TM 11-6625-298-14
of the resistance of an insulating material is
of surface conditions. A measurement taken with a
negative and numerically large. Therefore, even a
G U A R D connection and compared to a
small increase in temperature will cause a
measurement of the same equipment taken without
relatively large decrease in insulation resistance.
a GUARD connection will establish whether low
Always make measurements at the same tem-
insulation resistance readings are caused by poor
perature, if possible, because insulation resistance
insulation or bad surface conditions. If readings
drops at high temperatures. For example, the
taken with guarded connections are higher than
insulation resistance between the stator winding of
those without guarded connections, the surface of
a slow-speed generator and the frame is 100
the equipment has a low resistance.
megohms at 80 F., but falls to 10 megohms at
140 F.
All tests shall be made with the hand generator
(2) Moisture and humidity. Chemically pure
crank rotated at approximately 160 revolutions per
water is a non conductor; however, the impurities
minute. Since the meter scale is calibrated in
that water absorbs from the atmosphere and other
megohms, the position of the pointer on the scale
sources cause it to be partially conductive.
indicates insulation resistance values directly in
Therefore, moisture will become a conductor in
megohms. Indications shall be read with the eyes
parallel with the insulation and will cause a
directly above the pointer and the scale to prevent
decided drop in insulation resistance. This is a
errors. Types of operation described in subsequent
temporary condition and may be remedied by
paragraphs are necessarily general to cover
drying the equipment.
equipment of all types and designs. Before testing
(3) Chemical fumes. Strong acid or alkali
any specific equipment, refer to the technical
fumes attack insulation and permanently lower its
manual covering the equipment for definite in-
insulation resistance.
sulation resistance standards.
(4) Surface conditions. Dirt, especially when
a. Unit of Measurement. Insulation resistance is
mixed with oil, or with a copper or carbon dust,
that property of an insulating material which
forms a low-resistance path across the insulation,
opposes the passage of electrical current. It is
lowering its resistance.
expressed in ohms or megohms. Insulation
c. Values of Insulation Resistance. Since in-
resistance increases as the thickness of the in-
sulation resistance is affected by varying con-
sulating material increases but decreases as the
ditions (b above), no constant rules can be
area of material under test increases. For example,
established to govern its value. Certain minimum
if an insulating material covering a given surface
values have, however, been established arbitrarily
and having a resistance of 100 megohms is ex-
for different types of cable and equipment (para 3-
tended to 10 times its area, the insulation
12). Actual values of insulation resistance are not
resistance will drop to 10 megohms. Thus, one mile
as important as changes in insulation resistance.
Of* spinal-four cable may measure 1,000 megohms
Periodic tests should be made and the results
but 10 miles will measure 100 megohms. If the
recorded together with information regarding
insulating material is doubled in thickness,
climatic conditions at the time tests are made.
however, the area having an insulation resistance
of 100 megohms now will have a resistance of 200
Machines Rated 600 Volts or Less
megohms, and the area having an insulation
resistance of 10 megohms will have a resistance of
a. Connect a test lead from the LINE terminal
20 megohms.
of the ohmmeter to a lead, an exposed conductor,
or the commutator of the machine (fig. 3-1).
b. Conditions Affecting Insulation Resistance.
(1) Temperature. The temperature coefficient
