farther and farther apart, thereby lengthening the arc until it finally breaks.
Air-break switches are usually mounted on substation structures or on poles and are operated from the ground level (fig. 4-35). In a three-phase circuit all three switches. one in each phase. are opened and closed together as a "gang," as the system is called. The switches can be operated by a handle connected to the rod extending from the switch to the base of the pole. Remember, the opening of a three-phase gang switch is a two lineman project. One lineman opens the switch; the other lineman is for safety - he watches to ensure that all phases open. Many of the air rods extending from the air-break switches are operated by mechanized equipment connected to the switch- operating rod from a remote location. The automation ofdistribution circuits has resulted in the installation of many mechanized operators for key break switches so that they can be controlled from a central operations center. Switches for underground distribution circuits usually are installed in pad-mounted switchgear. The switches are operated with the cabinet doors closed to provide protection for the lineman or cableman.
Disconnect Switch. - A disconnect switch is an air-break switch not equipped with arcing horns or other load-break devices. The disconnect switch cannot be opened until the circuit in which it is connected is interrupted by. some other means, such as a portable load break tool attached to a hot-line tool. If a disconnect switch is opened while current is flowing in the line. an arc is likely to be drawn between the blade and the stationary contact where the arc might easily, jump across to the other conductor or to some grounded metal and cause a short circuit. The hot arc also could melt part of the metal. thereby, damaging the switch.
Figure 1-35. - Gang-operated three-pole air-break switch with arc interrupters (in the open position).
Disconnect switches are used to complete a connection to or isolate the following:
1. Two energized transmission or distribution lines
2. Transmission or distribution lines from substation equipment
3. Substation equipment
4. A distribution feeder circuit and a branchcircuit
Disconnect switches are frequently used to isolate a line or an apparatus, such as a transformer. to complete maintenance work. In most circumstances, it is necessary to test the equipment for high voltage and, if proved de-energized, to ground it before the maintenance work is performed.
OIL SWITCH. - An oil switch is a high-voltage switch whose contacts are opened and closed in oil. The switch is actually immersed in an oil bath and contained in a steel tank. The reason for placing high- voltage switches in oil is that the oil will 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. the oil will quench the arc. Furthermore. if an arc should form in the oil. it will evaporate part of the oil because of the high temperature and will partially fill the interrupters surrounding the switch contacts with vaporized oil. This vapor develops a pressure in the interrupters which assists in quenching or breaking the arc by elongating the arc.
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 (fig. 3-36): but if the voltage of the line is high. the three poles of the switch are placed in separate oil tanks. The poles are placed in separate tanks to make it impossible for an arc to form between any two phases when the switch is opened or closed. An arc between phases would be a short circuit across the line and would probably blow up the tank.
When an oil switch is to open the circuit automatically because of an overload or short circuit. it is provided with a trip coil. This trip coil consists of a coil of wire and a movable plunger. In low-voltage circuits carrying small currents. this coil is connected in series with the line. When the current exceeds its permissible value. the coil pulls up its plunger. The plunger trips the mechanism, and a spring opens the switch suddenly.Continue Reading