sprinkling systems, flushing systems, machinery- cooling water systems, washdown systems, and other systems as required. The fire-main system is used primarily to supply the fireplug and the sprinkling system; the other uses of the system are secondary. Naval ships have three basic types of fire-main systems: the single-main system, the horizontal- loop system, and the vertical-loop system. The type of fire-main system in any particular ship depends upon the characteristics and functions of the ship. Small ships generally have straight-line, single-main systems. Large ships usually have one of the loop systems or a composite system, which is some combination or variation of the following three basic types: • The single-main fire-main system consists of one main that extends fore and aft. The main is generally installed near the centerline of the ship, extending forward and aft as far as necessary. • The horizontal-loop fire-main system con- sists of two single fore-and-aft, cross-connected mains. The two mains are installed in the same horizontal plane but are separated athwartships as far as practical. • The vertical-loop fire-main system consists of two single fore-and-aft, cross-connected mains. The two mains are separated both horizontally and vertically. As a rule, the lower main is located below the lowest complete watertight deck, and the upper main is located below the highest complete watertight deck. • A composite fire-main system consists of two mains installed on the damage control deck and separated athwartships. A bypass main is installed at the lower level near the centerline. Cross connections are installed alternately between one service main and the bypass main. Communications Equipment Damage control communications are vital to a ship’s survival during emergency conditions. If adequate damage control (DC) communications are not maintained, the entire DC organization could break down rapidly and fail to perform its primary functions. The  following  communication  methods  are used for DC communications: Sound-powered battle telephone circuits Interstation two-way intercoms Ship’s service telephones Ship’s loud speaker or general announcing system Voice tubes Messengers Detailed information on each method can be found in Damage Controlman 3 & 2, NAVED- TRA 10572. Electrical Systems DCPOs perform periodic maintenance (PM) on electrical systems within their division spaces. They must know the location of fuse boxes and circuit breakers that control the circuits. Proper tag out procedures are also required to prevent harm to other crew members while the PM is performed. You should be able to identify the following electrical systems in your areas: • Lighting • Lighting panels • Electrical power outlets • Motors and fans • Casualty power terminals and panels • Battle lanterns Watertight Closures The watertight integrity of a naval ship is established when the ship is built. This original watertight integrity may be reduced or destroyed through enemy action, storm damage, collision, standing, or negligence. As a duty DCPO, you will be responsible for ensuring the proper material condition is set. Later in this chapter we will discuss in depth the material conditions and required reports. 7-6