As we discussed in substations and transformers lecture, transmission lines are usually branched out into several outgoing lines and connect with other substations. We also talked about interconnections to form large reliable networks and the role of switchgear in these interconnections and branching. The switchgear components are; busbars, switches, circuit breakers, current and voltage transformers, surge arresters, grounding switches. These components are rated in accordance with relevant requirements, as well as expected mechanical and electrical loads. Because modern systems are predominantly remote controlled, they are accompanied by additional facilities for control and monitoring. Besides measuring and calculating the quantities of energy supplied to consumers, these systems also incorporate mechanisms, providing protection against overvoltage, overcurrent, and short-circuit. At the core of any switchgear is the busbar. Designed usually as a short open strip, the busbar can also take the form of a conduit called internally by oil if very high currents need to be conducted. Serving as a network node in physical terms, the busbar constitutes the start and the end of the individual lines, which are called branches similar to tree branches. The branches can be connected and disconnected via switching devices called circuit breakers. A circuit breaker is an automatically operated electrical device designed to protect and the electrical circuit from damage caused by excess current typically resulting from an overload or short circuit. Its basic function is to interrupt current flow after a fault is detected. Unlike a fuse, which operates once and then must be replaced, a circuit breaker can be reset either manually or automatically to resume normal operation. Circuit breakers are made in varying sizes, from small devices that protect low current circuits or individual household appliance, up to large switchgear designed to protect high voltage circuits feeding an entire city. The generic function of a circuit breaker or a fuse is to automatically remove power from a faulty system. When switching device disconnects a circuit under load or even at no load, but with voltage present, an arc will appear between the contacts. The higher the voltage or current, the bigger is the arc. The arc is very dangerous for personnel and equipment and must be extinguished. At low voltages, the arc is de-energized in the air between the contacts. Circuit breakers are available for low voltage, medium voltage, and high voltage applications. Low voltage, less than 1,000 volts, these types are common in domestic, commercial, and industrial applications. Medium voltage circuit breakers rated between 1 and 72 kilovolts, maybe assembled into metal enclosed switchgear lineups for indoor use or maybe individual components installed outdoors in a substation. Electrical power transmission networks are protected and controlled by high voltage breakers. They are usually 69 kilovolts or higher. High voltage breakers are nearly always solenoid-operated, with current sensing protective relays operated through current transformers. In substations, the protective relay scheme can be complex, protecting equipment and buses from various types of overload and faults. High voltage breakers are broadly classified by the medium used to extinguish the arc; oil, air blast, vacuum, SF6, CO2. Due to environmental and cost concerns, overinsulating oil spills, most new breakers use SF6 gas to quench the arc.