Air Switches As their name implies, air switches are switches whose contacts are opened and use air to insulate their contacts when current flow is interrupted.

Figure 4-35.- Types of lightning arresters.

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. 

Circuit Reclosers 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 trips again, and after a short interval, recloses the system for the second time. If the fault has not cleared on the third time, the system will open and stay open. The recloser also has a manual lever or electronic control to set the recloser on what is commonly referred to as "single-shot" action. When linemen are working in the general area of a circuit, they place the recloser in the single-shot mode. Then should a mistake be made, causing the circuit to trip, it will not reset itself automatically.

POWER DISTRIBUTION EQUIPMENT The equipment most commonly used for construction and maintenance of Navy power distribution systems consists of the utility truck, aerial bucket truck, earth auger, pole trailer, wire trailer, and various manual lifting devices. 

UTILITY TRUCK The utility truck is a specially designed piece of equipment that, used effectively, provides the lineman with transportation, tool and material storage, and the

Figure 4-38.- Internal and external components of an oil circuit breaker. capability to accomplish most tasks safely, efficiently, and quickly. 

AERIAL BUCKET TRUCK The aerial bucket truck comes in various shapes and sizes. It is used at most naval installations or battalions for maintenance or construction of the power distribution system. The aerial bucket truck provides the power lineman with an efficient, comfortable, and safe working platform and is especially useful in areas where concrete or steel poles are used. The aerial bucket truck has storage bins for material and tools. Depending on the make of the vehicle and the desired use, the truck may have a telescoping single-or double-arm boom, it may be equipped with hydraulic outrigger jacks for stability, and the bucket may be powered by the truck engine or an auxiliary engine mounted on the back of the truck. Because of the various designs of bucket trucks, each one will have some capabilities that are not common to all. When using an aerial bucket truck, you should follow the manufacturer's operation, maintenance, and testing procedures at all times. This promotes smooth operation and extends the usable life of the vehicle. When you are operating the bucket, safety of operation should be your prime concern. Although the bucket is insulated, all personnel should maintain a constant awareness of the hazards of operating in and around high-voltage power lines.