SF6 Circuit Breaker and its maintenence

First of all what is a Circuit Breaker?

A CB is an element which automatically opens circuit under fault conditions in order to prevent damage to the equipment.

Principle of CB:

It consists of two electrodes which are fixed and movable. Whenever Fault occurs the trip coils of the CB get energised and the movable contacts are pulled apart, which will open the circuit. But under normal conditions these contacts remain closed.

SF6 Circuit Breaker

It is basically a suplhur Hexaflouride Circuit Breaker and this SF6  gas is used for arc quenching mechanism. SF6 is electronegative gas and has high tendency to absorb electrons. (arc mechanism will be covered later)

Due to its electronegativity the low arc time constant , the SF6 gas regains its dielectric strength rapidly after final current zero, the rate of rise of dielectric strength is very   high and time constant is very small.

In SF6 CB, the gas is made to flow from high pressure zone to low pressure zone through a convergent-divergent nozzle.The mass flow is a function of nozzle throat diameter, the pressure ratio and time of flow. The nozzle is located such that flow of gas covers the arc. The gas flow attains almost supersonic speed in the divergent portion of the nozzle, thereby the gas takes away the heat from the periphery of the arc, causing reduction in the diameter of arc. Finally the arc diameter becomes almost zero at the current zero and the arc is extinguished. The arc space is filled with fresh SF6 gas and the dielectric strength of the constant space is rapidly recovered due to the electronegativity of the gas.

Image Source: Google Images

Handling Non faulted SF6

The procedures for handling nonfaulted SF6 are well covered in manufacturer’s instruction books. These procedures normally consist of removing the SF6 from the circuit breaker, filtering and storing it in a gas cart as a liquid, and transferring it back to the circuit breaker after the circuit breaker maintenance has been performed.No special dress or precautions are required when handling nonfaulted SF6.

Handling Faulted SF6

Toxicity

Faulted SF6 GAS- Faulted SF6 gas smells like rotten eggs and can cause nausea and minor irritation of the eyes and upper respiratory tract. Normally, faulted SF6 gas is so foul smelling no one can stand exposure long enough at a concentration high enough to cause permanent damage.

Solid Arc Products– Solid arc products are toxic and are a white or off-white, ashlike powder. Contact with the skin may cause an irritation or possible painful fluoride burn. If solid arc products come in contact with the skin, wash immediately with a large amount of water. If water is not available, vacuum off arc products with a vacuum cleaner.

Normally, at least once a year or after every 500 operations, the circuit breaker must be maintained. During

maintenance, the moving parts of the mechanism must be lubricated carefully. The insulating parts are to be wiped

out by a clean and dry cloth. When maintaining, the circuit breaker should be open and high voltage sides must be

grounded. Auxiliary power supply should also be disconnected. On saline areas near seaside, the insulating parts

of the circuit breaker must be carefully cleaned, at least once every two months. If not, the microscopic salt particles

drawn by wind from the sea will create conductive layers on the insulating surfaces and may cause surface flashover.

Before maintenance, first circuit breaker, then isolator should be opened and grounded carefully. The maintenance

of circuit breaker must be done after checking the open position of isolator contacts by eye.