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
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.
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,