Figure 4-29. - Capacitors, open-rack installation.

Experience has shown that the following schedule normally will keep the equipment operating properly:

TYPE CONTROL YEARS

Time clock 3

Voltage 3

Dual temperature 5

Temperature only 8

Time clock and temperature 8

The capacitors switches usually are removed from the distribution line by the lineman and replaced with a spare unit during the season in which they normally are not operated. The capacitor switches can be maintained efficiently in a distribution shop by the lineman.

Protective/Interrupting Devices

A power distribution circuit, like any other electrical circuit, requires the use of special devices to provide control and to protect the system from internal or external influences that may damage the circuit. Overcurrent/overvoltage protection and personnel safety, requirements are provided in a power distribution system by the use of lightning arresters, cutouts, fuses, air switches, and oil switches.

LIGHTNING OR SURGE ARRESTERS. - A surge arrester is a device that prevents high voltages, by providing a low-impedance path to ground for the current from lightning or transient voltages. and then restores normal circuit conditions.

Surge arresters perform a function on the electric system similar to that of a safety valve on a steam boiler. A safety valve on a boiler relieves high pressure by blowing off steam until the pressure is reduced to normal. When the pressure is reduced to normal, the safety valve closes and is ready for the next abnormal condition. When a high voltage (greater than the normal line voltage) exists on the line, the arrester immediately furnishes a path to ground and thus limits and drains off the excess voltage. Furthermore, when the excess voltage is relieved, the action of the arrester must prevent any further flow of power current. The function of a surge arrester is, therefore, twofold-first, to provide a point in the circuit at which the overvoltage impulse can pass to earth without injury to line insulators, transformers, or other connected equipment and, second, to prevent any follow-up power current from flowing to ground.

DISTRIBUTION CUTOUT. - A distribution cutout provides a high-voltage mounting for the fuse element to protect the distribution system or the equipment connected to the system (fig. 4-30). Distribution cutouts are used with installations of