Insulation Test of Power Cables and Wires

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

e. Operate the hand generator crank. If the

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.

Figure 3-4. Connections for cable testing.

3-11. Insulation Test of Power Cables and Wires

(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.

(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

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

3-6