The cable was tested with a TDR and an undocumented
splice was found. Close examination of the splice
showed it to be totally saturated with water. The cable
was re-spliced and the lines were now quiet.
The greatest percentage of twisted pair problems fall in
the moisture-in-the-cable category. How to locate the
problem, why one pair may be affected but not another,
and how much of the cable is affected are all problems
which have to be addressed.
A TDR will find water in the cable. It shows up as a
lowering of the cable impedance. Most times, though,
it is difficult to accurately tell how much of the cable is
affected. In filled cable, moisture cannot migrate inside
the cable so it is typically a point problem in the cable
or splice case. In air-core or pulp cable, moisture can
migrate along the cable.
By testing the cable from both ends and recording the
distance to fault in all pairs, it is possible to determine
approximately how much cable is affected.
When testing through water, measurements up to the
water are very accurate. After the water, distance read-
ings may be erroneous due to a VOP change caused by
the water. Although the moisture may be 20 or 30 feet
wide, each pair usually becomes impregnated at differ-
ent points. The range of these points will indicate the
length of the problem.
Water can seep into the conductors through pin holes in
the plastic insulator around the conductors. When test-
ing each pair, the footage to the problem may read
different for each pair. This is because the water has
penetrated through the conductor insulation at different
points and affects the conductors at different footages.