man_digiflex_com_en_02_5.doc 31
(2) Resistance faults: If a cable is not properly connected in a junction
box, increased contact resistance will occur. This type of fault
generates a positive reflection in a size which is dependent on the
quality of the connection resp. its resistance.
(3) Shield interrupt means that the metal shield of a cable is broken or has
become detached. To locate such faults and to suppress noise signals
on the display, connect the test leads to the shield and to as many
conductors of the cable as possible. This type of fault generates a fault
reflection the size of which is dependent on the resistance of the
interrupt.
(4) Crossed lines (”split pairs”) and restored reversed lines occur when a
telecommunication cable is twisted or untwisted. Usually this may
happen at a cable joint. This phenomenon is one of the main causes
of crosstalk interference. Reversed wires in an untwisted cable will
generate a positive error reflection. If an individual cable is twisted, a
negative error reflection appears. As the distance between the points
of crossing and crossing is usually very small, both reflections occur
almost simultaneously and appear as a single weak reflection.
(5) A humidity-induced fault is brought about by water penetrating into a
defective line. At the beginning of a wet section a negative reflection
similar to a short-circuit can be observed, followed at the end of the
wet section by a small positive fault reflection typical of an open cable
end. In some cases, e.g. if the moisture gradually increases or
decreases with distance, these points of reflection spread into a
constant up and down in the horizontal traceform. In an old cable
which is wet along its entire length it may be difficult to detect a
moisture-induced fault because the beginning and the end of the zone
of moisture exposure are not clearly marked. It is only in rare cases
that the fault reflection becomes apparent as clearly as for a short-
circuit or broken line. Moisture in a cable filled with petrolatum may
possibly result in very small fault reflections as the volume of moisture
is only limited.
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