Figure 4-49. - Underground gas-testing apparatus.

When using either ammeters or voltmeters: ALWAYS start at the HIGHEST meter range. Then drop down to a lower scale range if necessary. This practice protects the meter from injury if an attempt is made to read a high value in a low range. Damage to instruments also can be minimized if you form a habit of placing the range selector switch in the highest range position after you have finished using the instrument.

OBSERVE POLARITY on all direct-current measurements. Take care to connect the positive terminal of the source to the positive terminal of the meter and the negative terminal of the source to the negative terminal ofthe meter. This action ensures that the meter polarity matches the polarity of the circuit in which the meter is placed.

Be careful to avoid dropping a meter or subjecting it to excessive mechanical shock. Such treatment may damage the delicate mechanism or cause the permanent magnet to lose some of its magnetism.

Care must be taken to avoid connecting the ohmmeter across circuits in which a voltage exists, since such connection can result in damage to the instrument. TO ENSURE THE REMOVAL OF ALL VOLTAGE TO THE EQUIPMENT UNDER TEST, DISCONNECT THE SOURCE OF THE INPUT VOLTAGE BY REMOVING THE POWER PLUG. Furthermore, ALL CAPACITORS MUST BE DISCHARGED before the ohmmeter prods are connected in the circuit. Charges remaining on capacitors after the applied voltage has been removed can severely damage the instrument.

Always turn ohmmeters OFF when finished. This action will avoid discharge of the internal battery if the test leads are shorted inadvertently.

It is important that you remember to USE A LOW- VOLTAGE MEGGER TO TEST LOW-VOLTAGE INSULATION. Application of high voltage may initiate insulation breakdown. low-voltage meggers should not be used to test high-voltage insulation because an inaccurate reading may result from the comparatively small output voltages available from this instrument. Be careful whether using high or low range meggers. Dangerous voltages exist at meter terminals and leads.

DIGITAL MULTIMETERS

There are a lot of different types and styles of autoranging digital multimeters that are designed for the professional at work in the field. These instruments stand up to the use and abuse of everyday service and electrically insulate the user from potential shock hazards. They have electronic overload protection against accidental application of voltage to resistance and continuity circuits. These characteristics,