PRINCIPLES OF NAVAL ENGINEERING 



must be manually shut down. Hence, the emer- 

 gency switchboard and distribution system are 

 always energized either by a ship's service 

 generator or by the emergency generator. 

 Therefore, the emergency distribution system 

 can always supply power to a vital load if both 

 the normal and the alternate sources of the 

 ship's service power to this load fail. The 

 emergency generator is not started if the emer- 

 gency switchboard can receive power from a 

 ship's service generator. 



A feedback tie from the emergency switch- 

 board to the ship's service switchboard (fig. 

 20-24) is provided on most ships. The feedback 

 tie permits a selected portion of the ship's 

 service switchboard load to be supplied from 

 the emergency generator. This feature facili- 

 tates starting up the machinery after major 

 alterations and repairs and provides power to 

 operate necessary auxiliaries and lighting dur- 

 ing repair periods when shore power and ship's 

 service power are not available. 



CASUALTY POWER 



The casualty power distribution system is 

 provided for making temporary connections to 

 supply electric power to certain vital auxiliaries 

 if the permanently installed ship's service and 

 emergency distribution systems are damaged. 

 The casualty power system is not intended to 

 supply power to all the electrical equipment in 

 the ship but is confined to the facilities neces- 

 sary to keep the ship afloat and to get it away 

 from a danger area. The system also supplies a 

 limited amount of armament, such as antiair- 

 craft guns and their directors, that may be 

 necessary to protect the ship when in a damaged 

 condition. The casualty power system for rig- 

 ging temporary circuits is separate and distinct 

 from the electrical damage control equipment, 

 which consists of tools and appliances for cut- 

 ting cables and making splices for temporary 

 repairs to thepermanently installed ship's serv- 

 ice and emergency distribution systems. 



The casualty power system includes portable 

 cables, bulkhead terminals, risers, switchboard 

 terminals, and portable switches. Portable 

 cables in suitable lengths are stowed in con- 

 venient locations throughout the ship. The bulk- 

 head terminals are installed in watertight 

 bulkheads so that the horizontal runs of cables 

 can be connected on the opposite sides to trans- 

 mit power through the bulkheads without the loss 



of watertight integrity. The risers are per- 

 manently installed vertical cables for trans- 

 mitting power through the decks without impair- 

 ing the watertight integrity of the ship. A riser 

 consists of a cable that extends from one deck 

 to another with a riser terminal connected to 

 each end for attaching portable cables. 



CONTROL AND SAFETY DEVICES 



The distribution of electric power requires 

 the use of many devices to control the current 

 and to protect the circuits and equipment. 



Control devices are those electrical acces- 

 sories which govern, in some predetermined 

 way, the power delivered to any electrical load. 

 In its simplest form, the control applies voltage 

 to (or removes it from) a single load. In more 

 complex control systems, the initial switch may 

 set into action other control devices that govern 

 the motor speeds, the compartment tempera- 

 tures, the depth of liquid in a tank, the aiming 

 and firing of guns, or the direction of guided 

 missiles. 



Switchboards make use of hand-operated 

 (manual) switches as well as electrically oper- 

 ated controls. Manually operated switches are 

 those familiar electrical items which can be 

 operated by motions of the hand, as with a 

 pushing, pulling, or twisting motion. The type 

 of action required to operate the manually 

 operated switch is indicated by the names of 

 the controls— push-button switch, pull-chain 

 switch, or rotary switch. 



Automatic switches are devices which per- 

 form their function of control through the 

 repeated closing and opening of their contacts, 

 without requiring a human operator. Limit 

 switches and float switches are representative 

 automatic switches. 



The Navy uses many different types of 

 switches and controllers, which range from the 

 very simple to the very complex. A typical a-c 

 across-the-line magnetic controller is shown 

 in figure 20-25. 



The simplest protective device is a fuse, 

 consisting of a metal alloy strip or wire and 

 terminals for electrically connecting the fuse 

 into the circuit. The most important charac- 

 teristic of a fuse is its current-versus-time 

 or "blowing" ability. Three time ranges for 

 existence of overloads can be broadly defined 

 as fast (5 microseconds through 1/2 second), 

 medium (1/2 second to 5 seconds), and delayed 

 (5 to 25 seconds). 



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