1-15DIFFERENTIAL VOLTMETERSIt is a seldom-known fact that the Fluke 893 ac-dc differential voltmeter can be used for measuringextremely high resistances from 10 megohms to 106 megohms with a typical accuracy of 5%. Thismeasurement method, however, requires some basic calculations on your part. The obvious advantage ofthe differential voltmeter is its capability of measuring extremely high resistances. Consult the Fluke 893technical manual for initial switch settings and a more detailed explanation of its operation.CAPACITOR MEASUREMENTSCapacitance is that property of a circuit that produces an electrostatic field when two conductingbodies separated by a dielectric material have a potential applied to them. Capacitors are made bycompressing an insulating material (dielectric) between two conductors (plates). The farad is the basicmeasurement of capacitance. It is dependent upon the area of the plates, the distance between the plates,and the type of dielectric used. Electrically, the farad is a measure of 1 coulomb of potential charged by 1volt. A coulomb (the amount of current flow maintained at 1 ampere that passes a given point of a circuitin 1 second) is a large charge. Most capacitors are measured in millionths of a farad (microfarad),expressed as F, or in one-millionth of a microfarad (picofarad), expressed as pF.Capacitors incur various losses as a result of such factors as resistance in the conductors (plates) orleads, current leakage, and dielectric absorption, all of which affect the power factor of the capacitor.Theoretically, the power factor of an ideal capacitor should be zero; however, the losses listed abovecause the power factors of practical capacitors to range from near 0 to a possible 100%. The averagepower factor for good capacitors, excluding electrolytics, is 2% to 3%. Current leakage, which is aninverse function of frequency, is important only at the lower frequencies and becomes negligible at higherfrequencies. Dielectric absorption (sometimes referred to as dielectric viscosity) results in losses thatproduce heat. The effect of this type of loss is the same as resistance in series with the capacitor.You have probably learned the hard way that some capacitors can retain a charge long after thevoltage has been removed. The electrical charge retained by capacitors in de-energized electronic circuitsis, in many cases, sufficient to cause a lethal shock. Be sure you and those working with you consider thishazard before performing any type of maintenance on any electrical or electronic circuit and beforemaking connections to a seemingly dead circuit. Use extreme caution prior to working on or near de-energized circuits that employ large capacitors. Be safe—discharge and ground all high-voltagecapacitors and exposed high-voltage terminal leads by using only an authorized shorting probe, as shownin figure 1-11. Repeat discharge operations several times to make sure that all high-voltage terminationsare completely discharged. It is of the utmost importance that you use only an authorized safety shortingprobe to discharge the circuits before performing any work on them. An authorized general-purposesafety shorting probe for naval service application may be requisitioned using the current stock numberlisted in the ELECTRONICS INSTALLATION AND MAINTENANCE BOOK (EIBM), GeneralNAVSEA 0967-LP-000-0100, Section 3, Safety Equipment. Certain electronic equipment are providedwith built-in, special-purpose safety shorting probes. These probes are not considered general purpose.Use them only with the equipment for which they are provided and only in a manner specified by thetechnical manuals for the equipment. It is considered to be poor practice to remove them for useelsewhere.
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