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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 constitute a test of the operation of a 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.

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.

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.

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.

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 the magnets of 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.

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 excessive 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).
  • In checking electron tubes with a tube testes 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 or other highly inductive circuits before you open the circuit

Q.11 Which quantity (voltage or current) determines the intensity of an electrical shock? answer.gif (214 bytes)

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.

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.
  • Observe the following procedures when measuring voltages in excess of 300 volts:
  • Turn off the equipment power.
  • Short-circuit or ground the terminals of all components capable of retaining a charge.
  • Connect the meter leads to the points to be measured.
  • Remove any terminal grounds previously connected.
  • Turn on the power and observe the voltage reading.
  • Turn off the power.
  • Short circuit or ground all components capable of retaining a charge.
  • 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:

  • It should be dry, without holes, and should not contain conducting materials.
  • 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.
  • 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.
  • 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.
  • 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.

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.

Figure 1-4. - Representative safety shorting probe.

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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 equipments are 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.

Q.12 What tool is used to de-energize capacitors in a circuit that has been disconnected from its power source? answer.gif (214 bytes)




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