An air-circuit breaker switch can have both blade and stationary contacts equipped with arcing horns (fig. 4-36). These horns are pieces of metal between which the arc forms when a circuit-carrying current is opened. These arc horns are drawn further and further apart until the arc finally breaks. Air-break switches are usually mounted on substation structures or on poles and are operated manually from the ground. In a three-phase circuit all three switches - one for each phase - are opened and closed together.
An air-disconnect switch is not equipped with arcing horns or other load-break devices. It therefore cannot be opened while current is flowing (fig. 4-37). If the disconnect switch should be opened while current is flowing in the line, an arc would likely be drawn between the blade and its stationary contacts. The hot arc would melt part of the metal, thereby damaging the switch. The purpose of a disconnect switch is to isolate a line or a piece of equipment for the purpose of making the disconnected line or equipment dead electrically, thus making it safe for repairs, tests, or inspections.
Figure 4-36. - Gang-operated air-circuit breaker.
Figure 4-37. - Air-disconnect switch.
Oil Switches An oil switch is a high-voltage switch whose contacts are opened and closed in oil. Oil switches may be used as disconnect, circuit breakers, or reclosers. The switch is actually immersed in an oil bath, contained in a steel tank, as shown in figure 4-38. The reason for placing high-voltage switches in oil is that the oil may help to break the circuit when the switch is opened. With high voltages, a separation of the switch contacts does not always break the current flow, because an electric arc forms between the contacts. If the contacts are opened in oil, however, the oil helps to quench the arc. Oil is an insulator and, therefore, helps to quench the arc between the contacts. The three lines of a three-phase circuit can be opened and closed by a single oil switch. If the voltage is not extremely high, the three poles of the switch are generally in the same tank. But if the voltage of the line is high, the three poles of the switch are placed in separate containers.
The circuit reclosers most commonly used in power distribution are electronic reclosers, oil reclosers, or vacuum reclosers. These reclosers basically operate in the same manner.
Reclosers come in single- or three-phase models and can either be pole-mounted or installed in a substation. These reclosers are for overload protection and are designed to open a circuit in an overload condition and then automatically reclose the circuit. If the fault on the system has cleared, the recloser remains closed. If the fault has not cleared, the recloser tripsContinue Reading