Why Do Underground Cable Joints Fail?

Published 15 Oct 2026

Underground cable joints fail when their electrical insulation, conductor connections or protective sealing systems can no longer perform their intended function. Common causes include moisture ingress, incorrect cable preparation, insulation damage, poor conductor connections, excessive electrical stress and unsuitable installation conditions.

Although underground power cables are designed for long-term service, a cable joint introduces several interfaces that must remain electrically, thermally and mechanically reliable throughout the installation’s operating life. Where these interfaces deteriorate, the result can be partial discharge, overheating, insulation breakdown or complete cable failure.

For LV, MV and HV cable networks, joint failure can result in unplanned outages, expensive excavation works, damage to connected equipment and disruption to critical electrical infrastructure. Understanding why failures occur is essential for improving cable installation quality and long-term network reliability.

This technical guide examines the causes of underground cable joint failure, explains why faults can develop prematurely and identifies the preventative measures that help improve reliability across LV, 11kV, 33kV and higher-voltage cable systems.

Thorne & Derrick supply MV & HV cable joints, terminations and connectors, resin cable joints and specialist cable jointing tools for cable installation, jointing and repair applications.

Quick Answer: Why Do Underground Cable Joints Fail?

Underground cable joints fail because of electrical, thermal, mechanical or environmental deterioration affecting the joint assembly. The most common contributing factors include moisture ingress, incorrect cable preparation, contaminated insulation, poor conductor connections, inadequate electrical stress control and physical damage.

In MV and HV cable systems, defects such as insulation scoring, voids or incorrectly positioned stress-control components can contribute to partial discharge and insulation breakdown. On LV systems, moisture ingress, unsuitable joint selection and poor electrical connections can also result in premature failure.

A joint may initially operate successfully despite containing a defect. Over time, electrical loading, temperature changes and environmental exposure can accelerate deterioration until the joint can no longer withstand normal operating conditions.

What Causes Underground Cable Joint Failure?

An underground cable joint connects two or more cable sections while maintaining conductor continuity, electrical insulation, screening or earthing arrangements, and environmental protection appropriate to the cable system.

Unlike a continuous length of power cable, a joint contains multiple assembled components and interfaces. These must operate together reliably throughout the cable circuit’s service life.

A cable joint failure can develop from a single serious defect or several interacting conditions. For example, moisture entering through a damaged outer seal may affect insulation integrity, while an incorrectly installed conductor connector can produce excessive localised heat.

Common causes include:

• Moisture ingress – water entering the joint through compromised sealing arrangements.
• Incorrect cable preparation – damage to insulation, screens or conductors during installation.
• Insulation contamination – dirt, moisture or foreign material trapped at critical interfaces.
• Poor conductor connections – excessive electrical resistance caused by unsuitable or incorrectly installed connectors.
• Incorrect electrical stress control – unsuitable or incorrectly positioned components affecting electric field distribution.
• Mechanical damage – excavation, ground movement or external pressure damaging the cable joint.
• Unsuitable joint selection – joint kits incompatible with the cable construction or operating environment.

The electrical damage observed after a fault may not be the original cause. Consequently, cable joint failure analysis should examine both the initiating defect and the mechanisms that allowed deterioration to progress.

Moisture Ingress & Water Damage In Underground Cable Joints

Moisture ingress is an important cause of underground cable joint deterioration, particularly where cable routes are exposed to saturated ground, groundwater, flooding or repeated wetting and drying.

Buried cable joints may remain surrounded by damp soil throughout their service life. Their environmental protection must therefore be suitable for the intended installation conditions.

How Does Water Enter An Underground Cable Joint?

Water can enter a cable joint through:

• Damaged outer seals – protective systems compromised during installation or service.
• Incorrect sheath sealing – inadequate protection around the cable entry points.
• Mechanical damage – cracks, punctures or deformation of protective components.
• Cable sheath defects – water migration through damaged cable components.
• Inadequate preparation – contaminated or poorly prepared surfaces affecting sealing performance.

Once inside, moisture may contribute to corrosion of metallic components and deterioration of susceptible insulation systems. The extent of damage depends on the cable construction, joint design, voltage and location of the moisture.

Can Water Cause Insulation Breakdown?

Yes. Moisture can contribute to electrical insulation deterioration, particularly when combined with contamination, defects and sustained electrical stress.

In certain polymeric insulation systems, prolonged moisture exposure under electrical stress may contribute to water treeing, a degradation mechanism that can affect insulation performance over time.

Water treeing and electrical treeing are different processes, although both are relevant when investigating deterioration in susceptible cable insulation systems.

Correctly specified resin cable joints and other approved sealed jointing systems provide environmental protection where they are suitable for the cable construction and application.

Incorrect Cable Preparation & Installation Defects

Cable preparation is one of the most important stages of underground cable joint installation. Incorrect preparation can introduce defects that remain undetected until the cable circuit is energised or has been operating for some time.

For MV and HV polymeric cable systems, preparation may involve removing the outer sheath, metallic screen, semi-conductive screen and primary insulation to specified dimensions.

Every operation must preserve the integrity of the remaining cable components.

Common Cable Preparation Mistakes

• Insulation scoring – cuts or scratches produced by unsuitable stripping tools or incorrect adjustment.
• Incorrect screen removal – poorly controlled removal of semi-conductive insulation screens.
• Contamination – dirt, moisture or conductive particles remaining on prepared insulation.
• Incorrect preparation dimensions – cable layers removed beyond or short of the specified positions.
• Incorrect component positioning – insulation or stress-control components installed in the wrong locations.
• Unsuitable tooling – equipment incompatible with the cable diameter, construction or preparation method.

At medium and high voltages, relatively small defects can become significant where they affect electrical field distribution or insulation integrity.

Why Specialist Cable Jointing Tools Matter

Purpose-designed cable jointing tools support repeatable preparation dimensions and controlled removal of insulation, sheaths and semi-conductive screens.

Specialist tools from manufacturers such as Alroc are used for MV and HV cable preparation, including insulation stripping, screen removal and related operations.

Correct tool selection helps reduce the risk of damaging cable layers during preparation and supports consistent installation quality.

Insulation Breakdown & Partial Discharge In Cable Joints

Insulation breakdown occurs when an electrical insulation system can no longer withstand the stresses imposed upon it.

In underground cable joints, the insulation must operate alongside conductor connections, cable screens, stress-control components and multiple insulation interfaces.

Defects within these interfaces can create concentrated electrical stress, potentially contributing to deterioration and eventual electrical breakdown.

What Is Partial Discharge In A Cable Joint?

Partial discharge (PD) is a localised electrical discharge that only partially bridges the insulation between conductive elements.

It may occur in susceptible defects such as voids, contaminated interfaces or locations experiencing excessive electrical stress.

Repeated partial discharge can progressively damage surrounding insulation and contribute to long-term dielectric deterioration.

However, not every insulation defect generates detectable partial discharge, and specialist assessment is required to interpret test results correctly.

Why Is Electrical Stress Control Important?

In screened MV and HV cable systems, the cable screen helps establish the electrical field distribution around the insulation.

Where the screen is interrupted at a joint, the jointing system must provide suitable electrical field control.

Incorrectly positioned, damaged or incompatible stress-control components may result in localised electrical field enhancement.

These considerations are particularly important for 11kV cable joints, 33kV joints and higher-voltage cable accessories where electrical field control and insulation preparation are essential to reliable performance.

Poor Conductor Connections & Overheating

An underground cable joint must maintain a reliable electrical connection between conductors throughout its intended service life.

Poor conductor connections can introduce excessive electrical resistance, resulting in localised heating whenever current flows through the connection.

Potential causes include:

• Incorrect connector selection for the conductor material or cross-section.
• Unsuitable compression dies or crimping equipment.
• Incorrect installation of mechanical shear-bolt connectors.
• Inadequate conductor preparation or contaminated contact surfaces.
• Connector damage or deterioration under operating conditions.

How Does A High-Resistance Cable Joint Fail?

When current flows through a connection, electrical resistance produces heat.

For a given current, greater resistance results in increased heating, illustrated by the relationship:

Excessive localised heating may contribute to deterioration of surrounding insulating materials, connector interfaces and protective components.

Repeated thermal cycling can place additional mechanical stress on the connection, particularly where materials expand and contract at different rates.

Correct connector selection, approved installation procedures and compatible tooling are therefore essential to reliable cable joint performance.

Environmental & Mechanical Factors Affecting Underground Cable Joints

Underground cable joints operate in conditions that can change considerably throughout the cable circuit’s service life.

Ground movement, thermal cycling, mechanical loading, moisture and accidental excavation damage can all affect the long-term integrity of a buried joint.

Ground Movement & Settlement

Ground settlement or movement of surrounding structures may place mechanical stress on buried cables and joints.

Where cables are inadequately supported or the joint is exposed to excessive movement, protective components and insulation systems may become damaged.

Thermal Cycling

Electrical loading causes cable temperatures to rise and fall. Different materials within the cable joint may expand and contract at different rates.

Correctly designed and installed joints accommodate their specified operating conditions, but defective interfaces or unsuitable components may become vulnerable to repeated thermal movement.

Excavation & External Damage

Underground electrical infrastructure can be damaged during excavation, civil engineering works or other construction activities.

External damage may immediately affect the joint or compromise its protective sealing, allowing deterioration to develop later.

Appropriate cable route protection, accurate installation records and controlled excavation practices help reduce the risk of damage.

Why Do Underground Cable Joints Fail Prematurely?

A premature cable joint failure occurs when the joint loses its required performance significantly earlier than expected for its intended operating conditions.

Some premature failures originate from defects introduced during installation that are not immediately detected.

A joint may initially withstand normal operating voltage despite containing a small insulation imperfection, contaminated interface or inadequately assembled connection.

Over time, electrical loading, moisture exposure, thermal cycling or mechanical stress may allow the defect to develop into a major fault.

Typical Causes Of Early Cable Joint Failure

• Incorrect joint kit specification – the selected accessory does not match the cable construction or electrical system.
• Inadequate installation cleanliness – contamination remains at critical insulation interfaces.
• Damaged insulation – scoring or incorrect screen preparation introduces electrical weaknesses.
• Incorrect connector installation – unsuitable tools or methods compromise conductor continuity.
• Insufficient environmental protection – inadequate sealing permits moisture ingress.
• Undetected installation defects – faults remain unidentified before energisation.
• Operating conditions outside design limits – loading, temperature or environmental conditions exceed the joint system’s capabilities.

Premature failure does not automatically establish poor workmanship as the root cause. Investigations should also consider cable condition, joint design, operating history, external influences and potential manufacturing defects.

How Are Underground Cable Joint Failures Investigated?

Cable joint failure analysis aims to identify the original cause of deterioration rather than simply the damage produced by the final electrical fault.

Severe electrical failures can destroy or obscure evidence of the initiating defect, making detailed investigation essential.

Initial Assessment

Specialist engineers may review:

• Electrical protection and circuit fault records.
• Previous cable testing and inspection results.
• Installation documentation and jointing records.
• Cable loading and operating history.
• Ground conditions and evidence of external damage.

Before intrusive investigation, the affected circuit must be isolated and made safe under the applicable electrical safety procedures.

Cable Fault Location & Diagnostic Testing

Depending on the cable construction and suspected fault, appropriate diagnostic methods may include:

• Insulation resistance testing – assessing insulation condition where appropriate to the cable system.
• Continuity and connection checks – investigating electrical continuity and suspected conductor defects.
• Time-domain reflectometry – helping identify certain impedance changes or fault-related features along a cable route.
• Partial discharge measurements – identifying and assessing detectable discharge activity in suitable cable systems.
• Specialist cable diagnostics – selected according to voltage rating, cable construction and test objectives.

No single test identifies every possible underground cable joint defect. Test methods, test voltages and interpretation must be appropriate to the installed system.

Physical Examination & Root-Cause Analysis

Where a failed joint is recovered, physical examination may identify:

• Electrical tracking or insulation breakdown.
• Thermal damage around conductor connections.
• Water ingress or corrosion.
• Incorrect cable preparation or screen removal.
• Improperly positioned joint components.
• Mechanical damage or sealing deterioration.

These findings should be assessed alongside the cable circuit’s installation and operating history. The most visibly damaged part of a failed joint is not necessarily where deterioration began.

How Can Underground Cable Joint Failure Be Prevented?

Preventing underground cable joint failure requires attention to the complete jointing process, from initial specification through to installation, testing and long-term operation.

Correct joint selection, compatible cable preparation tools and competent installation are all essential to reliable performance.

The Importance Of Competent Cable Jointers

Underground cable jointing is a specialist activity that requires appropriate training, equipment and installation experience.

This is particularly important for 11kV, 33kV and higher-voltage cable joints, where electrical field control, insulation preparation and cable system compatibility must be carefully managed.

Competent personnel working to approved installation procedures help reduce the risk of defects that could otherwise contribute to premature failure.

Do Different Cable Jointing Technologies Have Different Failure Risks?

Different cable jointing technologies are used across underground power distribution, industrial installations and infrastructure projects.

The suitability of each system depends on the cable construction, voltage rating, service environment and installation requirements.

No single cable jointing technology should be considered universally more reliable without reference to the joint design, cable compatibility, qualification, installation quality and operating environment.

For example, a correctly specified resin joint may provide a suitable solution for a particular LV underground installation, while another cable system may require an approved heat shrink or cold shrink arrangement.

Thorne & Derrick supply resin cable joints, 11kV cable joints and MV cable repair joints for compatible cable constructions and installation requirements.

Related Cable Jointing Products & Equipment

Correct cable joint specification and installation are essential to reducing the risk of premature failure. Thorne & Derrick supply specialist cable accessories and preparation tools used throughout LV, MV and HV cable installation and repair projects.

• MV & HV Cable Joints, Terminations & Connectors – cable accessories for power distribution, industrial and high voltage infrastructure applications.
• 11kV Cable Joints – jointing systems for compatible medium voltage cable constructions.
• Resin Cable Joints – resin-based cable jointing systems for suitable underground LV applications.
• MV Cable Repair Joints – specialist accessories for approved medium voltage cable repair applications.
• Cable Jointing Tools – equipment for cable sheath removal, insulation stripping, semi-conductive screen preparation and associated jointing operations.
• Alroc Cable Preparation Tools – specialist tools used by professional cable jointers for MV and HV cable preparation.

Correct equipment selection should consider the cable system as a whole, including electrical rating, conductor dimensions, insulation construction, installation environment and applicable project requirements.

Underground Cable Joint Failure FAQs

Conclusion

Underground cable joint failures can develop when electrical insulation, conductor connections or environmental protection become compromised. Moisture ingress, poor cable preparation, insulation contamination, incorrect electrical stress control and mechanical damage are important factors affecting long-term joint reliability.

For LV, 11kV, 33kV and higher-voltage cable systems, preventing premature failure requires correct joint specification, compatible cable preparation equipment, competent installation and appropriate testing.

Understanding the mechanisms behind cable joint failure helps electrical engineers, utilities and contractors improve installation practices, investigate faults and reduce the risk of unplanned outages.

Thorne & Derrick supply LV, MV and HV cable joints, cable repair accessories and specialist cable jointing tools to support underground cable installation, connection and repair projects throughout the UK and internationally.