Chapter 10 — SAFETY; TEST EQUIPMENT; TEST METHODS 



back-pressure, arm-lift method. Detailed infor- 

 mation on administering ai'tificial respiration can 

 be found in St andard Fir st Aid Training Coarse, 

 NavPers 10081, and in Handbook of Test Methods 

 and Practices, section 1. Whichever method you 

 use, the important objective is to start immedi- 

 ately. 



TEST EQUIPMENT 



Electrical equipment is designed to operate 

 at certain efficiency levels. Technical instruc- 

 tion books and sheets containing optimum per- 

 formance data — such as voltages and resistances 

 — are prepared for each Navy equipment. The 

 instructions are intended to aid the technician 

 in maintaining the equipment. 



As a Sonar Technician, you will work with 

 ammeters, voltmeters, ohmmeters, and electron 

 tube analyzers. You also may have occasion to 

 use such equipment as wattmeters, power factor 

 meters, capacitance-resistance-inductance 

 bridges, oscilloscopes, signal generators, and 

 other types of test equipment. Basic movements 

 and construction of measuring devices are dis- 

 cussed in Basic Electricity. 



GROUNDING TEST EQUIPMENT 



Equipment that measures electrical values of 

 component parts must never assume a ground 

 level different from that of the chassis of the 

 component. One reason is that measurement of 

 an electrical value of a component, by an equip- 

 ment having its own ground level value, would 

 possibly reflect the difference in potential be- 

 tween the component and the test equipment. 

 The other reason is that if the ground level of 

 the test equipment differs from that of the com- 

 ponent being measured, a condition of electrical 

 hazard may result, and maintenance personnel 

 may be shocked. For the ground levels of the test 

 equipment and component parts to be the same, 

 all electric and electronic test equipment must 

 be grounded. Self-contained test equipment, such 

 as multimeters, have two test probes and one 

 grounding cable. When the grounding cable is 

 attached to the component being tested, it is as 

 though the test equipment were part of the chassis 

 of the component. Touching the test equipment 

 in one hand and the chassis of the component in 

 the other does not, in itself, result in electric 

 shock if the grounding cable is attached securely. 



Externally powered equipment, such as a tube 

 tester, is grounded to the frame of the ship 

 through a standard three-wire electrical cord 



and plug. A firm connection to the power recep- 

 tacle is all that is needed to ground such equip- 

 ment so that it will be safe to the touch. Other 

 equipment— perhaps older or repaired test equip- 

 ment— may not be equipped with the three-wire 

 cord. If it is not so equipped, the test equipment 

 should be grounded, with a separate cable running 

 from its chassis or housing to the frame. Always 

 check to see that the ground connection is firm 

 and that it is sufficient to take any electrical 

 load supplied to it. 



VOLTMETER 



The voltage of a circuit, or the voltage drop 

 across part of a circuit, is measured with a 

 voltmeter. Vai'ious voltage ranges are obtained 

 by adding resistors of different values in series 

 with the coil of the meter's movement. The 

 voltmeter actually is a current-measuring instru- 

 ment, but it indicates voltage through measure- 

 ment of the current flow through a resistor of 

 known value. When using a voltmeter, the following 

 precautions must be observed to prevent damage 

 to the meter: 



1. Observe proper polarity in connecting the 

 meter to the circuit. 



2. Always connect the meter across (parallel 

 to) the circuit component being tested. 



3. Use a range (scale) large enough to pre- 

 vent full-scale deflection of the needle. 



The accuracy of the voltage readings depends 

 on the sensitivity of the meter. Sensitivity is the 

 meter's internal resistance, measured in ohms 

 per volt. A low-sensitivity meter will indicate 

 erroneous values when used in a high-resistance 

 circuit due to the loading effect of the meter. 

 Because the meter is connected in parallel, the 

 total resistance of the circuit is reduced and the 

 amount of current is Increased. The following 

 examples show how meter sensitivity affects 

 accuracy. 



• Figure 10-3 shows a series circuit. The value 

 of resistor Rl is 50,000 ohms, that of R2, 100,000 

 ohms. Source voltage is 150 volts. The circuit 

 has a current of .001 ampere, as determined by 

 Ohm's law, which means that 50 volts is dropped 

 across Rl, and 100 volts is dropped across R2. 

 When you place a voltmeter across R2, will it 

 read 100 volts ? Maybe. Assume the meter has a 

 sensitivity rating of 1000 ohms per volt, and you 

 use the 0— 100-volt scale. The meter resistance 

 is 100,000 ohms. When you connect the meter 

 across R2, you have two 100,000-ohm resistors 

 in parallel, and the equivalent resistance becomes 

 50,000 ohms. (The: total resistance of two parallel 



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