Chapter 1 Test Equipment Administration and Use
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1-1 UNCLASSIFIED 1 TEST EQUIPMENT ADMINISTRATION AND USE LEARNING OBJECTIVES
Learning objectives are stated at the beginning of each chapter. These learning objectives serve as a preview of the information you are expected to learn in the chapter. The comprehensive check questions are based on the objectives. By successfully completing the NRTC, you indicate that you have met the objectives and have learned the information. The learning objectives are listed below.
1. Upon completing this chapter, you should be able to: 2. Describe the Ship Configuration and Logistic Support Information System (SCLSIS). 3. State the differences between calibration and repair. 4. Explain the various calibration status labels used by the Navy. 5. List the procedures for obtaining repairs to test equipment. 6. Describe the Metrology Automated System for Uniform Recall and Reporting (MEASURE) System and the purpose of the Metrology Equipment Recall and Reporting (METER) card and recall schedule. 7. Describe major test equipment references available to you. 8. Explain the purposes and benefits of testing. 9. State the safety precautions involved in working with test equipment. 10. List three precautions you should observe to avoid damaging electric measuring instruments. 11. State the correct procedures for using a safety shorting probe. 12. Describe resistance, voltage, and current measurements in terms of purposes, methods, and instruments used. 13. Describe how capacitance and inductance are measured. 14. Explain the operation of bridges in the measurement of unknown resistances, capacitances, and inductances.
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1-2 UNCLASSIFIED 1.1 INTRODUCTION One purpose of this chapter is to acquaint you with the practical use of test equipment. The presence of adequate test equipment in your shop is not in itself a "cure-all" for making repairs to complex electronic equipment. You must know how to best use the equipment available. First, however, you must understand the basis of electronic theory and be able to apply it to the system under repair.
Another purpose of this chapter is to introduce you to calibration and repair procedures, and basic voltage and current measurements. You will also learn how ac bridges are used for precise measurements of resistance, capacitance, and inductance.
Much of the theory of operation and practical applications of the basic types of test instruments used in electrical and electronic circuits are found in the instruction books and technical manuals that accompany various equipments. You should read and understand these books before you attempt to use any test instrument. You should also know the established safety precautions to ensure your safety and safe equipment operating procedures to protect equipment from damage.
1.2 TEST EQUIPMENT IDENTIFICATION One of the first things you must learn as a maintenance technician is how to identify the various electronic equipment and components by their appropriate nomenclatures. You will find that several methods are used to identify test equipment used; this may be somewhat confusing to you at first. For example, a Tektronix Model 541A oscilloscope can also be identified as a CBTV-541A. The Joint Electronics Type Designation System (JETDS) is used by all branches of the military to identify equipment by a system of standardized nomenclatures.
Q-1. What system is currently used by all branches of the military to identify test equipment?
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1-3 UNCLASSIFIED 1.3 ELECTRONIC TEST EQUIPMENT CLASSIFICATION The Electronic Test Equipment Classification Board was established in 1973 to control the increased use of undesirable electronic test equipment (ETE) in fleet and shore activities. The board classifies electronic test equipments as GENERAL PURPOSE (GPETE) or SPECIAL PURPOSE (SPETE) and assigns responsibility for their management. Items classified as general purpose are managed by the Space and Warfare Systems Command (SPAWARSYSCOM). Items classified as special purpose are managed by the individual systems command that generates the requirement.
GPETE is test equipment that has the capability, without modification, to generate, modify, or measure a range of parameters of electronic functions required to test two or more equipments or systems of basically different design.
Special-purpose electronic test equipment (SPETE) is specifically designed to generate, modify, or measure a range of parameters of electronic functions of a specific or peculiar nature required to test a single system or equipment. These special test equipments are procured by the systems command that has the responsibility for the system/equipment requiring the SPETE for maintenance.
Q-2. Name the two classes of test equipment. Q-3. What test equipment is designed to generate, modify, or measure a range of parameters of electronic functions of a specific nature required to test a single system or equipment? Until the ETE classification board was established, the uncontrolled increase in use of nonstandard GPETE had resulted in loss of inventory control and increased support costs. NESEA has the responsibility for evaluating requests to purchase nonstandard GPETE and for recommending its approval or disapproval to NAVSEA. NAVSEA will then forward its final decision to the originating command for such requests.
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1-4 UNCLASSIFIED 1.3.1 Ship Configuration and Logistic Information System (SCLSIS) Program The Navy must maintain, update, and calibrate thousands of pieces of equipment. To do this, the SHIP CONFIGURATION AND LOGISTIC SUPPORT INFORMATION SYSTEM (SCLSIS) program was designed to keep track of all installed and portable equipment in the fleet. SCLSIS is used to keep up with the existence, location, and changes made to equipment. The SCLSIS program seeks to improve the quality of equipment reporting, provide information needed by other Navy management systems, and reduce record keeping. It is also designed to assist Navy supply systems that furnish spares, documentation, and training necessary to support installed and portable equipment.
Therefore, the inventory of assigned test equipment on board ship is directly related to SCLSIS records. Properly maintained SCLSIS records also show the complete inventory of test equipment on board by quantity, serial number, and location. The SCLSIS program has two basic elements: (1) VALIDATION, to establish a baseline data inventory, and (2) INVENTORY UPDATING, to correct errors or omissions and to document configuration changes.
Q-4. Name the two basic elements of the SCLSIS program.
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1-5 UNCLASSIFIED 1.4 CALIBRATION AND REPAIR PROCEDURES The difference between the terms calibration and repair needs to be addressed before we proceed further. Calibration is little more than checking, adjusting, or systematically aligning a test instrument to a known standard. To do this, you must ensure that the equipment you send to the calibration lab is in working order.
The calibration lab is where actual repair work becomes important. Obvious problems, such as open power cords, burned components, broken meters, and missing hardware, should be repaired or replaced before sending equipment to the calibration lab. Most calibration labs with which you will deal will be part of an intermediate maintenance activity (IMA) on board a tender.
1.4.1 Calibration Status You can determine the calibration status of any test equipment by checking the calibration label or tag located on the equipment. These calibration labels or tags advise you as to whether the item is usable and within its calibration interval. Tags and labels to be used in the METROLOGY CALIBRATION (METCAL) coordination program are listed in the following paragraphs. No other calibration labels or tags are authorized to be placed on test equipment.
1.4.1.1 Calibrated Label The CALIBRATED label, shown in view A of figure 1-1, has black lettering and a white background and comes in two sizes. It is the most commonly used label in the METCAL program. This label indicates that the instrument to which it is attached is within its applicable tolerance on all parameters. If there are any qualifying conditions for use of the instrument, one of the other labels described in the next paragraphs should be used.
1.4.1.2 Calibrated - Refer to Report Label The CALIBRATED - REFER TO REPORT label, shown in view B of figure 1-1, has red lettering and a white back ground. It comes in two sizes and is used when you must know the actual measurement values to use the instrument.
Q-5. What calibration label is used when actual measurement values must be known to use the test equipment?
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Figure 1-1 Calibration labels and tags
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1-7 UNCLASSIFIED 1.4.1.3 Special Calibration Labels Two SPECIAL CALIBRATION labels are shown in view C of figure 1-1 that have black lettering and a yellow background; the size and content of the labels are different. A SPECIAL CALIBRATION tag (figure 1-1, view C) is used with the smaller of the two labels. These labels or tag are used when some unusual or special condition in the calibration should be drawn to your attention.
Such special conditions may be deviations from usual calibration tolerances, multiple calibration intervals, or a requirement for in-place calibration. The special condition that resulted in the SPECIAL CALIBRATION label should be described on the large label when sufficient space is available on the instrument or on the tag when the small label is used. Brief descriptions of special conditions are provided in the following paragraphs.
Q-6. An instrument that must be calibrated in place requires what type of calibration label? In cases where you do not require full instrument capability, the calibration can be performed with reduced tolerances or cover less than all ranges and parameters. This approach is often used when the instrument does not meet full calibration tolerances on certain ranges or parameters, but can still meet user requirements. On the other hand, the special calibration may be for higher accuracy than usual on a short-term basis upon your specific request.
MULTIPLE CALIBRATION INTERVALS - Some instruments have components that require calibration less frequently than the rest of the instrument. For example, the attenuator in a signal generator may require calibration every 12 months, whereas the rest of the instrument parameters should be calibrated every 4 months. Since the attenuator calibration is time consuming and may require unavailable standards, use of the multiple- interval approach can save considerable time (man-hours) as well as permit the more frequent calibration to be performed at a lower level laboratory.
When a specific instrument has been designed for multiple calibration intervals, such information is provided in the applicable calibration procedure. The SPECIAL CALIBRATION label or tag is annotated with the words MULTIPLE INTERVAL, and the type of calibration performed is indicated; for example, partial 1 of 2, 2 of 2, complete calibration, and so forth. The calibration due date reflects the due date of the next partial or complete calibration.
CALIBRATION IN-PLACE - Some instruments should be calibrated in-place. Annotation on the SPECIAL CALIBRATION label or tag will alert both you and the calibrator that the instrument should not be removed, but should be calibrated in-place.
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1-8 UNCLASSIFIED 1.4.1.4 User Calibration Label Some test and measuring equipment (T&ME) should be calibrated by you instead of your referring the instrument to a calibration facility. For example, some instruments, such as hardness testers and densitometers, are provided with their own standards and should be calibrated each time used, or at least very frequently. Some instruments, such as oscillographic recorders, may require calibration before, during, and after each use.
Other automatic test equipment (ATE) has self-calibration tests that should be performed each time used or each day of use. Still other instruments are calibrated as part of checkout procedures performed daily or weekly and recorded in maintenance logs. Whenever recognized, the requirement for calibration by the user and the calibration interval (each use - daily, weekly, every 100 hours - each overhaul, and so forth) is indicated in the Metrology Requirements List (METRL).
The USER CALIBRATION label, shown in view D of figure 1-1, has black lettering and a white background and is affixed when the calibration is performed by the user; however, this label is not replaced at each calibration. When the label is first attached to the instrument, it is annotated as to the appropriate calibration interval. Records of calibrations performed, when other than each time used, should be by normal maintenance practices; that is, in the maintenance log, on maintenance action forms, and so forth.
1.4.1.5 Inactive - Calibrate Before Use Label In the event that an individual instrument due for recalibration will not be used for some time in the future, you may indefinitely postpone the recalibration by affixing an inactive label to the instrument. As shown in view E of figure 1-1, the INACTIVE - CALIBRATE BEFORE USE label has green lettering and a white background. The INACTIVE label remains on the instrument until it is recalibrated. The instrument is not to be used while bearing this label.
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1-9 UNCLASSIFIED 1.4.1.6 Calibration Not Required Label Test equipment standards and T&ME not requiring calibration are shown as CALIBRATION NOT REQUIRED. This label, shown in view F of figure 1-1, has orange letters and a white background. It is attached to and should remain on the instrument indefinitely unless its calibration requirements change. If the instrument is not listed in METRL, you should use the following criteria when placing instruments in the CALIBRATION NOT REQUIRED category:
• Instrument does not make quantitative measurements nor provide quantified outputs. • The device is "fail-safe"; that is, operation beyond specified tolerances will be apparent to the user. • All measurement/stimulus circuits are monitored during use by calibrated instruments or are dependent on external known or calibrated sources for performance within required limits. (When determining that an instrument falls into the CALIBRATION NOT REQUIRED category, you should annotate the label as to the authority for the decision, such as METRL, technical manual, letter or message from higher authority.)
1.4.1.7 Rejected - refer To Attached Tag Label In the event that an instrument fails to meet the acceptance criteria during calibration and cannot be adequately repaired, a REJECTED - REFER TO ATTACHED TAG label is placed on the instrument and all other servicing labels removed. This label, as shown in view G of figure 1-1, has black letters and a red background. In addition to the REJECTED label, a REJECTED tag, giving the reason for rejection and other information as required, is attached to the instrument. Both the label and tag remain on the instrument until it is repaired and recalibrated. The instrument is not to be used while bearing a REJECTED label.
1.4.1.8 Calibration Void If Seal Broken Label The CALIBRATION VOID IF SEAL BROKEN label, shown in view H of figure 1-1, has black letters and a white background. It is placed over readily accessible (usually exterior) adjustments to prevent tampering by the user when such tampering could affect the calibration. The label should not be used to cover adjustments or controls that are part of the normal use and operation of the instrument. This label may also be used to prevent removal and/or interchange of plug-ins, modules, subassemblies, and so forth, when such removal or interchange would affect the calibration.
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1-10 UNCLASSIFIED 1.4.2 Repair Procedures If you are unable to replace a known failed component with onboard spares, you can often locate the replacement component from other supply sources. The replacement component can then be delivered, along with the inoperative equipment, to the IMA. So by sending the repair part along with the equipment, you can reduce repair time considerably. This is particularly true when your unit is getting under way and no time is available for you to complete the repair before calibration. Most operational commands have a higher supply priority for purchase of repair parts than the IMA can use.
1.4.2.1 "No Reject" Policy IMAs have a "no reject" policy on test equipment to provide operational test equipment in a timelier manner. The "no reject" policy says, in effect, that test equipment submitted to the IMA for calibration, which is later found to require repair, will be repaired by the repair department of the IMA. Before this policy, any equipment found inoperative by the calibration lab was marked REJECTED, the reasons stated, and the equipment returned uncalibrated to the ship for repairs. The "no reject" policy does not relieve you of your responsibility to ensure your equipment is in working order prior to submitting it for calibration. Its purpose is to streamline the procedure and cut down delays in returning your equipment to you calibrated and ready to use.
1.4.2.2 Responsibility for Repair and Maintenance of Test Equipment Generally, the responsibility for repair and maintenance of test equipment is placed on maintenance personnel. In some cases, however, maintenance personnel are not authorized to make repairs. Then the test instrument must be sent to a shore repair/calibration facility.
Q-7. Responsibility for repair and maintenance of test equipment generally rests with what group of personnel? When test equipment is sent for calibration and repair, all accessories, such as probes, adapters, and calibration sheets, should be included. Only in emergencies or special situations should partial repair or calibration be attempted on test equipment designated as nonrepairable. Such emergency repairs should be noted on a tag attached to the unit and an entry made on the MEASURE card (discussed shortly). The equipment should then be sent at the earliest opportunity to an authorized facility so that permanent repairs can be made and the unit calibrated.
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1-11 UNCLASSIFIED 1.4.3 Stowage and Handling of Test Equipment Most electronic test equipment is precision equipment. Such equipment must be handled with care to properly perform its designed functions. Rough handling, excessive heat, moisture, and dust all affect the useful life of the equipment. Bumping or dropping a test instrument may ruin the calibration of a meter, cause short circuits, or damage electronic elements inside the case. Sharp bends, creases, or dents in coaxial test cables can alter the expected attenuating effect and cause false meter readings or measurements. Forced air cooling, dust filters, and heaters are used in many pieces of equipment. This test equipment requires clean air filters for proper ventilation and a warm-up period that permits units in the equipment to maintain calibrated standards.
Electronic test equipment should be stowed in a dry location with the dust cover (if provided) in place. Dust covers for spare plug-in units should be constructed for such stowage. For ease in performing maintenance, the test equipment should be stowed at a location convenient to equipment spaces. If possible, related test equipment should be mounted in the equipment spaces. This reduces the problem of finding adequate stowage space elsewhere.
In stowage spaces, individual pieces of test equipment should be held in place by stretch seat-belt-type straps. If bars are used to hold equipment on shelves, meters and control knobs should be protected by blocking the equipment to prevent it from rolling and sliding on the shelf. Test equipment too large for shelf stowage should be kept in stowage cases and tie-downs provided to secure the cases. Refer to Stowage Guide for Portable Test Equipment, NAVSEA ST000-AB-GYD-010/GPETE, to determine adequate stowage space and proper weight support requirements.
1.4.4 The Metrology Automated System for Uniform Recall and Reporting (Measure) For the sake of simplicity, we will use the more commonly used acronym MEASURE instead of the full name to describe this system in the next discussion.
MEASURE is a data processing system designed to provide a standardized system for the recall and scheduling of test, measurement, and diagnostic equipment (TMDE) into calibration facilities. It also provides for the documentation of data pertaining to the calibration actions performed by these facilities.
The primary reference document that describes the operation of the MEASURE system is Metrology Automated System for Uniform Recall and Reporting (MEASURE) Users Manual, OP 43P6A. The Chief of Naval Operations oversees this program and establishes policy and guidelines.
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1-12 UNCLASSIFIED Q-8. What Navy office oversees the MEASURE program? Each naval activity must ensure that the test equipment for which it has been assigned primary responsibility is submitted on a timely basis to a calibration activity for required calibration.
The MEASURE program is designed to assist these naval activities in the fulfillment of this responsibility. MEASURE does this by providing for the automatic scheduling and recall of all such test equipment for calibration.
Each activity submits an initial inventory, using the form shown in figure 1-2, to its Metrology Calibration Representative (METCALREP) for approval. The METCALREP then forwards the inventory to the Measure Operational Control Center (MOCC). The MOCC, based on the information contained on these inventory report forms, provides the necessary preprinted Metrology Equipment Recall and Reporting (METER) card. Figure 1-3 illustrates a MEASURE METER card.
Figure 1-2 MEASURE TMDF Inventory report form
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Figure 1-3 MEASURE METER card
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1-14 UNCLASSIFIED In part, the METER card is preprinted with information taken from the initial inventory data submitted on the inventory report forms together with such updated data as may appear on any prior METER card. The remaining information required is entered on the card by the user of the equipment or the calibration activity, as appropriate.
The METER card is used to report changes, additions, or deletions to the user activity’s inventory. It is also used to report changes in custody of the item of test equipment. The procedure for filling out the METER card is outlined in the appendixes of the MEASURE Users Manual. Blank METER cards can be obtained through the responsible METCALREP.
A computer printout recall schedule is also generated by the MEASURE system. The purpose of this printout is to list those items of equipment that are due for calibration. Each recall schedule is composed of a set of four identical copies. One set is provided to the calibration activity as an aid to workload planning; a second set is sent to the user’s activity. The recall schedule is one of several products/formats sent automatically by the MEASURE Operation Control Center to the user activity on a regular basis. The MOCC automatically distributes the following products to user activities at the intervals shown:
DOCUMENT TITLE TYPE DOCUMENT INTERVAL Format 215 Unmatched listing As required Format 310 Test equipment inventory Monthly Format 350 Test equipment inventory in sub- custodian order Monthly Format 804 Recall schedule for on-site equipment Monthly/Quarterly Replenishment cards Preprinted METER card As required Blank METER cards Initial issue
1.4.5 Test Equipment References Several publications that contain information concerning test equipment are required to be maintained aboard ship by type commander instructions. These requirements are usually found in the inspection checkoff list. Other publications, while not required by directive, are necessary to you as reference and study material so you will be able to administer an effective test equipment program. Technicians should become familiar with the publications/directives listed in appendix II of this module.
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1-15 UNCLASSIFIED 1.5 INTRODUCTION TO TROUBLESHOOTING Our military forces increasingly rely on electrical and electronic equipment to help perform their mission. The effectiveness of our tactical forces depends on many types of electronic systems, such as communications systems, detection systems, and fire control systems. The reliability of such equipment is determined by many factors; however, the primary factors are the quality of the equipment in use, the availability of spare parts, and the ability of maintenance personnel to perform adequate maintenance.
Maintenance is work done to correct, reduce, or counteract wear, failure, and damage to equipment. Maintenance of electrical and electronic equipment is divided into two main categories: PREVENTIVE (routine) and CORRECTIVE maintenance. Preventive maintenance consists of mechanical, electrical, and electronic checks to determine whether equipment is operating properly. It also consists of visual inspections of cabling and equipment for damage and to determine if lubrication is needed. Corrective maintenance isolates equipment failure by means of test techniques and practices; it also replaces defective parts and realigns or readjusts equipment to bring it back to proper performance.
Q-9. What are the two main categories of maintenance? Q-10. What type of maintenance involves isolating equipment troubles and replacing defective parts? Testing and troubleshooting are the areas of maintenance that require the greatest technical skill. Testing procedures are referred to as measurements, tests, and checks. The definitions of these terms often overlap, depending on their use and the results obtained. For example, a power measurement and a frequency check could constitute a test of the operation of the same radio transmitter.
Troubleshooting is a term which we in the electronics field use daily. But what does it mean? Troubleshooting is sometimes thought to be the simple repair of a piece of equipment when it fails to function properly. This, however, is only part of the picture. In addition to repair, you, as a troubleshooter, must be able to evaluate equipment performance. You evaluate performance by comparing your knowledge of how the equipment should operate with the way it is actually performing. You must evaluate equipment both before and after repairs are accomplished.
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1-16 UNCLASSIFIED Equipment performance data, along with other general information for various electronic equipments, is available to help you in making comparisons. This information is provided in performance standards books for each piece of equipment. It illustrates what a particular waveform should look like at a given test point or what amplitude a voltage should be, and so forth. This data aids you in making intelligent comparisons of current and baseline operating characteristics for the specific equipment assigned to you for maintenance. ("Baseline" refers to the initial operating conditions of the equipment on installation or after overhaul when it is operating according to design.)
Remember, maintenance refers to all actions you perform on equipment to retain it in a serviceable condition or to restore it to proper operation. This involves inspecting, testing, servicing, repairing, rebuilding, and so forth. Proper maintenance can be performed only by trained personnel who are thoroughly familiar with the equipment. This familiarity requires a thorough knowledge of the theory of operation of the equipment.
A logical and systematic approach to troubleshooting is of the utmost importance in your performance of electronics maintenance. Many hours have been lost because of time- consuming "hit-or-miss" (often referred to as "easter-egging") methods of troubleshooting.
1.5.1 General Test Equipment Information In any maintenance training program, one of your most important tasks is to learn the use of test equipment in all types of maintenance work. To be effective in maintenance work, you must become familiar not only with the common types of measuring instruments, but also with the more specialized equipment. Some examples of common types of typical measuring instruments are the ammeter, voltmeter, and ohmmeter; examples of specialized test equipment are the spectrum analyzer, dual-trace oscilloscope, and power and frequency meters.
1.5.2 Test Equipment Safety Precautions The electrical measuring instruments included in test equipment are delicately constructed and require certain handling precautions to prevent damage and to ensure accurate readings. In addition, to prevent injury to personnel, you must observe precautions while using test equipment. You can find a list of applicable instructions in appendix II of this module.
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1-17 UNCLASSIFIED 1.5.2.1 Instrument Precautions To prevent damage to electrical measuring instruments, you should observe the precautions relating to three hazards: mechanical shock, exposure to magnetic fields, and excessive current flow.
MECHANICAL SHOCK - Instruments contain permanent magnets, meters, and other components that are sensitive to shock. Heavy vibrations or severe shock can cause these instruments to lose their calibration accuracy.
EXPOSURE TO STRONG MAGNETIC FIELDS - Strong magnetic fields may permanently impair the accuracy of a test instrument. These fields may impress permanent magnetic effects on permanent magnets, moving-coil instruments, iron parts of moving-iron instruments, or in the magnetic materials used to shield instruments.
EXCESSIVE CURRENT FLOW - This includes various precautions, depending on the type of instrument. When in doubt, use the maximum range scale on the first measurement and shift to lower range scales only after you verify that the reading can be made on a lower range. If possible, connections should be made while the circuit is de- energized. All connections should be checked to ensure that the instrument will not be overloaded before the circuit is reenergized.
1.5.2.2 Other Instrument Precautions Precautions to be observed to prevent instrument damage include the following:
• Keep in mind that the coils of wattmeters, frequency meters, and power meters may be carrying large quantities of current even when the meter pointer is on scale. • Never open secondaries of current transformers when the primary is energized. • Never short-circuit secondaries of potential transformers the primary is energized. • Never leave an instrument connected with its pointer off-scale or deflected in the wrong direction. • Ensure that meters in motor circuits can handle the motor starting current. This may be as high as six to eight times the normal running current. • Never attempt to measure the internal resistance of a meter movement with an ohmmeter since the movement may be damaged by the current output from the ohmmeter. • Never advance the intensity control of an oscilloscope to a position that causes an excessively bright spot on the screen; never permit a sharply focused spot to remain stationary for any period of time. This results in burn spots on the face of the cathode-ray tube (CRT).
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1-18 UNCLASSIFIED • In checking electron tubes with a tube tester that has a separate "short test," always make the short test first. If the tube is shorted, no further test should be made. • Before measuring resistance, always discharge any capacitors in the circuit to be tested. Note and record any points not having bleeder resistors or discharge paths for capacitors. • Always disconnect voltmeters from field generating or other highly inductive circuits before you open the circuit.
Q-11. Which quantity (voltage or current) determines the intensity of an electrical shock? Situations can arise during the use of test equipment that are extremely dangerous to personnel. For example, you may have an oscilloscope plugged into one receptacle, an electronic meter plugged into another, and a soldering iron in still another. Also, you may be using an extension cord for some equipments and not others or may be using other possible combinations. Some of the hazards presented by situations such as these include contact with live terminals or test leads. In addition, cords and test leads may be cross connected in such a manner that a potential difference exists between the metal cases of the instruments. This potential difference may cause serious or fatal shocks.
Test leads attached to test equipment should, if possible, extend from the back of the instruments away from the observer. If this is not possible, they should be clamped to the bench or table near the instruments.
At times, you may use instruments at locations where vibration is present, such as near a diesel engine. At such times, the instruments should be placed on pads of folded cloth, felt, or similar shock-absorbing material.
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1-19 UNCLASSIFIED 1.5.3 Working on Energized Circuits Insofar as is practical, you should NOT undertake repair work on energized circuits and equipment. However, it could become necessary, such as when you make adjustments on operating equipment. In such cases, obtain permission from your supervisor, then proceed with your work, but carefully observe the following safety precautions:
• DO NOT WORK ALONE. • Station an assistant near the main switch or circuit breaker so the equipment can be immediately de-energized in case of an emergency. • Someone qualified in first aid for electrical shock should be standing by during the entire operation. • Ensure that you have adequate lighting. You must be able to see clearly if you are to perform the job safely and properly. • Be sure that you are insulated from ground by an approved rubber mat or layers of dry canvas and/or wood. • Where practical, use only one hand, keeping the other either behind you or in your pocket. • If you expect voltage to exceed 150 volts, wear rubber gloves. • DO NOT work on any type of electrical apparatus when you are wearing wet clothing or if your hands are wet. • DO NOT wear loose or flapping clothing. • The use of thin-soled shoes and shoes with metal plates or hobnails is prohibited. • Flammable articles, such as celluloid cap visors, should not be worn. • Remove all rings, wristwatches, bracelets, and similar metal items before working on the equipment. Also ensure that your clothing does not contain exposed metal fasteners, such as zippers, snaps, buttons, and pins. • Do not tamper with interlock switches; that is, do not defeat their purpose by shorting them or blocking them open. • Ensure that equipment is properly grounded before energizing. • De-energize equipment before attaching alligator clips to any circuit. • Use only approved meters and other indicating devices to check for the presence of voltage.
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1-20 UNCLASSIFIED • Observe the following procedures when measuring voltages in excess of 300 volts: o Turn off the equipment power. o Short-circuit or ground the terminals of all components capable of retaining a charge. o Connect the meter leads to the points to be measured. o Remove any terminal grounds previously connected. o Turn on the power and observe the voltage reading. o Turn off the power. o Short circuit or ground all components capable of retaining a charge. o Disconnect the meter leads. • On all circuits where the voltage is in excess of 30 volts and where decks, bulkheads, or workbenches are made of metal, you should insulate yourself from accidental grounding by using approved insulating material. The insulating material should have the following qualities: o It should be dry, without holes, and should not contain conducting materials. o The voltage rating for which it is made should be clearly marked on the material. The proper material should be used so that adequate protection from the voltage can be supplied. o Dry wood may be used or, as an alternative, several layers of dry canvas, sheets of phenolic (resin or plastic) insulating material, or suitable rubber mats. o Care should be exercised to ensure that moisture, dust, metal chips, and so forth, which may collect on insulating material, are removed at once. Small deposits of such materials can become electrical hazards. o All insulating materials on machinery and in the area should be kept free of oil, grease, carbon dust, and so forth, since such deposits destroy insulation.
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1-21 UNCLASSIFIED 1.5.4 Safety Shorting Probe A representative shorting probe is shown in figure 1-4. An approved shorting probe is shown in NAVSEA 0967-LP-000-0100, EIMB, General, Section 3.
CAUTION
Capacitors and cathode-ray tubes may retain their charge for a considerable period of time after having been disconnected from the power source.
Always assume there is a voltage present when working with circuits having high capacitance, even when the circuit has been disconnected from its power source.
An approved type of shorting probe should be used to discharge capacitors and cathode-ray tubes individually.
When using the safety shorting probe, always be sure to first connect the test clip to a good ground (if necessary, scrape the paint off the grounding metal to make a good contact). Then hold the safety shorting probe by the insulated handle and touch the probe end of the shorting rod to the point to be shorted out. The probe end is fashioned so that it can be hooked over the part or terminal to provide a constant connection by the weight of the handle alone. Always take care not to touch any of the metal parts of the safety shorting probe while touching the probe to the exposed "hot" terminal. It pays to be safe; use the safety shorting probe with care.
Some equipment is provided with walk-around shorting devices, such as fixed grounding studs or permanently attached grounding rods. When that is the case, the walk-around shorting devices should be used rather than the safety shorting probe.
Figure 1-4 Representative safety shorting probe
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1-22 UNCLASSIFIED Q-12. What tool is used to de-energize capacitors in a circuit that has been disconnected from its power source? 1.5.5 Working on De-Energized Circuits When any electronic equipment is to be repaired or overhauled, certain general safety precautions should be observed. They are as follows:
• Remember that electrical and electronic circuits often have more than one source of power. Take time to study the schematics or wiring diagrams of the entire system to ensure that all sources of power have been disconnected • If pertinent, inform the remote station regarding the circuit on which work will be performed. • Use one hand when turning switches on or off. • Safety devices, such as interlocks, overload relays, and fuses, should never be altered or disconnected except for replacement. In addition, they should never be changed or modified in any way without specific authorization. • Fuses should be removed and replaced only after the circuit has been de- energized. When a fuse "blows," the replacement should be of the same type and have the same current and voltage ratings. A fuse puller should be used to remove and replace cartridge fuses. • All circuit breakers and switches from which power could possibly be supplied should be secured (locked if possible) in the OPEN or OFF (safe) position and danger tagged in accordance with procedures in the Standard Organization and Regulations of the U.S. Navy, OPNAVINST 3120.32. • After the work has been completed, the tag (or tags) should be removed only by the same person who signed it (them) when the work began. • Keep clothing, hands, and feet dry if at all possible. When you must work in wet or damp locations, place a rubber mat or other nonconductive material on top of a dry, wooden platform or stool; then use the platform or stool to sit and stand on. Use insulated tools and insulated flashlights of the molded type when you are required to work on exposed parts.
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1-23 UNCLASSIFIED 1.5.6 Grounding of Power Tools and Equipment The possibility of electrical shock can be reduced by ensuring that all motor and generator frames, metal bases, and other structural parts of electrical and electronic equipment are at ground potential.
Normally, on steel-hull vessels, such grounds are inherently provided because the metal cases or frames of the equipment are in contact with one another and with the metal structure of the vessel. In some instances where such inherent grounding is not provided by the mounting arrangements, such as equipment supported on shock mounts, suitable ground connections must be provided.
The grounding wire used for this purpose is generally made of flexible material (copper or aluminum) that provides sufficient current-carrying capacity to ensure an effective ground. In this manner, equipment cases and frames that are not intended to be above ground potential are effectively grounded; also, the possibility of electrical shock to personnel coming in contact with metal parts of the equipment is minimized. The secondary purpose of grounding equipment is to improve the operation and continuity of service of all equipments.
Paint, grease, or other foreign matter can interfere with the positive metal-to-metal contact at the ground connection point. Therefore, all bonding surfaces (connection points or metallic junctions) must be securely fastened and free of such matter. In all instances where equipment grounding is provided, certain general precautions and preventive maintenance measures must be taken. A few of these precautions are listed below:
• Periodically clean all strap-and-clamp connectors to ensure that all direct metal- to-metal contacts are free from foreign matter. • Check all mounting hardware for mechanical failure or loose connections. • Replace any faulty, rusted, or otherwise unfit grounding strap, clamp, connection, or component between the equipment and the ground to the ship’s hull. • When replacing a part of the ground connection, make certain that the metallic contact surfaces are clean and that electrical continuity is re-established. • After completing the foregoing steps, recheck to be sure that the connection is securely fastened with the correct mounting hardware. Paint the ground strap and hardware in accordance with current procedures.
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1-24 UNCLASSIFIED Because of the electrical shock hazards that could be encountered aboard ship, plugs and convenience outlets for use with portable equipment and power tools normally are standard three-prong type. Both plugs and outlets are keyed so that the plug must be in the correct position before it can be inserted into the receptacle. To ensure that the safety factors incorporated in these devices are in serviceable condition and are safe for use, you must perform the following precautions and inspections:
• Inspect the pins of the plug to see that they are firmly in place and are not bent or damaged. • Check the wiring terminals and connections of the plug. Loose connections and frayed wires on the plug surface must be corrected and any foreign matter removed before the plug is inserted into the receptacle. • Use a meter to ensure that the ground pin has a resistance of less than 1 ohm equipment ground. • Do not attempt to insert a grounded-type plug into a grounded receptacle without first aligning the plug properly.
CAUTION
Never use a power tool or a piece of portable test equipment unless you are absolutely sure that it is equipped with a properly grounded conductor.
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1-25 UNCLASSIFIED 1.6 BASIC MEASUREMENTS Electronic measurements involve the fundamental electrical quantities of voltage and current and the inherent characteristics of resistance, capacitance, and inductance. In circuits being tested, voltage and current are dependent upon resistance, capacitance, and inductance for their distribution; therefore, voltage and current measurements are valuable aids in determining circuit component conditions and in the evaluation of symptoms. Practically any reading obtained from the use of test equipment will depend on these basic measured quantities of resistance, capacitance, and inductance.
1.6.1 Voltage and Current Measurements Voltage measurements may be made as part of either preventive or corrective maintenance. These measurements are made using a voltmeter. When compared with voltage charts, these measurements are a valuable aid in locating a trouble quickly and easily. However, if the sensitivity of the test voltmeter differs from that of the voltmeter used in preparing the chart, the voltage measurements must be evaluated before the true circuit conditions can be determined. (Sensitivity in voltmeters was discussed in NEETS, Module 3, Introduction to Circuit Protection, Control, and Measurement.)
Since many of the troubles you find in equipments and systems are the result of abnormal voltages, voltage measurements are a valuable aid in locating trouble. You can measure voltage with a voltmeter without interrupting circuit operation.
Point-to-point voltage measurement charts, usually found in equipment technical manuals, contain the normal operating voltages found in the various stages of the equipment. These voltages are usually measured between indicated points and ground unless otherwise stated. When you begin recording voltage measurements, it is a smart and safe practice to set the voltmeter on the highest range before measuring. This ensures that excessive voltages existing in the circuit will not cause overloading of the meter.
Q-13. On what range should you set the voltmeter prior to taking a voltage measurement? To increase accuracy, you should then set the voltmeter to the appropriate range for the proper comparison with the expected voltage in the voltage charts. When checking voltages, remember that a voltage reading can be obtained across a resistance, even if that resistance is open. The resistance of the meter itself forms a circuit resistance when the meter probes are placed across the open resistance. Therefore, the voltage across the component may appear to be normal or near-normal as you read the meter, but may actually be abnormal when the meter is disconnected from the circuit.
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1-26 UNCLASSIFIED If the internal resistance of the voltmeter is approximately the same value as the resistance being tested, it will indicate a considerably lower voltage than the actual voltage present when the meter is removed from the circuit. The sensitivity (in ohms per volt) of the voltmeter used to prepare the voltage charts is provided on those charts. If a meter of similar sensitivity is available, you should use it to reduce the effects of loading.
The following precautions are general safety measures that apply to the measurement of voltages. Remember that nearly all voltages are dangerous and have often proved fatal to careless technicians. When measuring voltages, be sure to observe the following precautions:
• Set test equipment to the HIGHEST range. • Make sure safety observer knows where to secure power for the equipment under test. • Connect the ground lead of the voltmeter first. • Use only one hand to take measurements (when possible), and put the other hand in your pocket or behind your back. • If the voltage to be measured is less than 300 volts, place the end of the test probe on the point to be tested; use the polarity switch to select positive or negative readings. • If the voltage to be measured is more than 300 volts, proceed as follows: o Shut off circuit power. o Discharge all filter capacitors with a shorting probe. o Temporarily ground the point to be measured. o Connect (clip on) the proper test lead to the high-voltage point. o Move away from the voltmeter. o Turn on circuit power and read the voltmeter. o Turn off circuit power. o Discharge all capacitors before disconnecting the meter.
Q-14. When taking a voltage measurement, which lead of the voltmeter should you connect to the circuit first? Current measurements are not often taken in the course of preventive maintenance or testing. This is because the ammeter (or other current-measuring instrument) must become an actual part of the equipment being tested. The circuit must be opened (desoldered) to connect the ammeter in series with the circuit being tested. Usually, you can take a voltage measurement and use this factor to calculate the circuit current by applying Ohm’s law.
Q-15. Is an ammeter connected in series or in parallel with the circuit under test?
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1-27 UNCLASSIFIED 1.6.2 Resistance Measurements Resistance measurements are a valuable aid to you in locating defective circuits and components during corrective maintenance. Maintenance handbooks for the equipment can often be used to help you take these measurements. These handbooks often contain resistance charts that are referenced to accessible test points within the equipment. Without these charts, taking resistance measurements in a complex circuit is a slow process. The process is slow because one side of the circuit component must often be desoldered to get a true resistance measurement. However, resistance tolerances vary so widely that approximate resistance readings are adequate for most jobs.
Once the most accessible test point is found, an ohmmeter is usually used to take the resistance measurement. Because of the degree of accuracy needed when an ohmmeter is used, proper calibration and understanding of the meter scales is a must. (Topic 2 of this module will discuss these requirements in detail.) When using an ohmmeter, you must observe the following precautions:
• The circuit being tested must be completely de-energized. • Any meters or transistors which can be damaged by the ohmmeter current must be removed before any measurement is made.
Q-16. What must be done to a circuit before you can use an ohmmeter for testing? 1.6.3 Capacitance Measurements Capacitance measurements are usually taken with a capacitance meter. Capacitance tolerances vary even more widely than resistance tolerances. Capacitance tolerances depend on the type of capacitor, the value of capacitance, and the voltage rating. The actual measurement of capacitance is very simple; however, you must make the important decision of whether to reject or to continue to use the capacitor after it has been tested.
The POWER FACTOR of a capacitor is important because it is an indication of the various losses of a capacitor. Power losses can be traced to the dielectric, such as current leakage and dielectric absorption. Current leakage is of considerable importance, especially in electrolytic capacitors.
Q-17. What is the term used to refer to the losses which can be traced to the dielectric of a capacitor?
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1-28 UNCLASSIFIED 1.6.4 Inductance Measurements Inductance measurements are seldom required in the course of troubleshooting. However, inductance measurements are useful in some cases; therefore, bridges (discussed in the next section) are available for making this test. You will find that many capacitance test sets can be used to measure inductance. Most capacitance test sets are furnished with inductance conversion charts if the test equipment scale is not calibrated to read the value of inductance directly.
1.6.5 Capacitance, Inductance, and Resistance Bridges You can measure capacitance, inductance, and resistance for precise accuracy by using ac bridges. These bridges are composed of capacitors, inductors, and resistors in a wide variety of combinations. These bridges are operated on the principle of a dc bridge called a WHEATSTONE BRIDGE.
1.6.5.1 Wheatstone Bridge The Wheatstone bridge is widely used for precision measurements of resistance. The circuit diagram for a Wheatstone bridge is shown in figure 1-5. Resistors R1, R2, and R3 are precision, variable resistors. The value of Rx is an unknown value of resistance that must be determined. After the bridge has been properly balanced (galvanometer G reads zero), the unknown resistance may be determined by means of a simple formula. The galvanometer (an instrument that measures small amounts of current) is inserted across terminals b and d to indicate the condition of balance. When the bridge is properly balanced, no difference in potential exists across terminals b and d; when switch S2 is closed, the galvanometer reading is zero.
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1-29 UNCLASSIFIED
The operation of the bridge is explained in a few logical steps. When the battery switch S1 is closed, electrons flow from the negative terminal of the battery to point a. Here the current divides as it would in any parallel circuit. Part of it passes through R1 and R2; the remainder passes through R3 and Rx. The two currents, I 1 and I2, unite at point c and return to the positive terminal of the battery. The value of I1 depends on the sum of resistance R1 and R2, and the value of I2 depends on the sum of resistances R3 and Rx. In each case, according to Ohm’s law, the current is inversely proportional to the resistance.
Figure 1-5 Wheatstone bridge
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1-30 UNCLASSIFIED R1, R2, and R3 are adjusted so that when S1 is closed, no current flows through G. When the galvanometer shows no deflection, there is no difference of potential between points b and d. All of I1 follows the a b c path and all I2 follows the a b c path. This means that a voltage drop E1 (across R1 between points a and b) is the same as voltage drop E3 (across R3 between points a and d). Similarly, the voltage drops across R2 and Rx (E2 and Ex) are also equal. Expressed algebraically,
E1 = E3
I1R1 = I2R3
and
E2 = Ex
I1R2 = I2Rx
With this information, we can figure the value of the unknown resistor R x. Divide the voltage drops across R1 and R3 by their respective voltage drops across R2 and Rx as follows:
I1R1 I1R2 = I2R3 I2Rx
We can simplify this equation:
R1 R2 = R3 Rx
then we multiply both sides of the expression by Rx to separate it:
Rx = R2R3 Rx
For example, in figure 1-5, we know that R1 is 60 ohms, R2 is 100 ohms, and R3 is 200 ohms. To find the value of R x, we can use our formula as follows:
Rx = R2R3 R1
Rx = 100 × 200 60
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1-31 UNCLASSIFIED Rx = 20,000 60
Rx = 333.33 ohms
1.6.5.2 Use of ac Bridges A wide variety of ac bridge circuits (such as the Wheatstone) may be used for the precision measurement of ac resistance, capacitance, and inductance. Let’s look at ac bridges in terms of functions they perform.
RESISTANCE BRIDGE - An ac signal generator, as shown in figure 1-6, is used as the source of voltage. Current from the generator passes through resistors R1 and R2, which are known as the ratio arms, and through Rs and Rx. Again, Rx is known as resistance. Rs has a standard value and replaces R3 in figure 1-6. When the voltage drops across R2 and Rs are equal, the voltage drops across R2 and Rx are also equal; no difference of potential exists across the meter and no current flows through it. As we discovered with the Wheatstone bridge, when no voltage appears across the meter, the following ratio is true:
Figure 1-6 Resistance bridge (ac)
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1-32 UNCLASSIFIED For example, if in figure 1-6 we know that R1 is 20 ohms, R2 is 40 ohms, and Rs is 60 ohms, we can find the value of Rx using our formula as follows:
Rx = R2Rs R1
Rx = 40 × 60 20
Rx = 2,400 20
Rx = 120 ohms
With the ac signal applied to the bridge, R1 and R2 are varied until a zero reading is seen on the meter. Zero deflection indicates that the bridge is balanced. (NOTE: In actual practice, the variables are adjusted for a minimum reading since the phase difference between the two legs will not always allow a zero reading.)
CAPACITANCE BRIDGE - Because current varies inversely with resistance and directly with capacitance, an inverse proportion exists between the four arms of the bridge in figure 1-7; the right side of our expression is inverted from the resistance bridge expression as follows:
R1 R2 = Cx Cs
Cx = R1Cs R2
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1-33 UNCLASSIFIED
Q-18. What effect does an increase in capacitance have on a capacitor’s opposition to current flow? Because R1 and R2 are expressed in the same units, the equation R1/R2 becomes a simple multiplication factor. This equation provides a numerical value for Cx and will be in the same units as Cs (farad, microfarad, and so forth).
Similarly, the following resistance ratio exists between the four arms of the bridge, just as in the resistance bridge expression discussed earlier:
R1 R2 = Rs Rx
or
Rx = R2Rs R1
Figure 1-7 Capacitance bridge
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1-34 UNCLASSIFIED Thus, both the unknown resistance and capacitance, Rx and Cx, can be estimated in terms of known resistance R1, R2, Rs, and known capacitance Cs.
In figure 1-7, for example, we know that R1 is 20 ohms, R2 is 40 ohms, Rs is 60 ohms, and Cs is 10 microfarads. We can find the values of Cx and Rx by using the respective formulas as follows:
Cx = R1Cs R2
Cx = 20 × 10 40
Cx = 200 40
Cx = 5 microfarads
and
Rx = R2Rs R1
Rx = 40 × 60 20
Rx = 2,400 20
Rx = 120 ohms
Q-19. When a bridge is used to measure resistance, what is the value of R x if R1 equals 80 ohms, R2 equals 120 ohms, and R3 equals 280 ohms?
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1-35 UNCLASSIFIED INDUCTANCE BRIDGE - The value of the unknown inductance Lx may be determined by means of the simple bridge circuit shown in figure 1-8. Ratio arms R1 and R2 are accurately calibrated resistors. Ls is a standard inductor with a known inductance; Rs is the known resistance, and Rx represents the resistance of the unknown inductor.
The ac signal is applied to the bridge, and variable resistors R1 and R2 are adjusted for a minimum or zero deflection of the meter, indicating a condition of balance. When the bridge is balanced, the following formulas may be used to find Lx.
(NOTE: The right side of this expression is NOT inverse as it was in the capacitance bridge.)
R1 R2 = Ls Lx
R2Ls R1 = Lx
Figure 1-8 Inductance bridge
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1-36 UNCLASSIFIED and
R1 R2 = Rs Rx
or
Rx = R2Rs R1
In figure 1-8, for example, the values of R1, R2, and Rs are 20, 40, and 60 ohms, respectively. The value of Ls is 10 millihenries. We can find the values of Rx and Lx by using their respective formulas as follows:
Lx = R2Ls R1
Lx = 40 × 10 20
Lx = 400 20
Lx = 20 millihenries
and
Rx = R2Rs R1
Rx = 40 × 60 20
Rx = 2,400 20
Rx = 120 ohms
Thus, both the unknown resistance and inductance can be estimated in terms of the known values for R1, R2, R s, and Ls.
Q-20. When an unknown capacitance is tested with a bridge, what is the value of C x if R1 equals 70 ohms, R2 equals 150 ohms, and Cs equals 550 microfarads?
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1-37 UNCLASSIFIED 1.7 SUMMARY The important points of this chapter are summarized in the following paragraphs:
The JETDS SYSTEM is jointly used by all branches of the military to identify equipments by a system of standardized nomenclatures.
GPETE is test equipment that has the capability, without modifications, to generate, modify, or measure a range of parameters of electronic functions required to test two or more equipments or systems of basically different design. All GPETE are listed in Standard General Purpose Electronic Test Equipment, MIL-STD-1364 (series).
SPETE is test equipment that is specifically designed to generate, modify, or measure a range of parameters of electronic functions of a specific or peculiar nature required to test a single equipment or system.
The SHIP CONFIGURATION AND LOGISTICS INFORMATION SYSTEM (SCLSIS) program is designed to keep track of equipment configuration changes in the fleet.
The SCLSIS program has two basic elements, VALIDATION and INVENTORY UPDATING.
The CALIBRATION STATUS of any items of test equipment can be determined by the information recorded on the calibration label or tag located on the equipment.
The CALIBRATED label, with black lettering on a white background, indicates the instrument to which it is attached is within tolerance on all scales.
The CALIBRATED—REFER TO REPORT label, with red lettering on a white background, is used when actual measurement values must be known to use the instrument.
The SPECIAL CALIBRATION label, with black lettering on a yellow background, is used when some unusual or special condition in the calibration should be drawn to your attention.
The USER CALIBRATION label indicates that you should calibrate the test and measuring instrument instead of sending the instrument to a calibration facility.
The INACTIVE—CALIBRATE BEFORE USE label is used when a piece of test equipment due for recalibration will not be used for some time in the future.
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1-38 UNCLASSIFIED The CALIBRATION NOT REQUIRED label is used on test instruments listed in the Metrology Requirements List (METRL) as not requiring calibration.
The REJECTED label is attached to a test instrument that fails to meet the acceptance criteria during calibration and cannot be repaired.
The CALIBRATION VOID IF SEAL BROKEN label is placed over readily accessible adjustments to prevent tampering by the user when such tampering could affect the calibration.
The MEASURE system is designed to standardize the recall and scheduling of test, measurement, and diagnostic equipment into calibration facilities and for the documentation of actions performed by the calibration facility.
MAINTENANCE is work done to correct, reduce, or counteract wear and damage to equipment.
PREVENTIVE MAINTENANCE consists of checks to determine weather equipment is functioning properly. It also consists of visual inspections of cabling and equipment for damage and to determine if lubrication is needed.
CORRECTIVE MAINTENANCE is used to isolate troubles by means of test techniques and practices that realign or readjust equipment or otherwise bring the equipment back up to proper performance.
SENSITIVITY of the voltmeter is always given on the voltage charts for a particular piece of equipment. You should always use a voltmeter of similar sensitivity to the equipment to diminish the effects of circuit loading.
CURRENT MEASUREMENTS are not often taken in the course of testing because the ammeter (or other current measuring device) must become an actual part of the equipment being tested. The circuit must be opened for necessary connection of the meter. Usually you can use a voltage measurement to calculate the circuit current by applying Ohm’s law.
You should observe the following PRECAUTIONS when using an ohmmeter:
1. The circuit being tested must be completely de-energized. 2. Any circuit components which can be damaged by ohmmeter current must be removed before any measurement is made.
The WHEATSTONE BRIDGE is used for precise measurement of resistance.
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1-39 UNCLASSIFIED The CAPACITANCE BRIDGE is used for measuring an unknown capacitance.
An INDUCTANCE BRIDGE is used to find the value of an unknown inductance
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1-40 UNCLASSIFIED ANSWERS TO QUESTIONS Q1. THROUGH Q20.
A-1. Joint Electronics Type Designation System (JETDS).
A-2. General-purpose electronic test equipment (GPETE) and special-purpose electronic test equipment (SPETE).
A-3. Special-purpose electronic test equipment.
A-4. Validation and updating.
A-5. CALIBRATED— REFER TO REPORT.
A-6. SPECIAL CALIBRATION label.
A-7. Maintenance personnel.
A-8. The Chief of Naval Operations.
A-9. Preventive and corrective maintenance.
A-10. Corrective maintenance.
A-11. Current.
A-12. Shorting probe.
A-13. Highest.
A-14. Ground.
A-15. In series.
A-16. It must be de-energized.
A-17. Power losses.
A-18. Opposition to current flow decreases.
A-19. 420 ohms.
A-20. 256 microfarads.