Medium Voltage (MV) cable accessories are used to terminate, join, or connect power cable and to manage electrical stresses. One challenge faced by utilities is assuring the reliability and maximizing the expected life of these components. EPRI has research underway to raise industry understanding and awareness of the function of cable accessories, and to illustrate the impact of various workmanship approaches on the anticipated performance of cable accessories as determined by the presence of partial discharge (PD).
This section describes a multi-year project investigating the performance of medium,voltage (MV) cable terminations. The function of a cable termination is to provide stress control at the cable shield terminus and separation between the cable shield and exposed conductor for external leakage insulation. In order for cable terminations to effectively control electrical stresses, they must be designed to do so at expected operating voltages, and built meticulously to specifications, as minor differences in workmanship may influence-electrical stress control efficacy. Some utilities have experienced in service failures of cable terminations operating at 35kV, and forensic examination has shown that termination designs and preparation practices may have been contributors. In this research, EPRI has performed research to better understand the contributors to in-service failures of MV terminations, and to understand and illustrate the impact of various workmanship approaches on the anticipated health of cable terminations, as determined by the presence of partial discharge (PD). The research was focused on 35 kV cable terminations, as higher operating voltages exacerbate the internal breakdown that will occur with the presence of partial discharge. Both heat shrink and cold shrink termination kits were examined. The cable type utilized for study was a 750 Al XLPE insulated cable with a strippable semiconducting insulation shield
Here is an update video describing MV Cable termination testing underway at EPRI’s Lenox Laboratory.
Utility companies that own and operate urban underground distribution
systems face a number of challenges. Installing, assessing and
maintaining underground infrastructure is challenging due to its
accessibility. Additionally, installed underground distribution
infrastructure is aging, and much of the early installed plant is
reaching the end of its useful life. Utility managers are charged with
managing costs, improving system reliability and resiliency, increasing
power throughput, and ensuring the health and safety of workers and the
public. In addition, significant changes to electric distribution
systems are underway, including the application new load types, new
distributed generation sources, new underground equipment and materials,
and automation technologies. At the same time, many utilities are facing
loss of institutional knowledge. EPRI’s Underground Assets research
project focuses on providing utilities the knowledge to acquire,
optimize, and maintain underground distribution infrastructure.
One challenge faced by urban underground utilities is assuring the
reliability and maximizing the expected life of key asset types such as
cable accessories, devices used to terminate, join, or connect power
cable and to manage electrical stresses. Beginning in 2019, EPRI has focused its
research on 35 kV cable terminations, used to connect underground cable
systems with overhead busbar, conductor, or apparatus.
The function of a cable termination is to provide stress control at the
cable shield terminus and separation between the cable shield and
exposed conductor for external leakage insulation [1]. In order for
cable terminations to effectively control electrical stresses, they
must, of course, be designed to do so at expected operating voltages,
and built meticulously to specifications, as minor differences in
workmanship may influence electrical stress control efficacy. Some
utilities have experienced in service failures of cable terminations
operating at 35kV, and forensic examination has shown that termination
designs and preparation practices may have been contributors.
Project Objectives
This research project seeks to raise industry understanding and
awareness of the function of cable accessories, and to illustrate the
impact of various workmanship approaches on the anticipated performance
of cable accessories as determined by the presence of partial discharge
(PD). The research is focused on 35 kV terminations, and
includes:
Examination of field aged cable terminations
Examination of newly prepared cable terminations, with and without
known defects
The research has been focused on 35 kV cable terminations, as higher
operating voltages exacerbate the internal breakdown that will occur
with the presence of partial discharge. Both heat shrink and cold shrink
termination kits were examined. The cable type utilized for study was a
750 Al XLPE insulated cable with a strippable semiconducting insulation
shield.
EPRI’s focus in examining newly prepared terminations was to vary the
approach to cutting back the insulation shield and to applying stress
relief materials (such as mastic tapes) at this cut back location, as
this is a key area of the termination where electrical stressed must be
properly managed.
Research Approach
Examination of Field Aged cable terminations
EPRI performed forensic analysis of aged medium voltage cable
terminations that had been in service in the field. This included
terminations which performed successfully, and those which were removed
because of performance issues, such as the presence of partial discharge
(PD) identified through testing. The purpose was to examine the factors
which contribute to the presence of PD in aged cable terminations to
inform specifications and work methods.
EPRI measured the levels of partial discharge present in each sample,
including the PD inception and the PD extinction voltage levels. EPRI
then dissected each sample to examine termination materials,
construction, condition, and workmanship to identify potential causes of
PD and to inform specifications and work methods.
Examination of Newly Prepared Cable Terminations
EPRI examined two types of 35-kV terminations, one heat shrink and one
cold shrink, built according to manufacturer instructions, while using
different work methods to prepare the cut back of the strippable
semi-conducting insulation shield, and varying the placement of tapes /
mastics provided by the manufacturer and applied to the insulation
shield cutback location.
EPRI measured the levels of partial discharge present in each sample,
including the PD inception and the PD extinction voltage levels, seeking
to better understand how various preparation approaches affect the
levels of PD measured in completed terminations.
Preparation of Terminations to be tested
Termination samples were prepared as follows:
For both the Heat Shrink and Cold Shrink Terminations, the
insulation shield “edge” was prepared using:
Knife cut (Knife Cut)
Cutting Tool cut, creating a square edge (Square Cut)
Chamfered Cut
For heat shrink terminations, the placement of stress reducing
mastic tapes was varied to reflect conflicting interpretations of
preparation instructions.
Tape centered half over the insulation and half over the semi
con insulation shield (Tape Centered)
Tape filling in the edge or corner, and then wrapped centered
over the insulation and semi con insulation shield (Tape Filled)
For cold shrink terminations, for one sample (with a Knife Cut),
EPRI varied the placement of the tube such that the placement of the
mastic (within the tube) was varied.
In all, ten (10) 35-kV terminations were prepared for testing.
2 - Terminations Assembled using Varied Work Practices
Overview
One driver of EPRI’s research into the performance of cable terminations
is the experience of cable termination failures by some utilities. One
utility, who had experienced four in-service failures of newly installed
35kV cable terminations, performed partial discharge (PD) testing of
installed cables through a service provider. These tests confirmed PD
with a magnitude of concern in many of the companion terminations.
Forensic analysis of two of the field failures by the utility company
indicated that the cables failed near the insulation shield cutback area
of each assembled termination. A group of eighteen (18) terminations
were removed from service for testing and root cause evaluation by EPRI.
PD testing at the EPRI facility in Lenox, MA confirmed that many of
these terminations were exhibiting a significant level of internal PD
activity. Not all terminations could be evaluated in this manner due to
limited cable length at the free end. Of those that could be tested, all
exhibited PD at a level well above the reasonably accepted value of 5 pC
(picocoulombs of apparent charge). Subsequent dissections of the 18
terminations removed from service revealed that the electric stress
control measures were inconsistently implemented for a number of reasons
including unclear instructions from the termination manufacturers, lack
of fundamental technical reference information, work practice concerns,
and significant installation constraints including tight bends and poor
access for termination installation. In addition, there was limited
familiarity with the particular cable and terminations. Similar problems
were found with three cold-shrink terminations that were assembled for
evaluation as an alternate. A synopsis of the forensic analyses is
presented in the following section.
To provide a fundamental basis for understanding the role of work
practices, termination designs, and assembly methods, a series of cable
terminations were assembled with varied work practices and subjected to
electrical testing, with particular emphasis on the occurrence of PD
and, where detected, on the magnitude and characteristics of the PD
activity. It is this effort that is the focus of this section.
Synopsis of Field Failures and Termination Partial Discharge Activity
A large number of cable terminations were installed at a facility that
was being upgraded. The affected cables included two distinct groups.
One group included cable rated for 46 kV service, energized at 34 kV,
with a 4/0 stranded aluminum conductor, insulated with tree-retardant
crosslinked polyethylene (TRXLPE) applied to the 133 % level [2]. The
conductor strands include a dual technology water-blocking treatment.
The second set of cable is rated for 35 kV service and also energized at
34 kV. These include a 750 MCM stranded aluminum conductor with the same
strand block technology, TRXLPE insulation at the 100 % insulation
level, and stranded copper concentric neutral conductors. Both types of
cable were manufactured by the same supplier. Both designs include a
strippable insulation shield and jacketed construction. Figure 1 and
Figure 2 present overviews of the heat-shrink and cold-shrink
terminations removed from service and submitted for analysis.
Figure 1:
Overview of selected heat-shrink terminations removed from service
Figure 2:
Overview of cold-shrink terminations assembled for testing
PD Testing following Termination Removal
Partial discharge testing was planned for the set of 18 samples removed
from service specifically for investigative purposes. Due to limited
spare length on the affected cable circuits, some the samples were
provided with attached cable lengths too short to support application of
stress control terminations, essential for such testing. Where the
lengths were suitable for installation of manually applied tape type
terminations, these were installed to support PD testing. This group
included 12 of the 18 terminations. It should be noted that all of these
terminations had been subjected to testing for PD in the field prior to
being removed from service. The high PD values measured provided a basis
for their removal.
PD testing of the 12 samples indicated PD inception at a level below
phase-to-ground potential. This indicated that the cables and
corresponding terminations would have been operating under conditions
with damaging internal electrical discharges. These tests indicated a
general problem as opposed to an isolated cause, limited to a small
fraction of the population.
PD initiation below service voltage was also measured in 2 of the 3 cold
shrink terminations. One of these had been dissected and could not be
tested. Forensic analysis suggested that this termination would also
have exhibited PD at or below service voltage.
The PD testing method and cable preparation method for this testing are
described in a following section.
Forensic Examination Summary
Following PD testing, all of the termination samples were dissected and
examined for a review of the assembly details, degradation or defects,
electrical damage, physical damage, or any other anomalies that might be
causative of the measured PD. Eighteen terminations were recovered from
service and three additional terminations, assembled by the same crews
using the same type of cable, were also provided for teardowns and PD
root cause evaluation. The terminations recovered from service were
designed with heat-shrinkable technology. The three independent
terminations were designed with cold-shrink technology. None of the
latter three had ever been energized.
All of the terminations recovered from service were systematically
dissected for examination of the internal components and assembly
practices, following PD testing, where possible. In each of the
previously-installed terminations, defects were found at the insulation
shield cutback. In some cases, the cutbacks were made reasonably well,
but the heat-shrinkable termination stress control material did not flow
into and fill the entire lower corner of the shield cutback where it
intersects the insulation. An example is shown in Figure 3. In this
example, the shield cutback exhibits some edge scalloping and exposed
insulation can be seen at the base of the vertical face of the cutback.
Additional examples of this prevalent condition are shown in Figure 4
and Figure 5, corresponding to the heat-shrink terminations. Exposure
of the insulation at this location inevitably leads to PD.
The same problem was found when the cold-shrink terminations were
dissected. In each case, the stress control putty material did not fully
conform to and fill the corner at the base of the insulation shield
cutback. An example from one the terminations is presented in Figure 6.
Complete filling of this interface with the shielding material is made
more complicated when the insulation shield cutback is not produced in a
uniform manner, without asperities, projections, and cuts into the
adjacent insulation. The assembly instructions draw superficial
attention to the details at this location and do not address the cable
insulation surface preparation at all. This includes insulation sanding
requirements, cleaning, and inspection. No detailed instructions are
provided regarding selection and use of insulation shield cutback tools.
No direction is provided about the ideal geometry for this interface.
This is the most highly stressed area of the cable-to-termination
interface. All 18 of the heat-shrink terminations were affected by
insufficient shielding at the insulation shield cutback. The same
problem was found in all 3 of the cold-shrink terminations.
Figure 3:
Example of insulation exposed at base of insulation shield cutback in heat-shrink termination
Figure 4:
Additional example of insulation exposure at insulation shield cutback base in heat-shrink termination
Figure 5:
Third example of insulation exposure at base of insulation shield cutback in heat-shrink termination
Figure 6:
Insulation exposure and ragged edge of insulation shield cutback in cold-shrink termination
Research Sample Production
Following the findings from examination of the terminations removed from
service, a study was undertaken to explore the role of cable preparation
methods for elimination of partial discharge at the insulation shield
cutback. Various field practices were reviewed and practical options for
preparation of this interface were considered. These approaches were
adopted as a basis for assembly and subsequent testing of terminations
that represent a range of insulation shield cutback profiles and
shielding material effectiveness. Heat-shrink and cold-shrink
terminations were prepared for these tests. The cable type used for this
matrix of test samples included the same cable used by the utility
company. This includes a TRXLPE insulation system and a strippable
insulation shield, as previously described.
Field crew members from a member utility company assisted EPRI with
preparation of the terminations for study. Cables staged for these
assemblies are shown in Figure 7. The terminations were prepared by
varying the type of edge profile formed when cutting back the strippable
insulation shield, the position of stress control tapes or mastics
provided by the manufacturer, and the method of stress control material
application to the insulation shield cutback area.
Figure 7:
Preparation of cable termination samples
EPRI utilized three different approaches for varying the type of ‘edge’
used when cutting back the insulation shield. These included a ‘knife’
cut, using a standard lineman’s knife as shown in Figure 8 and Figure 9; a ‘square’ cut, using a tool designed for scoring the semiconducting
insulation shield layer, as shown in Figure 10 and Figure 11; and a
‘chamfered’ cut prepared with a rat-tail file, as shown in Figure 12
and Figure 13. A guide was improvised for the rat-tail file by
application of a stainless steel constant force spring band, as used to
joined braided ground conductors to tape type neutral shields. The file
is shown resting against the spring.
Figure 8:
‘Knife cut’ of semiconducting shield layer using a lineman’s knife
Figure 9:
‘Knife cut’ of semiconducting shield layer using a lineman’s knife
Figure 10:
‘Square cut’ of semiconducting shield layer using a scoring tool
Figure 11:
‘Square cut’ of semiconducting shield layer using a scoring tool
Figure 12:
‘Chamfered cut’ of semiconducting shield layer using a rat-tail file
Figure 13:
‘Chamfered cut’ of semiconducting shield layer using a rat-tail file
EPRI also varied the approach used to apply the stress relieving mastic
(SRM) tapes provided with the termination kits. One approach, identified
as ‘tape centered’, was to center the tape over the insulation shield
cutback, with one half the tape covering the insulation and the other
half covering the semiconducting shield layer as shown in Figure 14.
This approach has the potential to create a void at the insulation
shield cutback location.
Figure 14:
Tape centered – completed wrap of tape centered over semicon layer and insulation
Another approach, identified as ‘tape filled’, included abutting the
tape to the insulation shield layer cutback edge and completing one wrap
onto the insulation surface with the tape filling in the edge or corner,
and then completing the application of the tape by centering over the
insulation and insulation shield as shown in Figure 15 and Figure 16.
Figure 15:
Tape filled – starting one complete wrap of the tape applied to insulation
Figure 16:
Tape filled – completing one wrap of the tape applied to insulation
With the cold-shrink termination design, the stress relief mastic
material is pre-positioned and bonded to the inner surface of the
shielded insulation tube, so that the use of an SRM tape is not
required. The proper application of the shrink tube depends on the
placement of the tube and a cutback of the semi conducting layer that
conforms to manufacturer requirements. Using one of the knife cut
samples, EPRI varied the placement of the shrink tube by one inch, such
that the mastic would be improperly applied. Figure 17 shows the
cold-shrink termination being positioned over the cable, prior to
removal of the internal expansion mandrel.
For all of these samples, the insulation surface was not sanded overall.
Localized sanding was applied only where there were visible indications
of semiconducting material remaining on the insulation surface. All of
the termination samples were prepared with 18 in. (46 cm) of cable
extending from the base for subsequent application of testing
terminations. In all, ten (10) 35-kV cable terminations samples were
prepared for testing. A summary description of these samples is
presented in Table 1.
Table 1:
Summary description of test samples prepared to explore assembly variables
Sample ID
Cable Preparation
Termination Type
A
Tool-cut square insulation shield cutback, stress control tape centered over cutback
Manually-applied terminations were installed onto the bare cable end of
each of the cable samples to support partial discharge testing. The
cables were prepared by removal of the jacket with a jacket-stripping
tool. The insulation was then stripped mechanically after preparing a
circumferential ring cut with a corresponding specialized tool. Axial
cuts were made through the surface of the insulation shield, then this
was peeled away. In all cases, the insulation shield terminus was
chamfered in stages, beginning with a utility knife to make a series of
angled cuts to achieve a long, shallow beveled profile. Next, the
insulation and beveled insulation shield terminus were sanded with 120
grit cloth-backed aluminum oxide abrasive to achieve a uniform
insulation surface and smooth beveled edge. The edge was carefully
tapered to achieve the thinnest transition possible. This is shown in
Figure 18. Semiconducting tape was applied over the tapered shield
profile, extending just over the edge onto the insulation to establish a
uniform, straight terminal edge. Application of the stress control tape
is shown in Figure 19. Next, insulating tape was applied carefully
abutting the semiconducting tape, and, by application of many layers in
a controlled manner, a conical profile was developed over the insulation
until the diameter of the cable was approximately doubled. Following
this, a layer of semiconducting tape was wrapped, beginning with an
overlap of the insulation shield and first layer of semiconducting tape.
Wraps continued until the tape reach the apex of the cone, thus forming
a divergent insulation shield profile. Insulating tape was then wrapped
over this assembly to build the insulation thickness against discharge
into the surrounding air. Technical references for assembly of hand-made
terminations can be found in the public domain [13]. A completed
termination on a cable undergoing high voltage PD testing is shown in
Figure 20.
Figure 18:
Tapered profile prepared at edge of insulation shield cutback
Figure 19:
Application of semiconducting stress control tape at tapered end of shield cutback
Figure 20:
Cable with handmade termination undergoing high voltage PD testing
Partial Discharge Testing Methods
Termination samples were subjected to PD testing using laboratory
conditions and equipment. The samples were energized to 34 kV service
voltage level, using a line-to-ground voltage of approximately 19.7 kV.
To maintain a discharge-free test, the high voltage lead wire was
shielded with a large corrugated metal tube. The free end of the cable
was terminated with a corona ring, attached directly to the conductor
with a threaded bolt connection. The outer diameter of the corona ring
used for these tests is 18 in. (46 cm). A test sample with these
shielded connections in shown in Figure 20.
The samples were energized using a special, low noise, variable ac power
supply, equipped with a voltage regulator. The transformer includes a
low noise porcelain bushing fitted with a corona ring at the output
terminal. The high voltage terminal was connected in parallel to an
instrumentation-grade capacitive voltage divider. This equipment is
shown in Figure 21.
PD measurement was made using an Omicron detection system and digital
bandpass filter network coupled to the voltage divider. Omicron analysis
software was used to record and analyze PD measurements from the
samples. A computer showing the Omicron analysis screen is presented in
Figure 22.
Figure 21:
Test set-up for PD detection and measurement
Figure 22:
Computer screen showing Omicron PD analysis in progress
Corona Camera Inspections
During development of the PD testing protocols, a corona camera was used
to scan the test leads, transformer connections, the hand-applied tape
terminations, and the combined cable and termination samples under test.
Figure 23 presents an example of a visible light image of a tape
termination with a superimposed image of partial discharge signals
converted to a visible light signal. The images are synchronized, thus
enabling source of corona and sporadic electrical discharge to be
identified for analysis or corrective action. In this example, the
hand-made terminations were determined to be under-insulated for use at
20 kV. These were subsequently re-insulated and found to pass this
inspection. All samples and all of the hand-applied terminations were
inspected with a corona camera during the PD testing to ensure that no
interference was competing with the measurements.
Figure 23:
Example of PD detected on hand-made termination surface
Partial Discharge Testing Results
The termination samples were subjected to phase-to-ground voltage level
during testing to determine if PD activity was present. The PD magnitude
was then measured and the sample was held at this voltage level for a
period of time. Following this, the voltage level was lowered
incrementally until the PD activity stopped. This value corresponds to
the PD extinction voltage or PDEV. The voltage was then raised again
until the first trace of PD activity initiated. This value is identified
as the PD inception voltage or PDIV.
Figure 24 presents a screen shot from the PD detection and measurement
system, applied to sample C, with PD pulses accumulated during a 10
minute hold at 20 kV. The sine wave corresponding to the applied voltage
is superimposed on the accumulated PD distribution. As noted in this
display, the PD activity is most prevalent at the ‘zero crossing’ where
the applied voltage crosses from negative to positive and back. This is
an expected response.
Figure 24:
Accumulated PD signal from termination sample C
PD testing was conducted in a set of test designed to initially raise
the applied voltage to service level (19.7 - 20 kV), determine if PD is
present, then monitor the PD during a 10 minute ‘soak’ period wherein
the applied voltage remained constant. The PDIV and PDEV values were
measured for each sample three times, with the average values reported.
These test results are summarized in Table 2.
Table 2:
Summary of PD tests of terminations prepared with various practices and a 10 minute hold
Sample ID
Cable Preparation
Termination Type
PDIV, average, kV
initial PD magnitude, average pC
PDEV, average, kV
PD at
20.4 kV,
10 minute
soak, pC
Test Comments
A
Tool-cut square insulation shield cutback, stress control tape centered over cutback
Heat-shrink
17.4
20 - 30
16.5
40 - 50
PD steadily increased in frequency and magnitude during testing
The results from these tests generally followed a logical path. The
termination prepared with a gross defect, sample G (cold-shrink tube
incorrectly positioned) exhibited a high PD level that was sustained
throughout the test period. This would be expected due to the loss of
electric stress control at the insulation and insulation shied
interface.
The samples prepared with chamfered or tapered insulation shield edges
(samples D, H, and J) were found to have no detectable PD activity at
service voltage, at any point during the testing. This is also expected
on the basis of fundamental electric stress control considerations.
The samples that would be expected to feature gaps the insulation and
insulation shield interface (Samples A and C) were found to produce PD
at the outset. PD persisted in Sample A, but mostly extinguished in
Sample C.
Elevated and persistent PD activity was found in the cold-shrink
termination samples with a square edge on the insulation shield cutback
and the shrink tube correctly positioned (Samples E and F). This finding
indicates that the stress control material will not adequately fill this
interface, independent of the two cutback methods evaluated. This
finding is also consistent with the teardown inspections of three trial
cold-shrink terminations.
The absence of PD in Samples B and I at least suggests that stress
control tape can be used to fill a square-cut insulation shield end, if
applied with attention to detail, combined with a properly prepared
cutback.
For those samples where PD is present, it will eventually break down the
polymers inside these terminations, though the rate would not be linear.
In the early stages, PD-initiated breakdown of the polymers would be
expected to produce gaseous decomposition products, most of which would
be non-polar (i.e., non-conductive). Gasses of this general nature would
accumulate under pressure where voids are present and electrical
discharges initiated. The effect of such gases would be to ‘quench’ PD
for a period of time that depends on many variables. The effect can be
seen by allowing a sample with initial, but declining PD magnitude to
’extinguish’. Sample B was subjected to such testing. Following PD
extinction, the same termination was re-energized at various intervals.
Only after approximately 24 hours of idle time did the PD reappear. This
outcome is expected since gases that form and occupy void space will
slowly diffuse through the polymer and escape. Results of this
experiment indicate that PD testing would be most effective on circuits
that had been off-line or never energized.
Longer-term degradation due to the cumulative effects of PD are
inarguably damaging, as shown by many years of research at EPRI. The
terminations prepared for this study have been preserved and will be
subjected to longer-term testing and research to provide a more
comprehensive understanding of the source, detection, and interpretation
of PD in cable accessories.
4 - References
EPRI Underground Distribution Systems Reference Book, 2016
Edition. EPRI, Palo Alto, CA: 2020. 3002018091.
Specification CS8-13 for Extruded Dielectric Power Cables Rated 5
through 46 kV, Association of Edison Illuminating Companies,
September 2013 and previous.
IEEE 1816-2013 IEEE Guide for Preparation Techniques of Extruded
Dielectric, Shielded Cables Rated 2.5 kV through 46 kV and the
Installation of Mating Accessories, IEEE, Sept. 2013
IEEE 1493-2006 IEEE Guide for the Evaluation of Solvents Used for
Cleaning Electrical Cable and Accessories, IEEE Dec. 2006.
IEEE 1617-2007 IEEE Guide for Detection, Mitigation and Control of
Concentric Neutral Corrosion in Medium-Voltage Underground Cables,
IEEE, Feb. 2008.
IEEE 48 IEEE Standard for Test Procedures and Requirements for
Alternating-Current Cable Terminations Used on Shielded Cables
Having Laminated Insulation Rated 2.5 kV through 756 kV or Extruded
Insulation Rated 2.5 kV through 500 kV, IEEE, Aug., 2009.
Handbook of Chemistry and Physics, 100th Edition, CRC Press, 2018.
AEIC CG-11 Guide for Reduced Diameter Shielded Power Cables Rated 5
through 46 kV, Association of Edison Illuminating Companies, Sept.
2013 and previous.
Why Are Terminations Required on Shielded Medium Voltage Cables,
Technical News, Edition 0016, The Okonite Company, 2004.
Splicing and Terminating of Portable Cables, Form INS-0083-0807,
General Cable Technologies Corp., 2007
Power Cable Splicing & Terminating, R. Goodman and W. Osborn, 3M
Corp., 2008.
Electric Power Connections for Substations, ANSI/NEMA Standard
CC1-2009, National Electrical Manufacturers Association, 2009.
Wire and Cable Selection and Technical Data, Document CM-621,
General Electric Corp., Jan. 1964.
