TM 11-6625-298-14
Section I. CONTROLS AND INSTRUMENTS
and the function of each control or instrument is
3-1. General
listed in table 3-1.
The ohmmeter controls and instruments used by
the operator are illustrated in figures 1-1 and 1-2
Control or instrument
Function
GROUND terminal . . . . . . . . . . . . . . . . . . . . .. Provides common ground connection between component being
tested and test equipment case.
LINE terminal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..Used when taking readings of external resistance or insulation
resistances connected across line and ground terminals.
GUARD terminal . . . . . . ......................................Used to guard against surface leakage resistance. Prevents
leakage from entering resistance measuring circuit.
Handcrank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . When rotated at approximately 160 rpm, generator produces
500 vdc.
Meter scale ZERO to INFINITY.
Provides a visual indication of resistance value under test.
Carrying handle . . . . . . ..........................................................................Used when carrying test equipment.
Test leads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Used to provide connections between test equipment and
component being tested.
Section Il. OPERATION UNDER USUAL CONDITIONS
FINITY if the test set is in good operating con-
3-2. General
dition.
Make all measurements with the hand generator
b. Zero Check. Short the LINE and GROUND
crank rotated clockwise fast enough (at ap-
terminals by connecting them and turn the hand
proximately 160 revolutions per minute (rpm)) to
generator crank at operating speed. The pointer
cause the centrifugal clutch to slip. When the
will indicate ZERO if the ohmmeter is in good
clutch slips, maximum steady voltage is being
operating condition.
delivered. The position of the pointer on the scale
indicates insulation resistance values in megohms.
3-4. Guard Connection
Be careful to read the scale directly above the
a. To obtain a true resistance value of the in-
pointer and the scale, otherwise, readings may be
sulation under test, both the insulation and the
in error because of parallax.The tests described in
measuring instrument must be protected against
paragraphs 3-6 through 3-16 are general and cover
surface leakage to eliminate possible errors. The
equipment of all types and designs. Before testing
ohmmeter has an internal protective or guard
the insulation of any equipment refer to the
system for this purpose. The guard system is
technical manual covering the specific equipment
connected to the GUARD terminal so that
under test for definite insulation resistance values
protection against surface leakage can be extended
and voltage ratings.
to the insulation under test. Any leakage current
between the base of the insulator and its terminal
3-3. Preoperational Check
will recarried back to the test set through the
Before connecting the ohmmeter to the equipment
GUARD terminal, and the internal guard circuit of
to be tested, make the following checks to
the test set will prevent this leakage from entering
determine whether the ohmmeter is operating
the measuring circuit.
properly.
b. When using guarded connections, readings
a. Full Scale Check. With no connection to the
are of the insulation resistance only and do not
take into account the parallel shunting resistance
operating speed. The pointer will indicate IN-
