TM 11-6625-298-14
the GUARD terminal of the ohmmeter and to a
the bushing. If the reading is considerably higher,
point approximately at the center of the bushing.
the low resistance formerly observed is caused by
Make this connection by wrapping the bushing
surface leakage. When low resistance is caused by
with tinfoil or with a cloth saturated with salt
surface leakage, the bushing is serviceable but
water and connecting the test clip to the tinfoil or
should be cleaned.
cloth.
CAUTION
Use the GUARD connection if insulation
Zreading taken with the GUARD terminal con-
tests are made on a damp or humid day.
nected is approximately the same as the first
Surface leakage always will be high under
reading, the low resistance is caused by a fault in
such conditions.
(a) Connect a test lead from the LINE
NOTE
terminal of the ohmmeter to the conductor to be
Disconnect equipment from the powerline
measured (fig. 3-5).
while insulation resistance measurements
(b) Connect a test lead from the GROUND
are being made.
terminal to the shield of the cable.
a Power Cables.
(c) Tie all other conductors together and
(1) Connections for measurement of one
connect them to the GUARD terminal. Use of the
conductor to all other conductors.
GUARD terminal prevents error caused by leakage
(a) Connect a test lead from the LINE
in the parallel circuits.
terminals of the ohmmeter to the conductor to be
(3) Operation.
measured (fig. 3-5).
(a) Turn the hand generator crank at
(b) Tie all other conductors together, and
operating speed. A short length of cable usually
connected them to the GROUND terminal of the ohm-
will have an insulation resistance value of 1,000
meter.
megohms or more. The insulation resistance value
(c) Connect a teat lead from the GUARD
of a long cable usually will be considerably lower
terminal to the cable sheath. Use of the GUARD
than that of a short cable.
terminal prevents error caused by parallel ground
NOTE
leakage.
When testing the insulation of a long
(2) Connections for measurement of one
cable, operate the hand generator for
conductor to ground.
approximately 1 minute before taking a
