JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III Year SWITCH GEAR AND PROTECTION Previous Question Paper (November/December - 2020 - R16)

The Symmetrical breaking capacity of a circuit breaker is the maximum root-mean-square (RMS) value of the symmetrical AC component of the fault current that the circuit breaker is capable of breaking at a specified recovery voltage. It is expressed in kA or MVA and represents the current flowing through the breaker contacts at the instant of contact separation, excluding the DC component.

The formula for breaking capacity in MVA is:

where V is the rated service voltage in volts and I is the rated symmetrical breaking current in amperes. This rating ensures the breaker can successfully interrupt the fault current without sustaining damage or failing to clear the circuit.

A Minimum-oil circuit breaker (MOCB) is designed to use a significantly smaller quantity of oil compared to bulk-oil circuit breakers, primarily using oil as an arc quenching medium rather than for insulation. The oil is contained in a small, insulated chamber, often made of porcelain or glass-reinforced plastic.

1. Arc Control Device: The breaker features a specialized arc control chamber (interrupter) where the fixed and moving contacts are located. This chamber is designed to force the oil into the arc path.
2. Arc Quenching: When the contacts separate, an arc is drawn. The heat from the arc vaporizes a portion of the oil, generating high-pressure hydrogen gas. This gas creates a turbulence that forces the oil into the arc, cooling it and de-ionizing the path to extinguish the arc at the current zero.
3. Construction: The main insulation to the ground is provided by solid insulation (porcelain), while the oil is restricted to the interrupting chamber. This reduces the fire hazard and the amount of oil required for maintenance.
4. Operation: The moving contact is typically operated by a spring or pneumatic mechanism. As the contact moves downward, the arc is drawn within the chamber, and the pressure generated by the vaporized oil ensures rapid arc extinction.

These breakers are compact, require less maintenance, and are highly effective for medium and high-voltage applications, though they are largely being superseded by SF6 or vacuum circuit breakers.