Low-Voltage Connectors

• 1: Heating of Crimp Connectors
• 1.1: Project Overview
• 1.2: Overheated Connector Examination
• 1.3: LV Connector Test Setup
• 1.4: LV Connector Findings
• 1.5: Future Research
• 1.6: References

1 - Heating of Crimp Connectors

This research is focused on better understanding the relationship between conditions that can cause heating, the resulting heating that may be identified by IR inspection, and the expected remaining life of the heated connectors. The results of this research will inform field practices and infrared thermography.

This section describes a multi-year project investigating the performance of low voltage (LV) cable accessories. The research has included investigation of the causes of heating in LV connectors and sought data to develop representative secondary connector load versus temperature characteristics. Research included the development of a testing facility at EPRI’s Lenox Laboratory to enable testing of LV connectors under various loading histories and measure the steady state thermal response to develop load characteristics, including the time to heat and temperatures measured with embedded thermocouples and Infrared (IR) imaging.

The research has focused on a one-way to one-way 500 MCM copper (Cu) to 500 MCM Cu crimp type connector used in a low voltage secondary network application. With the help of Con Edison, EPRI prepared 26 different connections for study, including samples with various intentional defects. The connectors were linked in series and connected to a programmable current power supply which enabled EPRI to control and vary the current to simulate various loading and heating conditions. Thermocouples were attached to each of the connections, enabling monitoring and recording of temperatures.

This research seeks to better understand the relationship between loading and temperature based on connector defect type and intentional workmanship variations. The results of this research will inform workmanship methods and provide information for improved interpretation of results from infrared thermographic inspections of low voltage connectors.

Update video describing low voltage connector test facility at EPRI’s Lenox Laboratory.

3002017510: Low Voltage Cable Accessory Research, 2019 Update

3002018974: Low-Voltage Cable Accessory Research, 2020 Update

3002021663: Low Voltage Cable Accessory Research, 2021 Update

3002024696: Infrared Thermography in Underground, 2022 Update

1.1 - Project Overview

Introduction

Utility companies that own and operate urban underground distribution systems face multiple challenges. Installing, assessing and maintaining underground infrastructure is inherently challenging due to limited accessibility and the inescapable environmental conditions. 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 operators of low voltage secondary network systems is assessing and assuring the health of low voltage (LV) connectors (splices). Utilities use low voltage connecters in urban underground manholes and vaults to join low voltage cable sections, using either a crimp type, or a bolted type connection. These connections will experience varied levels of loading throughout their service life and may be subjected to high levels of loading when a network is operating in a contingency (N-1 or N-2, for example) condition. Overheating of secondary cables and connectors in network secondary systems can lead to failures of either or both - a not uncommon failure scenario [1].

Overheating of secondary cables and connectors, and subsequent failures, may be due to various causes, including workmanship issues, component deficiencies, contamination at cable-connector interfaces and design errors. To assess the ongoing ‘health’ of low voltage connectors, many companies perform periodic inspections utilizing infrared imaging thermography (IR) to identify connectors that are ‘hot’, thus candidates for replacement. However, the relationship between loading, temperature, and expected remaining service life is not well understood. Further, the industry lacks information regarding the implications of various workmanship deficiencies on eventual connector heating and service life expectancy.

In 2019, EPRI launched research into the causes of heating in LV cable connectors and began gathering data that will be used to understand fundamental secondary connector load versus temperature characteristics. Research included examination of field-aged and overheated low voltage cable and cable accessory materials, and development of a dedicated testing facility at EPRI’s Lenox Laboratory. This facility will enable stressing LV cable connectors under various static and dynamic load conditions while measuring the steady state thermal response to understand thermal behavior, including the heating rate and temperatures measured through embedded thermocouples and IR imaging.

Project Objectives

The overall goal of this research is to better understand the relationship between current loading and temperature rise based on LV connector crimp practices and cable conditions to inform workmanship methods and to inform infrared thermographic inspection.

This multi-year project seeks answers to the following research questions:

• What is a typical secondary splice load vs. temperature characteristic?

• How is a ‘high’ (non-recoverable) temperature defined for a secondary splice?

• Do hot secondary cable connections continue to become hotter over time under constant load?

• How much time is required for an overheated connection to thermally destroy the overlying insulation and/or the insulation over the attached cables, considering historic and present materials?

• What are common causes of hot spots in LV connectors?

• What are the implications of workmanship to LV connector performance?

• Do some splice installation tools better mitigate workmanship errors?

Currently, the project is focused on analysis of a one-way to one-way 500 MCM Cu to 500 MCM Cu crimp type connections used in low voltage secondary network applications.

Research Approach

The EPRI research approach consists of two parallel efforts:

1. Analysis of aged, overheated or failed splices (connectors) of the type selected above.

2. Comparison of healthy (normal, properly built) and unhealthy (abnormal, improperly built) splices of the type selected above, by subjecting to varying loading and heating over time, and monitoring performance.

Analysis of aged, overheated, or failed connectors

EPRI will perform analysis of samples of secondary connections (500 MCM Cu – 500 MCM Cu crimp connections) that are removed because of identified hot spots through IR, or because of failure. These samples will be accompanied by information associated with the removed connectors, including:

• Device material specifications

• IR images

• Other data that shows temperature

• Any information about the age of the splice

• Any information about the operating environment of the connector, including loading history, and contingency loads,

• Samples will include the housings / coverings / insulation associated with the failed connectors and cables

Since 2019, EPRI received 11 connectors for analysis as described below.

Comparison of the healthy (normal, properly built) and unhealthy (abnormal, improperly built) splices

With the help of Con Edison, EPRI prepared 26 different connections for study, including samples with various known defects. The range of defects was based on a number of probable adverse conditions in the field, such as constraints imposed by cable bends and limited free lengths, as well as work practice challenges known to the Con Edison crews. Fundamental considerations were also applied, for example, in joining service-aged cables with non-pretreated conductors. The connector and adjoined cables were linked in series and energized with a current power supply that enables EPRI to control and vary the current in a programmed manner. Thermocouples were directly attached to each of the connections before the Con Edison standard heat-shrinkable insulating sleeve was applied. These sensors enable continuous monitoring and recording of temperatures. In addition, IR thermography is being used to record temperatures, while enabling comparisons to the actual connector heating as measured by the thermocouples.

EPRI will ’load-age’ tested units by applying a load / temperature cycle to simulate the effects of aging and to better understand the time required for a connection to increase in temperature, given its installation type (workmanship associated with the preparation of the connector), and operational parameters.

After aging the units (both the ’normal’ and ‘abnormal’ splices), EPRI will re-examine the temperature / load characteristic, including the time to heat and measured temperatures from both a thermocouple and IR camera, to determine if aging has an impact on the load / temperature characteristics. Testing will include applying a constant load to an aged splice to determine if the splice will become hotter over time, given a constant load.

The load-temperature characteristics gathered during these tests will inform the load needed to cause cable insulation damage associated with ‘healthy’ and ’non-healthy’ connectors.

The results of this research will inform workmanship methods and provide information for better interpreting results from infrared thermography scans of low voltage connectors.

1.2 - Overheated Connector Examination

Introduction

Low voltage (LV) connector heating can be driven by a number of factors including high levels of loading and component anomalies, such as a bad, and thus more resistive or reduced-ampacity, connection.

Secondary network systems are designed to carry varying levels of load (current) in normal modes of operation and in contingency conditions, where one or more of the medium voltage feeders that supplies a secondary network grid is out of service. By design, secondary cables and components are sized so that the levels of loading and load cycle anticipated in contingency conditions are manageable and within the emergency loading limits specified for the components. Many utility companies who operate network systems have developed specifications for 600V secondary cables of different types used in network mains that include normal and contingency ampacity levels. Con Edison, for example, provides detailed ampacity tables for various cable types, conductor sizes, load curves, duct bank loading, based on whether the cables are being operated in a normal, first (N-1), or secondary (N-2) contingency. For one particular anticipated winter load cycle curve, for example, 500 MCM Cu cable with a dual layer low smoke zero halogen jacket has a normal ampacity rating of 670A, a first contingency rating of 715A, and a second contingency rating of 750A [2].

In practice, situations may occur where these ampacity limits are exceeded. The network could fall into a contingency situation beyond N-2. The secondary system could have ‘open mains’, sections of secondary cables which comprise the mains that are out of service because of cable limiters which may have blown but are unidentified. This may result in higher than anticipated current flows in the remaining, energized cable sections. Finally, anomalies in the components that comprise the secondary itself may cause ‘hot spots’, areas within the secondary system which are prone to high heating, such as a poorly assembled LV connections.

Sample Description

A sample of an overheated LV connector and cable section was provided to EPRI for examination by Con Edison. The sample provided is a one-way to one-way 500 MCM Cu crimp connector of the type being studied in this research. Con Edison has experienced periodic failures of connectors of this type in their low voltage secondary network system, where these connectors are used to join 500 MCM Cu cables that comprise their network secondary mains.

Con Edison identified the sample through a proactive IR inspection program, aimed at identifying and remedying hot spots before they lead to failure. Con Edison has established a standard policy for performing IR on secondary distribution cables and splices. Within that policy, a “hot spot” is defined as “An isolated segment of cable or connection operating at a temperature greater than the rest of the cable length. If the maximum temperature is greater than 55°C (130°F) and 15°C (24°F) above the temperature of the cable at either duct edge, a hot spot condition is declared” [3].

Con Edison has established threshold values based on ANSI/NETA ATS-2009 Table 100.18, Thermographic Survey, Suggested Actions Based on Temperature Rise.

The sample provided experienced high levels of heating as shown by the IR images shown in Figures 1, 2 and 3 below. Note in Figure 2, that cracking of the cable jacket and insulation can be clearly seen in the IR image. Con Edison subsequently removed the overheated sample and provided it to EPRI for analysis.

Visual Examination

A visual examination of the sample corroborated the IR finding that cable section and connector were subjected to very high heating. The remaining cable insulation and jacket was cracked and partially disintegrated over the conductor surface. There was no remaining evidence of the heat-shrinkable insulation tube, indicating that it had burned away. The conductor was found heavily oxidized, as evidenced by the red surface color (cuprous oxide). This is shown in Figure 5.

One theory posited by Con Edison and being explored by EPRI is that the overheating of the cable / cable connector due to loading may have caused the shrink tube sealant material to melt, and that molten material from the inside of the heated shrink tube may have migrated into open spaces within the connector, including between the cable strands and between the outer cable strands and the connector barrel. The addition of this material may have increased the resistance of the connector assembly, thus increasing the heating rate and decreasing the corresponding ampacity. The heat-shrinkable tube includes an outer layer of insulation and an inner layer of a rubbery material (i.e., ‘mastic’ or ‘adhesive’) that melts then solidifies during installation to form a water-tight seal. Overheating of the materials from which the shrink tubes are made may also have led to the formation of reactive gaseous decomposition products which, when they come into contact with the copper cable strands and connector barrel, may also result in reduced conductivity (increased resistance) of the connector through formation of oxides or chlorides, for example.

Any change in performance of the connector (change in resistance) may not be linearly related to temperature and time. There are organic antioxidant compounds in the cable insulation and jacket materials that may inhibit a change in conductivity due to overheating – at least for a period of time. After the antioxidant has been consumed through heating, then the change in performance will accelerate. The temperature and time relationship depends on the chemistry of the insulation and jacket, as well as the type and concentration of the antioxidant or antioxidants.

To better understand the performance of both the heat-shrinkable tube materials and of the cable insulation/jacket when subjected to heating, EPRI will perform a series of thermal exposure tests and corresponding chemical analyses.

Initial tests, to establish some baseline indications of performance, were performed at the Lenox Lab, with a Con Edison representative in attendance. EPRI sectioned a new sample of the Con Edison LV cable (General Cable 500 MCM Cu with EAM insulation and LSZH jacket), into roughly one-inch long sections (see Figure 6). Each section was sequentially placed into a furnace for 45 minutes at a predetermined temperature to observe changes to the cable conductors, insulation, and jacket. In addition, EPRI sectioned the shrink tube material into small strips, roughly two inches long, and ¼ inch wide. These were also placed in the furnace to observe changes at various temperatures. To conduct these tests in a single day, coarse temperature increments were selected based on fundamental materials considerations, such as the oxidation onset temperature for copper. The exposure temperatures and corresponding cable response are summarized in Table 1.

Sample ID	Temp., °F	Temp., °C	Observations
Cable and Jacket	257	125	No visible changes
Heat-Shrink Tube	257	125	Slight softening of adhesive layer
Cable and Jacket	302	150	No visible changes
Heat-Shrink Tube	302	150	Continued softening of adhesive layer, outer sleeve not shrinking
Cable and Jacket	400	204	Initial oxidation of conductor strands; jacket and insulation not affected
Heat-Shrink Tube	400	204	Liquified mastic layer and outer sleeve shrinkage complete
Cable and Jacket	500	260	Initial indication of thermal damage to jacket layer
Heat-Shrink Tube	500	260	Tube fully recovered, liquified mastic layer
Cable and Jacket	600	316	Insulation and jacket layer decomposing and releasing smoke at end of 45 minutes exposure, evidence of liquification and boiling of insulation at conductor interface
Heat-Shrink Tube	600	316	Immediate combustion of heat-shrinkable tube and mastic
Cable and Jacket	700	371	Complete destruction including pyrolized ash
Heat-Shrink Tube	700	371	N/A

Photographs that document some of the thermal exposure tests are presented in Figure 7, Figure 8, and Figure 9. EPRI will continue this testing, subjecting cable samples to heating at smaller temperature increments for longer periods of time.

In total, EPRI examined 11 connectors that were returned from the field. The table below provides a description and the associated findings for each of these components.

EPRI Sample ID (PID if not included in this report)	Description	EPRI Findings	Overview Photograph
A (3002018974)	2 splices Cu-Al-Cu	Connector barrel too large even with sleeves, over-crimped, damage strands
B	1 splice and 1 limiter terminal	No connector issues found
C (3002018974)	Cu-Al splice	Over-crimped and damaged conductor strands; single crimp used at each end of barrel
D (3002018974)	crab and 3 splices	Poor copper conductor preparation
E	crab with burned splice	Poor assembly of crimp connector, two strands set aside; inadequate crimp pressure resulted
F (3002018974)	Cu - Cu splice	Crimps not made
G	Cu - Cu crimp splice	Conductor strand missing; consequential inadequate crimp force
H	splice with large and small diameter Cu conductors	Two strands missing from 7 conductor wire; crimp pressure non-uniform and asymmetric due to distorted geometry
I	small Cu to large Cu splice	Over-crimping damage of conductor
J	2 Al x 1 Cu Y-splice	singe crimps applied - two recommended by manufacturer and utility work practice
K	welding cable with crimped ring terminal	Ring connector bore drilled oversize from 0.3875 in. to 0.661 in.

Detailed examinations of the overheated connectors revealed the following concerns. All of these provide direction for improved practices in the field.

• Incorrect crimp barrel and conductor size coordination
• Inadequate cleaning of conductors prior to crimping
• Inconsistent crimping practices, including depth control and number applied
• Problems with interconnecting copper and aluminum conductors

1.3 - LV Connector Test Setup

Overview

For 2019, the research focused on a ‘one-way’ to ‘one-way’ compression type tin-plated high conductivity copper connector used join 500 MCM secondary cables (see Figure 1). This connector, custom-produced for Con Edison, is a common size and type used in their low voltage secondary network system and is very similar to other commercially-available crimp connectors. The design includes a straight barrel, without a center stop or any embossed or printed labels. The connector is 4 inches (10.16 cm) long, has an outside diameter of 1.06 in (2.69 cm), and an inside diameter of 0.834 in (2.12 cm).

Con Edison has experienced some in service failures of these connections caused by overheating due to a variety of factors including abnormally high loading and anomalies or defects in the preparation of the connection due to factors such as contamination and variations in workmanship.

Con Edison performs infrared thermographic scans of its in-service LV connectors to identify hot spots and candidates for replacement. One of the goals of this research is to better understand the relationship between loading and temperature both to inform workmanship associated with LV connectors and response decision-making based on IR findings.

The test set up includes 26 different connectors prepared for study, linked in series and connected to a current supply which enables EPRI to control and vary the current in a programmed manner. These connectors were built with Con Edison’s assistance with various known defects, described below, as the research will seek to identify and distinguish load versus temperature profiles over time associated with various defect types. Depending on the findings, it may be possible to leverage learnings from the research to predict both the likely type of defect, and expected remaining life based on an analysis of results from IR scans.

Preparation of Samples for Study

The connections for study were prepared by a Con Edison field crew at a Con Edison facility in the Bronx. As one of the goals of the research is to compare performance of connectors built with various defects, EPRI prepared a draft listing of defects targeted for study. This draft was reviewed with Con Edison Engineering personnel and field crew members and, based on their feedback, modified so that the chosen defects for study were those which are realistically representative of defects that may exist in the field.

All connectors were assembled onto the Con Edison standard stranded copper 500 MCM, FR-EAM insulated 600 V network cable. The connectors are, as described above, ‘one-way’ to ‘one-way’ compression type, manufactured with tin-plated high conductivity copper.

All crimps were made with a hydraulic, die-less hand-held tool, with a line-powered hydraulic pump. This crimping system is custom-manufactured for Con Edison and has been the standard tool for approximately forty (40) years. The device includes a flat-faced anvil and an opposing Vee block support. Crimping ‘shots’ are completed using visual cues only, with the press stroke controlled by the companion crew member who receives direction from the tool operator who determines when the crimp shot is complete. The same tool set includes a separate head for cable cutting, driven from the same hydraulic pump. Two people are required to run either the crimping tool or cutter, with one dedicated to pump control. The term ‘shot’ refers to a single crimp impression. The crimp is an indent style, with two normally applied over one side of the connector, co-aligned, and spaced uniformly between the center and end of the barrel.

The samples were assembled with the crimp defect under study applied to one end of the connector only, crimped to the end of an approximately 3 ft (91 cm) long section of cable. This resulted in various subassemblies which were later joined together in a test loop at an EPRI laboratory using a controlled depth concentric crimping tool. It is noted that Con Edison has begun implementing battery-powered hand-held crimping tools that include a controlled crimp depth operation. Evaluation of these tools by Con Edison is ongoing.

Table 1describes the various crimping defects prepared for study. Two or more samples were prepared with each defect type. Three samples were prepared for defect Type G, which includes crimps made over contaminated, corroded, or oxidized conductor strands. The cable used for the Type G defects was obtained from a scrap container at the Con Edison Bronx facility. An internal marker tape was partially recovered. This indicates a manufacturing date of 1961. The manufacturer’s name is incomplete due to where the cable was sectioned. This will be recovered at a later date when the corresponding cable is removed from testing. Finally, it is noted that defect Type M is a properly-prepared crimp connection. The 3 ft. (91 cm) cable lengths prepared at Con Edison were later interconnected at the EPRI site in Lenox, MA using a battery-powered, handheld Burndy concentric crimping tool.

Defect Code	Defect Description	How Implemented	Notes	Example
A	Dual under-crimp	Crimp duration reduced (pressure cannot be adjusted).	Simulates poor field practice. 2 samples prepared
B	Crimp placement error Type 1	One crimp made at edge of barrel over conductor, other crimp adjacent, but not overlapping. Both crimps offset, but normally spaced and at normal depth.	Simulates poor field practice. 2 samples prepared
C	Crimp placement error Type 2	The two crimps overlapping. Each crimp made to normal depth.	Simulates poor field practice. 2 samples prepared
D	Over-crimp combination	First crimp at cable end excessively deep; second crimp at normal depth. Both crimps positioned normally.	Simulates poor field practice. 2 samples prepared
E	Conductor strands partially excluded	Both crimps at normal depth and position; two conductor strands directed outside of connector.	Simulates poor field practice where 'stray' conductor strands may not be directed into barrel, with condition not visible due to position against vault wall. This may also simulate condition where conductor strands have been nicked and broken away during cable preparation. 2 samples prepared
F	Improper cable cutback	Cable jacket cutback length too short, resulting in incomplete insertion into barrel. This results in 'inboard' crimp incompletely engaging or missing conductor end. Both crimps approximately normal.	Simulates poor field practice 2 samples prepared
G	Crimps over contaminated, corroded, or oxidized conductor strands	Field-aged lengths of cable harvested from scrap dumpster at ConEdison. Three samples prepared with both crimps properly positioned at normal depth. Corroded conductor strands subsequently confirmed. Partially recovered market tape confirms 1961 vintage	Simulates common field experience where new and aged cables are joined. 3 samples prepared
H	Contaminated conductor strands	Prepared cable ends were immersed into a soil-water slurry then dried before crimping	Simulates field condition where the prepared cable end may be dropped in a wet vault before crimping. 2 samples prepared
I,J,K	Categories reserved for additional experiments using same cable and connectors	N/A	N/A
L	Oblique conductor cut	Cable cut at oblique angle. Both crimps properly positioned, at normal depth.	Simulates poor field practice 2 samples prepared
M	Normal assembly	Both crimps properly positioned, at normal depth	2 samples prepared
N	Single crimp	Single normal depth crimp placed at mid-length of barrel half.	Simulates poor field practice. If made in this manner, insufficient space remains for second crimp. 2 samples prepared
O	Separator tape incompletely removed	Separator tape was only partially removed and left in place on one side only. Both crimps positioned normally, at normal depth.	Simulates possible field error where tape may be present on side of cable against vault wall and hidden from view 2 samples prepared

Test Set Up – Lenox Laboratory

Overview

EPRI will ’load age’ the various connector samples, by applying a long-term continuously-repeated load cycle, to correlate temperature with loading, and to better understand the connector temperature rise kinetics, as related to the variety of assembly parameters and pre-existing cable conditions addressed in Table 1.

To conduct these tests, EPRI connected the test samples in a series loop with subsequent connection to a programmable current supply. This current supply can be programmed to vary the loading to simulate utility load cycles, and to vary the heating experienced by the test loop to simulate and accelerate aging of the connections. In order to understand the temperatures experienced in each of the connectors, EPRI applied thermocouples to every connection, thus enabling continuous monitoring and recording of the temperatures. EPRI developed data acquisition programming to support recording temperatures from each of the 26 defect connectors and two ambient temperature connectors at programmed intervals. In addition, EPRI mounted an IR camera to periodically record temperatures and IR images. This information will be used to correlate outside temperatures as determined through an IR camera with internal temperatures experienced in the connectors, as ascertained by the thermocouples. Finally, EPRI will periodically record more detailed imagery of each connector from a handheld, higher resolution IR camera to better understand the specific heating characteristics of LV connectors as they age.

Completing the Test Loop

With the support of Con Edison, EPRI had prepared 26 different samples assembled with various crimp defects for study to be connected into a test loop. To do so, each 3 ft. cable section was connected together to create a series loop. Figure 9 illustrates the final test loop configuration. The letters correspond to the Defect Code identified in Table 1 above. The numbers refer to the specific sample. For example, Connector A-1 refers to the first sample of defect Type A, the dual under-crimped connection. Defect Code A-2 refers to the second sample with defect Type A.

To prepare the test loop, EPRI connected each of the three-foot cable samples together. Each of these samples was prepared with the connector on the one end of the sample, with one side of the connector barrel crimped to the cable with the known defect and the other side of the connector barrel open. On the other end of the sample, the cable insulation was removed to expose bare conductor which would be joined to the adjacent sample. For consistency, and to eliminate potential human factor influences, EPRI joined the cable samples with a controlled depth, concentric Burndy compression type crimping tool, using the specific die identified for 500 MCM copper cable. This compression style crimp differs from the lateral indentation style crimp used by Con Edison. These differences are illustrated in Figure 12 and Figure 13.

Figure 17 shows the test loop under construction. This photograph shows the assembled loop segments prior to thermocouple installation, heating of the shrink tubes, and the installation of heat sinks.

To minimize the transfer of heat from one connector to another, EPRI installed split type heat sinks midway between each of the connectors. EPRI utilized split zinc anodes, used for marine propeller shafts, as heat sinks[^1]. To apply these, a section of the cable insulation was removed approximately midway between connectors and a piece of copper braid was used between the heat sinks and the conductor itself to ensure a tight connection and optimized heat transfer.

Baseline Resistance Measurements

The electrical resistance through the connector-cable sample combinations was measured at elevated current using a precision digital low voltage ohmmeter (DLRO), manufactured by Megger Corporation. A 200 A current was applied for all of these measurements. Three sequential measurements were made for every sample tested. The electrical contacts were rotated in place following each measurement in an effort to reduce the effects of surface resistance due to oxide films or contamination, for example. The measured values are presented in Table 2. Initial measurements were made through a crimp connector and through a 3 ft (91 cm) length of the Con Edison cable. Measurements through a crimp barrel indicated resistance values of 5.3 µΩ, 4.9 µΩ, and 5.4 µΩ, for an average of 5.2 µΩ. Resistance measurements through the cable section indicated values of 30.4 µΩ, 29.7 µΩ, and 28.4 µΩ, for an average of 29.5 µΩ. The latter values were considered higher than expected, possibly due to a surface film on the conductor strands. This is not an uncommon occurrence due to the presence of lubricant residues used for drawing the copper wire through a die, as well as waxy processing aids used in the rubber extrusion process. Since the surface of these cables is representative of field conditions, the measurements were subsequently made though assembled cable and connectors. Plastic deformation during the crimping process is intended to displace conductor surface films, including oxides and lubricants.

For reference, the resistance measured through a 3 ft (91 cm) length of the same cable, with Burndy crimp connections applied to each end was found to be 55.3 µΩ, 55.2 µΩ, and 55.1 µΩ, for an average value of 55.2 µΩ. The corresponding values obtained from crimps applied to each end of an identical cable, using a ’normal’ Con Edison crimp connection are as follows: 43.3 µΩ, 43.2 µΩ, and 43.2 µΩ, for an average value of 43.2 µΩ. The Con Edison crimping practice appears to exhibit lower interfacial resistance, however the crimping method does not engage all conductors in a uniform manner, as shown when Figure 12 and Figure 13 are compared. The concentric crimp is expected to exhibit greater comparative ampacity as a result of having full engagement of the entire conductor cross-section without gaps between adjacent conductor strands. The lateral crimping method radially displaces the conductors that do not lie immediately below the crimp impression.

Connection ID	Res. Test 1, µΩ	Res, Test 2, µΩ	Res, Test 3, µΩ	Res. Ave., µΩ
A-1	63.2	62.9	63.0	63.0
B-1	61.4	61.3	61.3	61.3
B-2	60.9	60.9	60.8	60.9
C-1	62.0	62.0	62.0	62.0
C-2	64.9	64.8	64.8	64.8
D-2	61.6	61.7	61.6	61.6
E-1	62.3	61.8	62.0	62.0
E-2	63.3	63.2	63.2	63.2
F-1	63.4	63.4	63.4	63.4
F-2	63.9	63.8	63.8	63.8
G-2	62.9	63.1	63.1	63.0
G-3	81.8	81.8	81.7	81.8
H-1	61.2	61.4	61.2	61.2
H-2	63.5	63.1	63.5	63.4
L-1	58.8	58.7	58.7	58.7
L-2	57.2	57.1	57.1	57.1
M-1	62.1	62.3	62.5	62.3
M-2	62.1	62.1	62.1	62.1
N-1	62.9	62.9	63.0	62.9
N-2	62.3	62.3	62.1	62.3
O-1	69.8	69.8	69.7	69.8
O-2	65.7	65.8	65.8	65.8

Final Assembly

Upon completion of the joining of the cable samples and installation of the heat sinks, the test loop was connected to the current supply, a Magna-Power Electronics TS Series IV programmable DC power supply. This current supply can be programmed to vary the current in the loop to simulate load cycling. The current supply control circuit adjusts the voltage as needed to maintain the desired current in the test loop.

To complete the test loop, the thermocouples were installed, then the heat-shrinkable tubes were installed and ‘recovered’ over the connections. One thermocouple was applied to each of the crimp connectors, on the back side of the barrel where the indent style crimps were applied. Two additional thermocouples were added to the test facility to provide ambient temperature readings, one in the front of the test room, and the other at the rear. The thermocouples were terminated into a data acquisition block, which, through programming, enables EPRI to monitor and record temperatures from each.

In order to obtain periodic infrared images of the test loop, EPRI permanently mounted an IR camera on the ceiling of the test facility. This camera is tied to data acquisition software which enables detailed recording of infrared images of the test loop on a predefined period. In addition, EPRI plans to supplement this imagery with more detailed thermal imaging of each connector using a higher resolution IR camera. At the outset, EPRI plans to capture images from the permanently mounted camera every 15 minutes and prepare more detailed images of each connector in the test loop at weekly intervals.

Data Acquisition / Test Approach

The load cycle used for testing will vary the loading between a higher and a lower current level to replicate cyclical heating and cooling of the connections as would be experienced in the field.

In total, seven load profiles were utilized over the course of 2+ years of testing, to attempt to heat the connectors, and bring about internal changes that would yield thermal runaway. These profiles are shown graphically below:

Over the course of the testing the load cycle was gradually made more severe to encourage connector The use of each cycle over the 1,271 cycles completed over the course of the testing.

EPRI controlled the current cycling through software mounted on a laptop computer and connected to the current supply. Temperature information from the thermocouples attached to the connectors was recorded through the data acquisition software at 1-minute intervals.

Images from the ceiling-mounted IR camera were also captured every 15 minutes and stored on the computer hard drive and were later used in the analysis.

1.4 - LV Connector Findings

Findings

The installation defects included in this testing were expected to reveal performance differences in terms of thermal behavior during thermal cycling. These defects may be classified into a small number of groups for analysis as follows:

Reference Connectors | Improper Crimping | Contaminated Conductors | Conductor Insertion

The analysis on the thermal data was performed considering each connector’s relative performance to the reference connector group over all completed cycles.

Reference Connectors

Within the full loop, two reference connectors were included that were used as performance benchmarks for assessing the population of connectors with installation errors. The progressive increase in operating temperature shown below occured as a result of the increasing “severity” of the load cycle. Averaging these two connectors leads to the “reference” connector that is used for the analysis

Reference connector maximum temperatures (M-1, M-2, and average) for all load cycle profiles

Connectors with Improper Crimping Related Defects

Six connector pairs were prepared with defects related to improper crimping. Such defects include crimp location, crimp depth, and number of crimps.

The performance (maximum temperature during each cycle) of this group of connectors relative to the “reference” connector over the completed cycles appears below. To interpret this figure, the zero line denotes when connectors match the temperature of the “reference” connector. Temperatures that are above or below the zero line indicate the connector ran hotter (greater than zero) or cooler (less than zero) than the “reference” connector. The red regression lines are used to provide a visual indicator of the overall trend in the data – increasing, decreasing, or stable with respect to temperature. The slope of the regression line may be used to quantify the performance gap between these connectors and the “reference” connectors. Positive slope indicates worsening performance while negative slope implies improving thermal performance (more likely stable performance versus worsening performance of the “reference” connector).

Relative thermal performance of connectors with improper crimping-related installation defects

Taking the improperly crimped connector class as a whole, this group of connectors consistently remain more or less within 5°C of the “reference” connector. There are occasional thermal excursions which occur during individual cycles, but the relatively narrow temperature bandwidth is nonetheless surprising given the types and severity of the defects present.

Connectors with Contaminated Conductor Defects

This class of connectors includes a number of commonly occurring issues with contamination on the conductor. These issues include corrosion, dirt, and cable components not fully removed during stripping / prep. Using the “reference” connector comparison approach, the resulting thermal behavior for these connectors appears below. Note that the scale is substantially increased due to the elevated temperatures these connectors achieved over the “reference” connectors. Connector G-3 experiences over 20°C increase in temperature over the “reference” connector. This is the largest observed temperature difference to date in this research.

Relative thermal performance of contaminated conductor-related installation defects

Connectors with Conductor Insertion Defects

This class of connectors is focused on understanding the performance difference when there are issues with conductor insertion. These issues include not fully inserting the conductor into the connector that could result from an improper cable cutback as well as not inserting all conductor strands into the connector. In the latter case, two strands were intentionally bent 90 degrees away from the conductor resulting in a loss of conductor cross-sectional area. This conductor is specified as using 61 strands and so the loss of two strands represents a loss of 3.2% in cross section. The relative thermal behavior for these connectors is shown below:

Relative thermal performance of conductor insertion-related installation defects

The regression lines indicate some degradation in performance in three of four samples. These differences are larger than the improperly crimped connector group but smaller than those observed in the contaminated conductor samples. In two of the samples, the temperature remained consistently below the “reference” connector.

Effect of Other Installation Defects

A final installation defect was tested during this program – an oblique insulation cutback that did not prevent full insertion of the conductor within the connector. The relative thermal behavior for these connectors is shown below. As with the conductor insertion samples, the performance was mixed but relatively stable as indicated by the slightly positive sloping regression line. It is interesting to note that sample L-2 ran considerably cooler than the “reference” connector throughout the cycling. It is possible that this connector’s position near the entry door contributed to this behavior.

Relative thermal performance of oblique conductor cutback installation defect

Regression Line Comparison

For each defect class, the individual samples were fitted with linear regression lines to provide a visual and quantitative metric for evaluating the performance over the course of the cycling. The slopes of these regression lines may be considered as overall indicators of the additional heating these mis-installed connectors experienced. In addition to providing a consistent reference for the multiple load cycle profiles, this also helps account for any environmental conditions that occurred within the test area as a result of ambient temperature fluctuations. Using this approach, it is reasonable to assume that the temperature differences are primarily due to connector performance. An analysis of the resulting regression fit slopes reveals interesting behavior in this connector population.

The recorded slopes for each defect class can be displayed in boxplot format – the black dots (●) represent the recorded slopes for the individual connector samples within the same defect class while the boxes show the spread in the slope data by encompassing the 25^th to 75^th percentiles (middle 50%) of the recorded slope data. Boxes which appear small in height indicate the slope data are highly consistent within the class and non-overlapping boxes, in terms of the linear fit slope, imply the differences are likely statistically significant. Of the four defect classes, only the contaminated conductor class appears significantly different from the rest of the sample population with a median slope of 0.04°C/cycle (approximately 50 times greater than the second highest median slope).

Comparison of regression slopes for each defect class (boxes represent middle 50% of data while the black dots (●) represent individual connector slopes)

The remaining defect classes (conductor insertion, improper crimping, and oblique cable cutback) do not appear to generate connector performance differences at least within the testing completed during this program.

Summary

The findings can be summarized as:

• No connector experienced thermal runaway or altered its behavior significantly from established analytical models

• Analysis indicates that defects involving a contaminated conductor are more likely to lead to elevated temperatures during operation

• IR imaging is not as sensitive to the changes in connector thermal performance when connectors are covered by an insulating sleeve

• Identifying cycle maximum temperatures is not necessarily required for using IR information when making connector-to-connector comparisons – individual images do provide meaningful comparisons

1.5 - Future Research

This multiple year research project sought to better understand how field practices used to prepare low voltage (LV) connectors can impact the relationship between loading and temperature over time and contribute to connector aging and heating. Results from this research are aimed at informing both field connector preparation practices and methods for leveraging infrared thermography as a diagnostic tool in LV systems.

In 2020, EPRI completed and began applying load to a LV connector test set up comprised of connectors with various build defects. EPRI varied the loading to try to understand the relationship between loading and heating, and to bring about internal changes to the connectors to simulate conditions found in field aged components. As described the 2020 update report, analysis of the data revealed that the connectors under study performed normally and predictably, without indications of thermal runaway or other anomalies. This suggested that the load cycle utilized may not be sufficiently aggressive for effecting internal changes to the connector / cable system, where temperatures can ‘run away’ from normal expectations due to increased resistance or decreased ampacity.

In 2021, EPRI revised the loading profile to increase the connector maximum temperature excursion to attempt to bring about run-away heating, or until such time as a failure or incipient failure causes test interruption. No such behavior was observed during the additional loading profile that was utilized. The testing did reveal some performance differences between defect classes. In addition to the LV connector loop aging, further research was performed to understand the composition of the connectors used in the testing as well as forensic analyses of failed or overheating components from field.

In 2022, EPRI completed its study of the specific low voltage connectors under test since 2020. Research included subjecting the components to high current and associated heating in an attempt to bring about premature failure. EPRI performed analysis of data from the testing by defect classes under examination, including defects associated with conductor insertion, contaminated conductor, improper crimping, and an oblique conductor cut. At the conclusion of the testing, the connector samples were dissected and analyzed revealing evidence of thermal damage to the underlying copper conductors that could indicate the start of failure due to thermal runaway. Results from these analyses can be found in report 3002024696.

Also in 2022, EPRI began the design and construction of a test bed for examining heating in medium voltage cable joints with a focus on producing results that can inform the use of infrared thermography as an underground cable systems diagnostic tool.

For 2023, EPRI will complete the construction of the test bed and begin the MV cable joint study. The study will continue into 2024.

Other potential future research avenues under consideration include:

• Comparison of different crimping approaches, including radial and concentric crimping, to understand their effectiveness, consistency, and anticipated long term performance.

• Expanded forensics analyses that would include failed or overheating shear bolt connectors.

• Examination of the implications of the use of fire proof tapes on UG cable system infrared thermography.

• Expansion of the UG test bed to examine heating and IR in separable connectors.

• Expansion of the UG test bed to continue examination of heating in LV components.

1.6 - References

References

1. Managing Urban Underground Systems, EPRI, Palo Alto, CA: 2018. 3002012906.

2. Standard Ampacity Ratings for 600 Volt A.C. Mains Cables Installed Underground in Ducts, Consolidated Edison Company of New York, Inc, New York, NY: Dec 2016. Specification EO-6040, Rev 10.

3. Infrared Imaging on Secondary Distribution Cables and Splices, Consolidated Edison Company of New York, Inc, New York, NY: May 2018. Specification EO-10370, Rev 0.