Document: 990-0000046-1000
Page
20
4
4
2
2
0
0
0
0
-
-
F
F
S
S
S
S
i
i
d
d
e
e
S
S
c
c
a
a
n
n
S
S
o
o
n
n
a
a
r
r
2.
Short the two connector pins (or wires if un-terminated) of the shorted pair at both ends
and measure the total resistance between the ends.
R1+R2
=_______________
3.
Remove the shorts.
4.
Measure the resistance between a shorted pair on one end with an ohmmeter on the Rx1
scale.
R1+Rs
=_______________
5.
Measure the resistance from the other end.
R2+Rs
=_______________
6.
Add the measurements of 4) and 5) above, subtract the measurement of 2), and divide the
result by 2.
Rs
=_________________
7.
Subtract the value of 6) from the measured values of 4) and 5).
R1
=_________________
R2
=_________________
8.
The distance to the short from end #1 is the ratio of (R1/R1+R2) times the total cable
length. Recheck from end #2 which is (R2/R1+R2) times the cable length.
Open Wire
An open wire in a cable is much more difficult to locate than a short circuit; therefore, a
capacitance bridge is recommended. Measuring the capacitance from the open wire to the shield
on both ends allows two different capacitance readings to be recorded. This represents a direct
ratio related to cable length and distance of break from each end. Before cutting the cable,
double-check the same ratio of capacitance using an adjacent good wire in a multi-conductor
cable. Capacitance may vary from wire to wire, depending on their separation.
Most breaks occur around the tow cable termination or where a previous repair has been made.
A cable break may be found or confirmed by laying out the cable and attaching an ohmmeter
across each end of the open wire. Then flex the cable first near the termination or repaired
section and then along its entire length until the break is reached. When flexing, the ends of the
broken wire may touch, giving a continuity reading on the meter.
Insulation Resistance Breakdown
Insulation breakdown is the most difficult fault to locate. Cable leakage is not necessarily
located near the end terminations. However, the area near each termination receives the most
abuse and is, therefore, subject to suspicion. Successive cutting of the cable end until leakage
disappears will prove successful in many cases.
