Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGmust be manually shut down. Hence, the emergencyswitchboard and distribution system arealways energized either by a ship's servicegenerator or by the emergency generator.Therefore, the emergency distribution systemcan always supply power to a vital load if boththe normal and the alternate sources of theship's service power to this load fail. Theemergency generator is not started if the emergencyswitchboard can receive power from aship's service generator.A feedback tie from the emergency switchboardto the ship's service switchboard (fig.20-24) is provided on most ships. The feedbacktie permits a selected portion of the ship'sservice switchboard load to be supplied fromthe emergency generator. This feature facilitatesstarting up the machinery after majoralterations and repairs and provides power tooperate necessary auxiliaries and lighting duringrepair periods when shore power and ship'sservice power are not available.CASUALTY POWERThe casualty power distribution system isprovided for making temporary connections tosupply electric power to certain vital auxiliariesif the permanently installed ship's service andemergency distribution systems are damaged.The casualty power system is not intended tosupply power to all the electrical equipment inthe ship but is confined to the facilities necessaryto keep the ship afloat and to get it awayfrom a danger area. The system also supplies alimited amount of armament, such as antiaircraftguns and their directors, that may benecessary to protect the ship when in a damagedcondition. The casualty power system for riggingtemporary circuits is separate and distinctfrom the electrical damage control equipment,which consists of tools and appliances for cuttingcables and making splices for temporaryrepairs to thepermanently installed ship's serviceand emergency distribution systems.The casualty power system includes portablecables, bulkhead terminals, risers, switchboardterminals, and portable switches. Portablecables in suitable lengths are stowed in convenientlocations throughout the ship. The bulkheadterminals are installed in watertightbulkheads so that the horizontal runs of cablescan be connected on the opposite sides to transmitpower through the bulkheads without the lossof watertight integrity. The risers are permanentlyinstalled vertical cables for transmittingpower through the decks without impairingthe watertight integrity of the ship. A riserconsists of a cable that extends from one deckto another with a riser terminal connected toeach end for attaching portable cables.CONTROL AND SAFETY DEVICESThe distribution of electric power requiresthe use of many devices to control the currentand to protect the circuits and equipment.Control devices are those electrical accessorieswhich govern, in some predeterminedway, the power delivered to any electrical load.In its simplest form, the control applies voltageto (or removes it from) a single load. In morecomplex control systems, the initial switch mayset into action other control devices that governthe motor speeds, the compartment temperatures,the depth of liquid in a tank, the aimingand firing of guns, or the direction of guidedmissiles.Switchboards make use of hand-operated(manual) switches as well as electrically operatedcontrols. Manually operated switches arethose familiar electrical items which can beoperated by motions of the hand, as with apushing, pulling, or twisting motion. The typeof action required to operate the manuallyoperated switch is indicated by the names ofthe controls—push-button switch, pull-chainswitch, or rotary switch.Automatic switches are devices which performtheir function of control through therepeated closing and opening of their contacts,without requiring a human operator. Limitswitches and float switches are representativeautomatic switches.The Navy uses many different types ofswitches and controllers, which range from thevery simple to the very complex. A typical a-cacross-the-line magnetic controller is shownin figure 20-25.The simplest protective device is a fuse,consisting of a metal alloy strip or wire andterminals for electrically connecting the fuseinto the circuit. The most important characteristicof a fuse is its current-versus-timeor "blowing" ability. Three time ranges forexistence of overloads can be broadly definedas fast (5 microseconds through 1/2 second),medium (1/2 second to 5 seconds), and delayed(5 to 25 seconds).514