Submission for mod.telecom (telephone cable fault location)
la...@kitty.uucp
> > Maintaining proper tip/ring polarity on all outside cables is very
> > important to the telephone company, since various kinds of test equipment
> > and procedures used from the central office can pinpoint precise location
> > of a cable fault; an important assumption for many of these fault-locating
> > procedures is that the outside cables always have tip/ring correspondence.
>
> I've always wanted to know how the locating of cable faults works -
> i have some hypotheses, but maybe you would care to edify myself and
> other readers on this subject?
There are a number of fault-locating methods used by operating
telephone companies (and others who maintain long runs of cable); these
methods have in common that measurements are taken from one or both ends
of the cable only - with the location of the fault between the two locations
being the result of the measurement.
Fault location in telephone cable is not as simple as just making
resistance or capacitance measurements. One of the reasons is that there
is often a foreign potential present across tip-to-ring, tip-to-ground, and
ring-to-ground. Depending upon the type of fault (like a wet cable splice),
all three of these measurements may be different. What this means is that
a simple ohmmeter circuit or simple two-terminal Wheatstone bridge is
useless since these resistance measurement techniques will be seriously in
error when made in the presence of a foreign potential.
As a result, various other bridge measurements have been developed
over the years; many of these are three-terminal measurements which are also
made to ground in order to compensate for the presence of foreign potentials
on the suspect pair. Examples of these techniques are the Varley Loop,
Murray Loop, Fisher Loop, Moody Loop and Hilborn loop. Some of the above
procedures also require the use of pairs in the same cable which are known
to be good (like the Moody Loop).
All of the above methods are used with DC bridge excitation and
measure the location of crosses (conductor-to-conductor) and grounds
(conductor-to-ground). Some of the above methods are used with AC bridge
excitation (like the Murray Loop) and can therefore measure the location
of open pairs and the presence of "split" pairs (caused by an incorrect
splice).
The selection of the actual test method is generally based upon
the preference and experience of the craftsperson making the test.
A fault location test on a bad pair will generally begin as
follows:
1. Isolate pair from apparatus at both ends, and "cord" pair to a
test position in the central office.
2. Measure voltage from: tip-to-ring, tip-to-ground, ring-to-ground,
tip-to-battery and ring-to-battery. Test battery voltage is
usually a current-limited -48 volts, but other test voltages are
also used.
3. If there is little foreign potential (a good craftsperson knows
the difference between actual potential and momentary potential
caused by capacitive charging of the subject cable pair) in the
above measurements, it is safe to assume that fault is either an open
or a cross, and is isolated from ground. Measure resistance from
tip-to-ring using a simple Wheatstone bridge circuit. If there is
a cross (i.e., the pair is open, so anything less than normal leakage
resistance is a cross), use the resistance measurement with a circuit
layout card (shows path, cable gauges, loading, and distance from
central office) and calculate location of short based upon knowing
the actual resistance of the cable from the central office to any
point.
If the resistance is high (like normal leakage resistance, >> 100 K
ohms), have craftsperson at other end of line short pair from
tip-to-ring. Measure resistance using Wheatstone bridge circuit.
If resistance is the expected loop resistance, then pair looks
good for tip-to-ring DC continuity, but may have leakage to ground.
If resistance is still high, then pair is open.
If pair is open, then measure capacitance using bridge in Murray
Loop configuration with AC excitation (or use capacitive "kick"
method of foreign potential measurement - if you want to be crude
about it). Using capacitance measurement and known capacitance
of cable, determine the distance of the "good" section of the pair
under test. All outside telephone cable, regardless of wire gauge
- either polyethylene or paper insulted - is intentionally designed
to give a known capacitance per foot; the most common value is 0.083
uF/loop-mile.
4. If there was significant foreign potential or the presence of a
ground as detected in (2) above, then the situation gets hairy; now
is the time to use some of the other bridge techniques mentioned
above. The foreign potential currents (as opposed to voltages above)
may be measured to get a handle on the cause of the foreign potential
(usually a cross with another working pair). These bridge techniques
may also require a craftsperson at the other end to selectively
ground, cross, or connect the suspect pair to other pairs that are
known to be good.
5. If a pair looks good from a DC standpoint, but still has transmission
trouble, then capacitance measurements are taken from each wire in
the pair to ground to see if it might be crossed with a wire from
another pair, or have some other type of fault. Also, resistance
measurements may be again applied, but with a high range in the
order of the cable leakage resistance, to see if something is amiss.
The above five steps are the "traditional" methods used by telephone
companies, and are still used in the majority of test centers.
Other (and at times better) techniques include:
1. Capacitance measurement through electronic means that are reasonably
immune to foreign potentials on the pair under test. Direct-reading
test sets have been available for a number of years that read fault
distance directly in feet (you dial in the normal capacitance of the
cable per unit length). For example, I have a Hewlett-Packard
4910-F open fault locator test set which has paid for itself many
times over in locating cable faults; it will resolve down to _feet_.
2. Using a time-domain reflectometer (TDR, also sometimes called "sweep"
testing) will give a good picture of a cable pair condition under
many conditions. The TDR works by sending a short, high-energy
pulse down a cable, and displaying the return echo on an oscilloscope
display. Use of a TDR requires dialing in the dielectric constant
of the cable (not a problem, usually) so that the normal propagation
velocity of the cable is known. The display is calibrated directly
in feet or meters, so that the precise location is the fault can be
pinpointed. Poor splices, leakage to ground, etc. show up well on
a TDR, along with any bridge taps. A TDR has some limitations, in
that the measurement generally cannot pass through more than one
loading coil. Most TDR's are used for coaxial cable measurement,
but TDR's for twisted pairs are available from Biddle, Northeast
Electronics, W&G Instruments, etc.
3. Sending an audio-frequency or VLF signal into a cable pair and
following its progress using a hand-held receiver held in close
proximity to the cable. This is obviously not too handy for
faults which could be miles away :-), but it is inexpensive and
useful for one-person use in the field where distances are
relatively short.
In summation, the above will give you some idea as to what is involved
in cable-fault location; some of the procedures may form an outline for people
reading this article to actually use themselves under the right circumstances.
Overall, there is no magic procedure for cable-fault location, with the actual
method being a matter of personal experience, discretion, and nature of the
available fault-locating apparatus (not unlike writing a computer program :-).
<> Larry Lippman @ Recognition Research Corp., Clarence, New York
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