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FAULT FINDING
SOLUTIONS
Table of Contents
Section
Page
Section
Page
I
Cable Characteristics
When Cable Insulation is Bad . . . . . . . . . . . . . .2
Why a cable becomes bad . . . . . . . . . . . . . . .3
V
Surge Generators, Filters and Couplers
Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19
Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
Proof/Burn . . . . . . . . . . . . . . . . . . . . . . . . . .21
Surge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21
Ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21
Arc Reflection Filters and Couplers . . . . . . . . .22
II
Fault Locating Procedures
Test the cable . . . . . . . . . . . . . . . . . . . . . . . . .4
Fault resistance and loop test . . . . . . . . . . . .4
TDR tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
DC hipot test . . . . . . . . . . . . . . . . . . . . . . . . .5
Fault resistance and loop test . . . . . . . . . . . .5
TDR tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
DC Hipot test . . . . . . . . . . . . . . . . . . . . . . . . .6
Cable Route . . . . . . . . . . . . . . . . . . . . . . . . . .6
Locate - pinpoint the fault . . . . . . . . . . . . . . . .6
Locate Faults in Above Ground
Primary Cable . . . . . . . . . . . . . . . . . . . . . . . . . . .6
VI
Localizing Methods
Sectionalizing . . . . . . . . . . . . . . . . . . . . . . . .24
Resistance ratio . . . . . . . . . . . . . . . . . . . . . .24
Single phase, coaxial power cable
with neutral bridges over splices . . . . . . .25
Single phase PILC cable with bonded
Three-phase PILC . . . . . . . . . . . . . . . . . . . .26
Arc reflection . . . . . . . . . . . . . . . . . . . . . . . .27
Surge pulse reflection . . . . . . . . . . . . . . . . .28
III
Cable Route Tracers/Locators
VII Locating or Pinpointing Methods
Electromagnetic/Acoustic Surge Detection . . .31
VIII Solutions for Cable Fault Locating
Underground Utility Locating
Suitcase Impulse Generator . . . . . . . . . . . . . . .37
Cable Analyzer . . . . . . . . . . . . . . . . . . . . . . . . .38
IV
How to See Underground Cable Problems
Time domain reflectometry . . . . . . . . . . . . .12
Differential TDR/radar . . . . . . . . . . . . . . . . .13
Low-voltage TDR/cable radar . . . . . . . . . . . .13
Faults that a low-voltage
TDR will display . . . . . . . . . . . . . . . . . . . . . .13
Landmarks that a low-voltage
TDR will display . . . . . . . . . . . . . . . . . . . . . .13
Velocity of propagation . . . . . . . . . . . . . . . .14
Gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16
Cursors . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16
Zoom . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16
Pulse width . . . . . . . . . . . . . . . . . . . . . . . . . .16
Three-stake method . . . . . . . . . . . . . . . . . . .17
Fault Finding Solutions
Table of Figures
Figure
Page
Figure
Page
1
Good insulation . . . . . . . . . . . . . . . . . . . . . . .2
28
TDR used to measure distance to
2
29
3
Bad insulation . . . . . . . . . . . . . . . . . . . . . . . .2
30
4
31
TDR used to localize distance to
fault relative to a landmark . . . . . . . . . . . . .18
5
Fault region simplified diagram . . . . . . . . . . .3
6
pen or series fault on the cable . . . . . . . . . .3
32
Three-stake method . . . . . . . . . . . . . . . . . . .18
7
Test for insulation (fault) resistance
33
34
Energy vs. voltage for a 4-µF, 25-kV
surge generator . . . . . . . . . . . . . . . . . . . . . .19
8
Loop test for continuity using a
Megger insulation tester . . . . . . . . . . . . . . . .4
35
Energy vs. voltage for a 12-µF, 16-kV
9
TDR test for cable length . . . . . . . . . . . . . . . .5
10
36
Energy vs. voltage for a constant
energy 12-µF, 16/32-kV surge generator . . .20
11
37
12
Cable under test . . . . . . . . . . . . . . . . . . . . . . .7
38
13
Using an ohmmeter to measure
39
14
Hookup showing ground rod at
40
Resistive arc reflection diagram . . . . . . . . . .23
41
15
Hookup with far end of cable
under test isolated . . . . . . . . . . . . . . . . . . . . .9
42
Basic Wheatstone Bridge . . . . . . . . . . . . . . .24
16
Current coupler connection to
neutral on primary jacketed cable . . . . . . . . .9
43
Murray Loop Bridge application . . . . . . . . .24
44
17
Inductive coupling to neutral on
primary jacketed cable . . . . . . . . . . . . . . . . .10
45
Coaxial power cable with neutral
bridges over splices . . . . . . . . . . . . . . . . . . .25
18
Use of return wire to
46
Electromagnetic detection in single-phase
19
Circling path with receiver . . . . . . . . . . . . . .10
47
Electromagnetic detection of faults on
three-phase power cable . . . . . . . . . . . . . . .26
20
No interference, no offset between
magnetic field center and center
48
49
Arc reflection and differential arc
reflection methods of HV radar . . . . . . . . . .27
21
Depth measurement using null method
with antenna at 45-degree angle . . . . . . . .11
50
Surge pulse reflection method
22
Offset caused by interference from
nontarget cable . . . . . . . . . . . . . . . . . . . . . .11
51
23
52
24
53
25
TDR used to measure length of cable
54
26
TDR used to measure length of cable
with far end shorted . . . . . . . . . . . . . . . . . .14
55
56
AC voltage gradient . . . . . . . . . . . . . . . . . . .33
27
TDR measuring distance to a
57
DC voltage gradient . . . . . . . . . . . . . . . . . . .33
Fault Finding Solutions
1
Cable Characteristics
SECTION I
GOOD CABLE INSULATION
When voltage is impressed across any insulation
system, some current leaks into, through, and
around the insulation. When testing with dc high-
voltage, 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 insula-
tion will be considered.
For shielded cable, insulation is used to limit cur-
rent 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.
WHEN CABLE INSULATION IS BAD
When the magnitude of the leakage current
exceeds the design limit, the cable will no longer
deliver energy efficiently. See Figure 3.
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 physi-
cally 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 con-
dition 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 environ-
ment 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 replace-
ment of the cable to restore service.
µAmps
Inductance
Series Resistance
R
S
L
HV
Test
Set
Parallel
Resistance
R
P
Z
0
Z
0
Capacitance
C
Figure 1: Good insulation
The electrical equivalent circuit of a good run of
cable is shown in Figure 2. If the insulation were
perfect, the parallel resistance R
P
would not exist
and the insulation would appear as strictly capaci-
tance. Since no insulation is perfect, the parallel or
insulation resistance exists. This is the resistance
measured during a test using a Megger
®
Insulation
Tester. Current flowing through this resistance is
measured when performing a dc hipot test as
shown in Figure 1. The combined inductance (L),
series resistance (R
S
), capacitance (C) and parallel
resistance (R
P
) as shown in Figure 2 is defined as
the characteristic impedance (Z
0
) of the cable.
Figure 2: Equivalent circuit of good cable
mAmps
HV
Test
Set
Figure 3: Bad insulation
2
Fault Finding Solutions
Cable Characteristics
SECTION I
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 cata-
strophic 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.
Phase Conductor
Spark Gap
G
Fault Resistance
R
F
Shield or Neutral
Figure 5: Fault region simplified diagram
Spark Gap
G
mAmps
Phase Conductor
HV
Test
Set
Fault Resistance
R
F
Fault
Figure 4: Ground or shunt fault on the cable
Shield or Neutral
Figure 6: Open or series fault on the cable
Fault Finding Solutions
3
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