04-06-2013, 02:59 PM
FAULT FINDING SOLUTIONS
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GOOD CABLE INSULATION
When voltage is impressed across any insulation
system, some current leaks into, through, and
around the insulation. When testing with dc highvoltage,
capacitive charging current, insulation
absorption current, insulation leakage current, and
by-pass current are all present to some degree. For
the purposes of this document on cable fault
locating, only leakage current through the insulation
will be considered.
For shielded cable, insulation is used to limit current
leakage between the phase conductor and
ground or between two conductors of differing
potential. As long as the leakage current does not
exceed a specific design limit, the cable is judged
good and is able to deliver electrical energy to a
load efficiently.
Cable insulation may be considered good when
leakage current is negligible but since there is no
perfect insulator even good insulation allows some
small amount of leakage current measured in
microamperes. See Figure 1.
Why A Cable Becomes Bad
All insulation deteriorates naturally with age,
especially when exposed to elevated temperature
due to high loading and even when it is not physically
damaged. In this case, there is a distributed
flow of leakage current during a test or while
energized. Many substances such as water, oil and
chemicals can contaminate and shorten the life of
insulation and cause serious problems. Cross-linked
polyethylene (XLPE) insulation is subject to a condition
termed treeing. It has been found that the
presence of moisture containing contaminants,
irregular surfaces or protrusions into the insulation
plus electrical stress provides the proper environment
for inception and growth of these trees
within the polyethylene material. Testing indicates
that the ac breakdown strength of these treed
cables is dramatically reduced. Damage caused by
lightning, fire, or overheating may require replacement
of the cable to restore service.
CABLE FAULTS DESCRIBED
When at some local point in a cable, insulation has
deteriorated to a degree that a breakdown occurs
allowing a surge of current to ground, the cable is
referred to as a faulted cable and the position of
maximum leakage may be considered a catastrophic
insulation failure. See Figure 4. At this
location the insulation or parallel resistance has
been drastically reduced and a spark gap has
developed. See Figure 5.
Occasionally a series fault shown in Figure 6 can
develop due to a blown open phase conductor
caused by high fault current, a dig-in or a failed
splice.
LOCATE FAULTS IN BURIED PRIMARY CABLE
After all clearances have been obtained and the
cable has been isolated in preparation for cable
fault locating, it is strongly recommended that a
fixed plan of attack be followed for locating the
fault. As in diagnosing any complex problem, following
a set step-by-step procedure will help in
arriving at the solution or, in this case, pinpointing
the fault efficiently.
At the very start, it is a good idea to gather as
much information as possible about the cable
under test. Information that will help in the fault
locating process is:
■ Cable type — is it lead covered, concentric neutral
(bare or jacketed), tape shield?
■ Insulation type — is it XLPE, EPR, Paper?
■ Conductor and size — is it CU, AL, stranded,
solid, 2/0, 350 MCM?
■ Length of the run — how long is it?
■ Splices — are there splices, are the locations
known?
■ T-taps or wye splices — are there any taps, are
the locations known, how long are branches?
After obtaining the cable description the acronym
“TALL” can help you remember the procedure for
finding cable faults in buried cable.
DC Hipot Test
After a surge generator is connected to the cable
under test, do a quick dc proof test to be sure the
cable is faulted and will not hold voltage. Make a
note of the kilovolt measurement when the fault
breaks down. This will be an indicator of what
voltage will be required when surging in order
break down the fault when doing prelocation or
pinpointing. If there are transformers connected
to the cable under test, a proof test will always
indicate a failure due to the low resistance path to
ground through the transformer primary winding.
A dc proof test in this case is not a valid test.
LOCATE FAULTS IN ABOVE GROUND PRIMARY
CABLE
Some faults can be found by searching for obvious
physical damage to the cable especially if the cable
section is short. If necessary, connect a surge generator
and walk the cable and listen for the discharge.
If the cable is very long it might take a
good deal of time to walk the cable while the
surge generator is on. To reduce the total time
spent and to minimize high-voltage exposure to
the cable, use a localizing technique before
attempting to pinpoint the fault.
Once the fault is localized, a listening aid is used
to zero in on the thump that occurs when the
surge generator breaks down the fault. For metalto-
metal (bolted) faults on non-buried cable, an
electromagnetic impulse detector may help to pinpoint
the fault. The use of electromagnetic
impulse detectors is discussed in detail in Section
VI.
OVERVIEW
Before attempting to locate underground cable
faults on direct buried primary cable, it is necessary
to know where the cable is located and what
route it takes. If the fault is on secondary cable,
knowing the exact route is even more critical.
Since it is extremely difficult to find a cable fault
without knowing where the cable is, it makes
sense to master cable locating and tracing and to
do a cable trace before beginning the fault locating
process.
Success in locating or tracing the route of electrical
cable and metal pipe depends upon knowledge,
skill, and perhaps, most of all,
experience. Although locating can
be a complex job, it will very likely
become even more complex as
more and more underground
plant is installed. It is just as
important to understand how the
equipment works as it is to be
thoroughly familiar with the exact
equipment being used.