Megger BTDR1500 Guia Del Usuario página 10

6.5.3 Bridge Taps
Bridge taps occur when another pair of conductors is connected to a pair in the main cable to
form a branch or party line. At the branch or bridge junction, a short circuit type fault will occur
due to the characteristic impedance halving at that point. If a pair of conductors has a large
number of taps, then the waveform displayed will be difficult to evaluate if specific knowledge
of the cable network is lacking.
6.5.4 Load Coils
Load coils are used on telephone lines to increase the line inductance, so improving the
transmission characteristics of long lines. The inductive load coils appear as open circuits to
a cable fault locator. To test beyond the coils, a new test site further upstream has to be
chosen.
6.6 TDR APPLICATION NOTES
The BTDR is intended for use on de-energised circuits only. For operator safety the
instrument is double insulated, it also incorporates safety terminals. For complete list of the
Safety Standards adhered to, please refer to the specification (8.1). Please refer to the
enclosed Quick Reference Guide for a list of typical waveforms relating to various cable
features.
6.6.1 Metallic Shorts
These are caused by metallic contact between two conductors of a cable pair. This produces
a strong downward pulse. See the Application Card supplied with the BTDR.
6.6.2 Sheath Shorts
These are caused by a conductor in a cable making metallic contact with the metallic sheath
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of the cable. To locate a sheath short, disconnect the sheath from earth and then connect one
terminal to the sheath. Connect the other terminal to each conductor in turn until you locate
the shorting conductor.
6.6.3 Crossed Conductors
When multiple twisted pair circuits pass through the same junction box, there is a possibility
crossing conductors from adjacent pairs. This produces waveforms similar to metallic shorts
but with reduced amplitude. A crossed conductor can be located from either adjacent pair but
is more pronounced if the BTDR is connected across both crossed conductors.
6.6.4 Metallic Open Circuits
This is caused when one or both conductors of a pair are disconnected or broken and
produces a strong upward fault pulse.
6.6.5 Resistive joints or Splices
These are caused by poor joints or the joining of two cables at a junction box. They produce
upward going fault pulses whose amplitude depends on the quality of the joint.
6.6.8 Water Ingress Faults
When a cable's sheath is damaged, water can soak into the cable and contaminate the
insulation medium. The affect this contamination has is to cause a drop in cable impedance
at the start of the water ingress (downward pulse) and a corresponding increase in cable
impedance at the end of the ingress (upward pulse). If the contamination is gradual then the
impedance change is also gradual and so the pulses shape more extended and rounded. If
the whole cable is contaminated then the fault can be difficult to locate, as there is no
impedance change.