LEARNING OBJECTIVE: Recall general information concerning electrical and electronic system maintenance and repair.
The failure of a circuit to function properly usually is caused by a break in the circuit (open), a short circuit (a circuit that permits current to bypass a part of the circuit), or a grounded circuit (an undesired path of current).
Any of these faults affects the current and voltage values and causes the circuit to function improperly.
Open circuits may result from dirty or loose connections, improperly installed wire, mechanical damage, faulty installing or repair, and vibration. If connections are clean and tight, no resistance is added to the circuit.
Short circuits are low-resistance paths or shortcuts that cause the current to bypass the load. The current from the source passes through the short instead of the load, causing the load to function improperly. Most shorts are accidental. They occur when vibration wears away the insulation, when salt water gets into connection boxes, when heat melts away insulation, or when carelessness brings two conductors together.
A grounded circuit is one in which one side of the path is connected to ground either intentionally or accidentally. An intentionally grounded circuit uses a ground that is the hull of the ship, equipment chassis, and so forth, as one side of the line or one conductor. If the "hot side" conductor of a grounded source touches ground accidentally, a short circuit results. Power circuits in the Navy are not grounded and must be insulated from ground at all times. One side of this circuit may be grounded accidental y and no harm will result, but if both sides are grounded, a short circuit is the result. An ungrounded circuit has a safety feature. If anyone accidentally touches one side of an ungrounded circuit, there will be no path for current flow through the body to the other side of the source. This danger is one reason why power circuits in the Navy are insulated from ground.
TYPES OF CIRCUIT CHECKS
There are three basic circuit checks used to locate shorts, grounds, and open circuits within electric and electronic equipment: (1) voltage (volt) checks, (2) current (amperes) checks, and (3) resistance (ohms) checks.
Voltage checks reveal the amount of potential force present to move electrons in a circuit.
Current checks show the actual amount of current flowing through the circuit.
Resistance checks tell the resistance characteristics of the circuit; that is, how much opposition the circuit offers to the flow of current.
With the proper use of test equipment, failures in electric and electronic circuits can be detected and isolated to specific components by using one of these three checks.
Since volts, amperes, and ohms are units of electrical measure (Ohm's law is explained in NEETS, Module 1), some measuring device must be used to measure them. One device used for this purpose is the multimeter. The Simpson AN/PSM-4A and 260 series are the most commonly used types of multimeters. The specific operating procedures for these meters are contained in the manufacturer's manual. Multimeters and other test equipment are also covered in NEETS, Module 16, Introduction to Test Equipment, NAVEDTRA 172-16-00-84.
GROUND DETECTION INDICATOR
Another type of current measuring device you should be familiar with is the ground detection indicator. A ground detection indicator is a continuous monitoring system designed to detect a first ground. Because the power distribution and control circuits are powered by ungrounded currents, the first ground will not always cause a malfunction. However, if the first ground is not discovered and a second develops, the result could be an illogical and destructive sequence of gun mount operation. The reason for this action is that the two grounds act as an electrical conductor between the grounded components. A ground detection indicator will produce audio and visual indications if a gun system develops a short or ground.
For example, if a ground were to occur in the 5"/54 automatic loading system and it was not corrected, perhaps nothing would happen. Operations might be normal. If a second ground develops, however, and some safety interlocks are energized due to the grounded circuit, the equipment could be damaged and personnel might be injured. The elimination of grounds and potential grounds is preventive maintenance and the responsibility of the GM.
When the ground detector indicator indicates that some part of the gun system is grounded, the GM must determine the location and cause of the ground by using a multimeter. Ground detection indicators are being installed on all new gun and missile systems.