TM 11-6625-298-14
Each wire should give a reading as high as that
b. Insulation Resistance Requirements for
given in the table.
(5) If the insulation resistance of any in-
sulation resistance requirements for telephone
dividual wire is less than that given in the table,
cables based on a standard required insulation
disconnect all other bunches from the sheath and
resistance of 500 megohms per mile of cable. Given
measure the wire against these bunches and
values are the minimum insulation between one
against the sheath. This will determine if the
wire and the sheath, with all other wires of the
trouble is between wires, or between the wire and
cable grounded to the sheath at a temperature of
the sheath, or both.
60 F.
(6) After testing all the bunches, measure a
NOTE
few wires of each bunch against the remaining
The set shall be operated for one minute
wires of the bunch to determine whether any of the
before readings are taken.
conductors have a low insulation resistance.
Length
Length
Required
Required
of
of
insulation
insulation
cable ( ft )
resistance (meg)
cable (mi)
resistance ( meg)
500 . . . . . . . . . . . . . . . . ...
500
5,250 . . . . . . . . . . . . . . . . . . . .
1...............
1,000 . . . . . . . . . . .
2,640 . . . . . . . . . . . . . . . . .
250
2.......................
1,500 . . . . . . . . . . . . . . . . . .
1,760 . . . . . . . . . . . .
167
3..........
2,000 . . . . . . . . . . . . . . . . . .
125
1,230 . . . . . . . . . . . . . . . 4 . . . . . . . . . . . . . . . . . .
3,000 ................................
100
880. . . . . . . . . . . . . . . . . . . . .
5
4,000 . . . . . . . . . . . . . . . . . .
88.3
660 . . . . . . . . . . . . . . . . . . . . . 6
5,000 . . . . . . . . . . . . . . . . . .
71.3
528 . . . . . . . . . . . . . . . . . . 7
.
.
.
.
.
.
6,000 . . . . . . . . . . . . . . . . . .
62.5
440 . . . . . . . . . . . . . . . . .
8.......................
7,000 . . . . . . . . . . . . . . . . . .
55.5
377 . . . . . . . . . . . .
9........
8,000 . . . . . . . . . . . . . . . . . .
50
330 . . . . . . . . . . . . . . . . .
10
9,000 . . . . . . . . . . . . . . . . . .
45.4
293 . . . . . . . . . . . . . . . . . . . .
11
41.7
10,000. . . . . . . .
264 . . . . . . . . . . . . . . . . . . . . . 12 . . . . . . . .
c. Insulation Resistance Requirements for
installation, should show an insulation resistance
Spiral-Four Cable.
above 1,000 megohms. For lengths greater than 1
mile, the minimum insulation resistance can be
(1) General. Spiral-four cable consists of four
individually insulated conductors in a braided
found by dividing 1,000 by the number of miles in
metal sheath. Cable WC-548 has a heavy rubber
the cable. For example, if a 10-mile length of Cable
jacket; Cable WF-8/G has a polyethylene plastic
WF-8/G is to be tested 1,000 10 = 100
covering. For a complete test of the insulation
megohms, the minimum insulation resistance for
resistance of spiral-four cable, measure each
10 miles.
conductor in turn with the other three conductors
NOTE
connected to the braid. Thus a complete test can
Manufacturing specifications require a
be made in four measurements.
minimum resistance of 5,000 megohms per
(2) Cable WC-548.
1,000 feet. Insulation resistance above
1,000 megohms is indicated by INFINITY
(a) Insulation resistance requirements for
various lengths of new cable WC-548 are as
on the meter scale of the ohmmeter.
follows:
(b) A cable line may develop low insulation
resistance and deteriorate to a point where it can
Cable length (mi)
Insulation resistance(meg)
no longer be useful. When this happens, the cause
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...750
1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ...200
of the drop in insulation resistance to below one
25 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1O
megohm must be located and removed.
d. Computing Minimum Insulation Resistan-
(b) In the case of working cable, the in-
sulation resistance of one 1/4-mile Cable Assembly
length or size of cable may be found as follows:
CC-358 should not drop below 10 megohms, and
(1) The required minimum insulation
the insulation resistance of a 25-to 30-mile repeater
resistance between one wire, one foot long and the
section should not drop below 1 megohm.
cable sheath is 2,640,000 megohms. To find the
(3) Cable WF-8/G.
required minimum insulation resistance for one
(a) New Cable WF-8/G, either in 1/4-mile
insulated conductor of any length, divide 2,640,000
lengths of Cable Assembly CX-1065/G of four
by the length of the conductor expressed in feet.
cable assemblies coupled together during initial
