The following logic inputs are involved with the two-step modes of control used to re-extend the launcher rail:

1. QCX4-1-Step load selected

2. QCX6-1-Step exercise selected

3. SIR1-1-Launcher rail loaded

4. SIE14-2-Not elevation not at load position (90 degrees)

5. SIR1-2-Not launcher rail loaded

6. SIA2-1-Arming device retracted and forward-motion latch locked

7. SML2-1-Launcher rail extend push button depressed

The remainder of the circuit is the same as the auto-not-unload signal path.

Solid-State Control Circuit-Summary

The solid-state circuits we studied in figures 5-78 and 5-79 were relatively simple in nature. But, again, they are not "hard" if you do the following:

1. Understand the sequence of equipment operation.

2. Understand how solid-state devices function (and use the truth tables).

3. Practice to develop your skills.

SHIP'S POWER AND DISTRIBUTION

LEARNING OBJECTIVE: Recall general information concerning ships power distribution and safety precautions for working on and around electrical or electronic equipment.

As with compressed air, the electric power you need to run your equipment is generated and controlled by

the ships engineers. Ships power is referred to as either 60 or 400 cycle (or Hz). All electrical power of the ship begins as the 440 volt, 60-cycle, three-phase output of the ships generators. This output, as shown in figure 5-80, is directed through the main switchboards of the ship to the various sections of the ship. Gun mount power drive motors normally operate off this power. Transformers are used to regulate some of the output to 115-volt, 60 Hz ac for lighting and other common uses. Other circuits convert some of the output to 400 Hz. Gun mounts use 400 Hz ships power to operate the synchros in positioning equipment, fuze setters, and sights.

Gun mounts normally have more than one source of electrical power. Both the primary and secondary sources are made available to the equipment through a large switch called an automatic bus transfer (ABT). The ABT powers the equipment from the primary source as long as it (power) is available. Should the primary source be interrupted for any reason, the ABT switches to the secondary source. Some gun maintenance will require the power supply to be secured from this switch box.

It is beyond the scope of this text to explain the details of electric power generation and distribution. Our intent is to give you the fundamental knowledge of its existence and impact on your equipment. For further information about power generation and distribution, refer to the Navy Electricity and Electronics Training Series (NEETS), Module 5, Introduction to Generators and Motors, NAVEDTRA 172-05-00-79, and ships drawings.

SAFETY

Some common safety features of electrical or electronic equipment are interlock switches, bleeder resistors, current-limiting resistors, insulating controls, and power line safety devices, such as fuses. Keep in mind that these features cannot always be relied upon to function. Do not develop a false sense of security just because an equipment has safety features. The following list of safety precautions will help prevent electric shock or bums when working on or near electrical or electronic equipment.

1. Do not block high-voltage protective cutouts on doors or covers to keep the circuit energized with the cover off. It is intended that work be performed on such electrical equipment while the circuit is de-energized.

2. Always ground the provided ground lead located at the plug of portable tools, such as electric

Figure 5-80.-Shipboard power distribution on a FFG-7 class ship.

drills, to protect yourself from shock in case a ground occurs within the tool.

3. Always be sure all condensers (capacitors) are fully discharged before commencing work on a de-energized high-voltage circuit. Use an insulated shorting bar for this purpose.

4. Tag the switch OPEN (open the switch and place

a tag on it stating "This circuit was ordered open for repairs and will not be closed except by direct order of .") at the switchboard supplying power to the circuit you will be working on. When possible, remove the fuses protecting the circuit and place them in your toolbox for safekeeping until the job is complete.

5. Always remove fuses with fuse pullers, and never remove fuses until after opening the switch connecting the circuit to the source of supply. Never replace a fuse with one larger than the circuit is designed to handle.

6. Observe utmost caution when inspecting behind an open-back switchboard in an energized state.

7. Never use an incandescent test lamp unless its voltage rating is greater than the highest voltage that may be tested.

8. Always test a supposedly de-energized circuit with a voltage tester before commencing work on the circuit.

9. Never work on an electric circuit or network without first thoroughly acquainting yourself with its arrangement and with its points of power feed.

Electric shock may cause instant death or may cause unconsciousness, cessation of breathing, and burns. If a 60 Hz alternating current is passed through a person from hand to hand or head to foot, the effects when current is gradually increased from zero are as follows:

1. At about 1 milliampere (0.001 ampere) the shock can be felt.

2. At about 10 milliamperes (0.010 ampere) the shock is severe enough to paralyze muscles so that a person is unable to release the conductor.

3. At about 100 milliamperes (0.100 ampere) the shock is fatal if it lasts for 1 second or more.

It is important to remember that current is the shock factor, rather than the amount of voltage.

You should clearly understand that the resistance of the human body is not great enough to prevent fatal shock from a voltage even as low as 115 volts. In many cases, voltages less than 115 volts are fatal. When the skin is dry, it has a high resistance. The resistance may be high enough to protect a person from fatal shock even if one hand touches a high voltage while another part of the body touches the chassis or another ground.

Contact resistance decreases when the skin is moist and body resistance may drop to as low as 300 ohms.

With this low resistance, a very low voltage could supply enough current to cause death.

SUMMARY

In this chapter we have looked at some of the components that make up electronic gun control circuits. We have also examined the predominate types of control circuits used to control gun system operation-transistorized circuits and logic circuits. We described the basics of circuit operation and the procedures for troubleshooting when circuits fail. We described some of the fundamentals of synchros and some general circuit failure principles. The Mk 75, Mk 45, and the Mk 13 control systems were only briefly described. Finally, we addressed some common safety precautions for working with electricity. Remember, you should refer to the publications cited within the chapter for additional specific information on individual subject areas.