Device or switch used for controlling the flow of current in an electrical circuit
Circuit Breaker
True/False
Circuit breakers must be optimized for two opposing duties.
True
True/False
When the switch (CB) is closed, it must carry current without significant loss; when it is open, it must insulate without
significant leakage.
True
An electrically conductive gas formed by ionization.
Plasma
An air gap of one centimeter can withstand about volts at normal atmospheric pressure and temperature
30,000
True/False
Air is ionized and turned into plasma at almost 5000°C
True
Cooling is effectively used in high-voltage circuit breakers because the rate at which an arc column loses heat is proportional to its cross section; the thinner the column is, the faster it loses heat.
inversely
Circuit breakers that employ air as the interrupting medium are of two general types: the breaker and the breaker (ACBs
magnetic, air-blast
True/False
All three phases of the Magnetic CB are separated from each other and electrical ground by box type barriers of insulating material
True
True/False
Magnetic circuit breakers are typically installed in outdoor stations
False - Indoor
The advantage of magnetic ACBs is that air (not under high pressure) is used for high current
interrupting
The disadvantages of magnetic ACBs are that they are and are to maintain than OCBs because of the auxiliary equipment required.
noisy, more expensive
circuit breakers are the most effective type of CB for interrupting large (high) amounts of current
Air-blast
Typically in air-blast circuit breakers, such as the G.E., a single pair of air-blast contacts can interrupt a circuit of some
100kV
Oil circuit breaker contacts are submerged in high-grade .
mineral oil
How is the arc cooled in an oil CB
The contact separate , then an arc is developed in the oil
The heat of the arc evaporates the surrounding oil and breaks it down into carbon and s substantial volume of Hydrogen at high pressure
The expanding hydrogen cools and extinguishes the arc
What the advantages and dis-advantages of OCBs
They are dependable and rugged
They are heavy do to the weight of the oil
They must be kept free of moisture and foreign substances
Carbon produced by arcing is very detrimental to the dielectric strength of oil
Oil becomes flammable when heated beyond its flashpoint
Oil is potentially hazardous to the environment
Oil circuit breakers are used on the DWP Power System on voltages ranging from to
4.8kV, 500kV
The contacts of vacuum breakers are typically made of
copper-tungsten
True/False
In air-blast circuit breakers, the arc is extinguished by a blast of air moving through a nozzle at supersonic velocity
True
How do SF6 Circuit Breakers extinguish an ARC
The sulfur hexafluoride molecule, SF6 readily acquires any free electron, forming a heavy negative ion. This tends to remove free electrons and thus prevents reignition of the arc after it had lost its conductivity.
How do Vacuum Circuit Breakers extinguish an ARC
The vaporized particles fly almost unimpeded to the walls of the chamber. So when current rises from zero, there are virtually no particles present of any kind to be ionized and to provide a pathway between the open contacts
How do Magnetic circuit breakers extinguish the arc
As the contacts separate, the arc initially strikes across the shortest distance between the horns but it is then driven steadily outward by the perpendicular magnetic field until it extends from one tip to the other when the horns are fully separated. Finally the arc is pushed away from the horns, and
thereafter it can be lengthened in open air or in an arc chamber, where it can be split up into many short segments by metal plates
