if their output rating is 220 KV, then breakers need 50 thousand Kg of oil.

In bulk oil circuit breaker, oil performs mainly two functions. Firstly, it acts as an arc extinguishing medium and secondly, it insulates the live parts of the breaker from earth. The quantity of oil requires for arc extinction is only about one-tenth of the total and the rest being used for the insulation.

These large quantities of oil are subject to the carbonisation, sludging, etc., which occurs due to arc interruption and other causes reducing the insulating properties and requires regular maintenance.

Bulk oil circuit breaker needs a large tank which increases expenses and also increases the weight of the circuit breaker. Because of the following disadvantage the low oil circuit breaker is developed which use minimum oil for arc extinction.

Minimum Oil Circuit Breaker

In this type of circuit breaker minimum oil is used as an arc quenching medium and it is mounted on a porcelain insulator to insulate it from the earth. The arc chamber of such type of circuit breaker is enclosed in a bakelised paper. The lower portion of this breaker is supported by the porcelain and the upper porcelain enclosed the contacts.

This circuit breaker is of the single breaker type in which a moving contact tube moves in a vertical line to make or break contact with the upper fixed contacts mounted within the arc control devices.

A lower ring of fixed contacts is in permanent contact with the moving arm to provide the other terminal of the phase unit. Within the moving contact, the tube is a fixed piston. When the moving contact moves downwards, it forces the insulating oil to enter into the arc control devices . Thus, the arc gets extinguished.

Minimum oil circuit breaker requires less space as compared to bulk oil circuit breaker which is an important feature in large installations. But it is less suitable in places where the frequent operation is required because the degree of carbonisation produced in the small volume of oil is far more dangerous than in the conventional bulk oil circuit breakers and this also decreases the dielectric strength of the material.

The low oil circuit breakers have the advantages of a requirement of the lesser quantity of oil, smaller space requirement, smaller tank size, smaller weight, low cost, reduced risk of fire and reduced maintenance problems. Minimum oil circuit breaker suffers from the following drawbacks when compared with the bulk oil circuit breakers

• Increased degree of carbonisation due to a smaller quantity of oil.
• The dielectric strength of oil decreases due to a high degree of carbonisation.
• Difficulty in removal of gases from the contact space-time