Other Instrument Precautions

1-13 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. 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). � 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