Chapter 10 -SAFETY; TEST EQUIPMENT; TEST METHODS 



moisture-soaked insulation tliat provides current 

 a leakage path to ground. 



Usually, a ground is indicated by a blown fuse 

 or tripped circuit breaker. A high resistance 

 ground, however, may occur where insufficient 

 current can flow to rupture the fuse or open the 

 circuit breaker. 



An ohmmeter is used in testing for grounds. 

 By measuring the resistance to ground at any 

 point in a circuit, it is possible to determine if 

 that point is grounded. Before conducting this test, 

 it is necessary to study the associated wiring dia- 

 grams for the presence of intentional grounds. 

 These grounds m-jst be disconnected before the 

 circuit can be tested accurately. A zero, or very 

 low, reading indicates the circuit is grounded. 



(a-c or d-c) to be measured and that the scale 

 setting is set to the proper range. Because defec- 

 tive elements may cause higher-than-normal 

 voltages to be present in the circuit, the highest 

 voltmeter range should be used first. Once a read- 

 ing is indicated, adjust the voltmeter scale pro- 

 gressively downward until the lowest scale 

 appropriate to the actual voltage may be used. 

 This system gives the most accurate measure- 

 ment and avoids damage to the meter movement. 

 Many schematics indicate proper voltages at 

 various test points. If a certain stage is suspected, 

 the voltage can be measured by placing the test 

 leads at the designated test points. The voltage 

 reading obtained by the meter should be the same 

 as that given on the schematic. 



Shorts 



A short circuit is similar to a grounded one, 

 except that it is between two or more conductors 

 and usually has a greater tendency to blow fuses. 

 Two conductors with frayed insulation may touch 

 each other, short-circuiting the current path. Too 

 much solder on the pin of a connector may cause 

 a short circuit to an adjacent pin. Again, the test 

 instrument used is the ohmmeter. Isolate the two 

 suspected circuits and connect the meter to them. 

 A high resistance reading indicates the circuits 

 are satisfactory. 



Shorts may occur in conductors besides those 

 in cables or in the wiring in chassis. Components 

 such as transformers, motor windings, and capa- 

 citors — to name a few — are also susceptible to 

 short circuits. These components should be 

 checked for a resistance reading, and this reading 

 compared with the figure given for the component 

 in the schematic or equipment technical manual. 



VOLTAGE 



CURRENT 



Current, expressed in amperes, is measured 

 in much the same way as voltage except that the 

 meter must be connected in series in the circuit 

 being tested. The ammeter is a low-resistance 

 instrument because, if it offered more than a 

 m'nute amount of resistance, it would reduce the 

 amount of current flow in the circuit and result 

 in an erroneous measurement. 



When the ammeter is connected in the circuit, 

 it is used in the manner prescribed for the volt- 

 meter. In other words, start with the higher scale 

 and progressively adjust the scale used untU a 

 reading is obtained on the lowest scale that gives 

 a meter indication. 



If a circuit is disconnected or opened to insert 

 the meter for a measurement, be sure and restore 

 that part of the circuit before further testing. 



Because of the time and effort expended in 

 connecting an ammeter into a circuit, it usually is 

 more advantageous to measure the voltage across 

 a resistor of known value and calculate the current 

 by using Ohm's law. 



The voltage test is accomplished only with 

 power applied to the equipment. Prescribed safety 

 precautions must be carefully observed, there- 

 fore, to prevent injury to personnel or damage to 

 the equipment. The voltage test is used not only 

 in isolating casualties to major units, but also 

 in determining malfunctions of subassemblies and 

 circuits. Before measuring the voltage of a cir- 

 cuit, a test should be made to ascertain that the 

 power source is supplying the normal voltage 

 required by the circuit being tested. In using the 

 voltmeter for this test, m,ake certain that the 

 meter used is adjusted for the type of voltage 



RESISTANCE 



Usually the resistance of a circuit, portion of a 

 circuit, or circuit element is measured with the 

 ohmmeter. First, power must be secured to the 

 circuit and the meter zeroed before connecting 

 the meter into the circuit. 



Selection of the proper scale is an Important 

 phase in making resistance measurements. To 

 check a circuit of low resistance, use the lower 

 range of the ohmmeter. If a high range is used, the 

 meter may indicate zero, even though some resist- 

 ance is present in the circuit. Conversely, to 



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