Chapter 10 — SAFETY; TEST EQUIPMENT; TEST METHODS 



letter (R) means the cable supplies electronic 

 equipment; the circuit letters (SK) denote scanning 

 sonar; and number 3 means it is the third cable 

 in the scanning sonar circuit. A complete list of 

 cable designations m.ay be found in chapter 9600 

 of NavShips Technical Manual. 



CABLES 



Insulation resistance tests (ground tests) must 

 be made periodically to determine the cable's 

 condition. Tests also should be made when the 

 cable sustains physical damage, when cables have 

 been disconnected for circuit or equipment 

 changes, after shipyard overhauls, and when the 

 cable has been subjected to oil or saltwater. The 

 500-volt megger is used for making the tests. 

 Following are the recommended test procedures. 



1. Disconnect the cable from the equipment. 



2. Measure the resistance between the cable 

 armor and ground. A zero reading should be 

 obtained. 



3. Connect all conductors in the cable together, 

 and simultaneously measure their resis- 

 tance to ground. If the reading is at or above 

 the acceptable minimum, no other readings 

 need be taken. (Usually 10 megohms indi- 

 cates an acceptable condition, but mini- 

 mums vary according to cable type, length, 

 and temperature; consult the appropriate 

 instruction book for the equipment.) If the 

 reading is below the accepted minimum, the 

 conductors must be separated and tested 

 individually to isolate the faulty conductor. 



RESISTORS 



A resistor is a device used to limit current 

 flow. It is of either the fixed or variable type. 

 Resistors are tested with an ohmmeter after the 

 resistor is disconnected from the circuit. This 

 action prevents damage to the meter from circuit 

 voltage, and ensures that only the resistor being 

 tested gives a reading on the meter. Before con- 

 ducting the test, set the meter to the proper scale, 

 touch the ends of the meter test leads together, 

 and adjust the meter to obtain a zero reading. 



Fixed Resistors 



Fixed resistors may be made of wire, wound 

 on a core and coated with ceramic, or they may 

 be made of carbon. They have either axial or 

 radial leads, and have a wattage rating according 

 to the amount of heat they dissipate. In general. 



Table 10-1.— Standard Color Code for Resistors 



♦Symbol designation alternate for color. 



20.373 



carbon resistors are rated at 1/2 to 2 watts. 

 Wire-wound resistors usually are rated above 2 

 watts. 



Resistance value is indicated by colored mark- 

 ings on the resistor. The standard color code 

 given in table 10-1 is used to interpret the resis- 

 tor m,arkings. With only slight effort you should be 

 able to memorize the color code in a short time. 



A resistor always has three (sometimes four) 

 resistance value indicators. Two methods are used 

 to color code fixed resistors. In figure 10-7, the 

 axial lead type is shown to the left; the radial lead 

 is shown on the right. Resistance value is deter- 

 mined as follows: color A indicates the first sig- 

 nificant figure, color B the second significant 

 figure, color C gives the multiplier (number of 

 zeros), and color D is the percentage of tolerance. 

 If there is no fourth color, the tolerance is 20 

 percent. Assume that an axial lead and a radial 

 lead resistor each has the following color code 

 marking: A— red, B— green, C— orange, and D — 

 gold. Referring to table 10-1, you find that the 

 first significant figure is 2, the second significant 

 figure is 5, and the multiplier is 1000 (three zeros). 

 The resistance value is 25,000 ohms, or 25K as 

 it sometimes is marked on a schematic diagram. 

 The tolerance is 5 percent. 



A fixed resistor is tested by placing an ohm- 

 meter lead on each resistor lead. For the resistor 



161 



