Most HT cable faults happen at joints and terminations, not in the cable itself. This guide explains the 12 most common causes of cable joint and termination failure on 11kV, 22kV and 33kV XLPE and PILC cables — moisture, poor cable preparation, stress-control errors, bad crimping, wrong kit selection and more — with warning signs, a prevention checklist and FAQs.
Quick Answer: Why Do Cable Joints and Terminations Fail?
Cable joints and terminations fail mainly because of installation workmanship, moisture and electrical stress — not because of the kit material itself. The most common causes are:
- ●Moisture getting into the joint through poor sealing or wet cable ends
- ●Damage during cable preparation — knife cuts in the insulation or a rough semiconductor screen cut
- ●Wrong position of the stress-control tube, which concentrates the electric field at the screen end
- ●Poor conductor connection — under-crimped lugs or ferrules that overheat
- ●Wrong kit for the job — wrong size band, voltage, cable type, or an indoor kit used outdoors
- ●Order the correct kit — voltage, XLPE/PILC, cores, size band, indoor/outdoor/joint
- ●Use only trained, experienced jointers and the instruction sheet supplied with the kit
- ●Joint in a dry, clean, covered area; keep cable ends sealed until jointing
- ●Use correct tools: semicon stripper, crimping tool with correct dies, LPG torch
- ●Check every dimension on the kit sheet before shrinking any tube
- ●Fill voids with mastic and shrink from the centre outwards
- ●Make solid earth connections for screen and armour
- ●Test IR before energising and record the values for future comparison
- ●Support terminations and protect joints mechanically
- ●Heat Shrink Cable Jointing Kits — Straight-Through and Transition
- ●Heat Shrink Cable Termination Kits — 1.1kV to 33kV
- ●Contact VIKCHEM
Utility fault records across India consistently show that a large share of underground HT cable faults occur at joints and terminations, which is why choosing an IS 13573 certified kit and following the installation procedure exactly matters more than any other step.
This guide is written by the engineering team of Vikram Power Technologies Pvt. Ltd. (VIKCHEM), Yamuna Nagar, Haryana — manufacturer of heat shrink cable termination kits and cable jointing kits since 1988.
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The 12 Most Common Causes of Cable Joint and Termination Failure
1. Moisture Ingress
Water is the number one enemy of a cable joint. Moisture can enter through an unsealed outer sheath, a damaged cable end left open in a wet trench, or condensation trapped during installation. Inside the joint it causes tracking, corrosion of the conductor and screen, and water treeing in XLPE insulation, leading to breakdown months or years later.
Prevention: keep cable ends capped until jointing, never joint in standing water or rain without a tent, dry the cable ends, and make sure every sealing mastic and adhesive-lined tube is fully recovered so the adhesive flows and seals.
2. Insulation Damage During Cable Preparation
A knife cut, nick or deep score in the XLPE insulation while removing the semiconductor screen creates a weak spot where the electric field concentrates. Even a small scratch can start partial discharge.
Prevention: use proper semicon stripping tools, never cut towards the insulation, and smooth the insulation surface with the abrasive supplied in the kit — not with a file or coarse paper.
3. Poor Semiconductor Screen Termination
The end of the semiconductor screen is where electrical stress is highest. A jagged, stepped or lifted screen edge, or semicon residue left on the insulation, creates a high-stress point and a tracking path.
Prevention: make a clean, straight, chamfered screen cut and remove all semicon traces from the insulation surface with the solvent wipes supplied.
4. Incorrect Stress-Control Tube Position
Heat shrink kits control the electric field at the screen end with a stress-control tube or stress-relief mastic. If the tube is too short over the screen, shifted, or not in full contact, the field is not graded and the insulation fails at the screen end — one of the most common causes of early termination failure.
Prevention: follow the kit's dimension sheet exactly for the overlap on the screen, and check the position before shrinking. See our XLPE cable jointing procedure for the correct sequence.
5. Under-Crimped or Wrong Lugs and Ferrules
A loose or under-crimped connector has high contact resistance. Under load it overheats, the heat damages the insulation and sealing around it, and the joint fails thermally. Using a copper lug on an aluminium conductor without a bimetallic transition causes corrosion and the same result.
Prevention: use the correct die and number of crimps for the lug size, use bimetallic lugs for aluminium-to-copper connections, or use shear-head mechanical lugs and ferrules which tighten to a set torque.
6. Voids and Air Gaps Under the Tubes
Air trapped between layers — at the connector, around the ferrule or under the stress tube — becomes a site for partial discharge, which slowly erodes the insulation until it fails.
Prevention: fill the connector area with the void-filling mastic supplied, and shrink every tube from the centre outwards so air is pushed out, not trapped.
7. Improper Heating (Under- or Over-Shrinking)
Under-heated tubes do not fully recover, leaving gaps and unsealed adhesive. Over-heated tubes scorch, crack or thin out. Both reduce the insulation and sealing performance.
Prevention: use a soft, yellow-tipped flame (LPG torch), keep it moving, and heat evenly around the circumference until the tube surface is smooth and the adhesive shows at the ends.
8. Contamination
Dust, sand, sweat, grease and moisture on the insulation surface create a conductive path along the interface. In outdoor terminations, pollution on the surface causes tracking and flashover.
Prevention: clean hands and tools, wipe the insulation with the cleaning solvent supplied just before shrinking, and do not touch cleaned surfaces.
9. Wrong Kit Size, Voltage or Cable Type
A kit that is too large will not shrink tightly; one that is too small cannot be installed correctly. A PILC kit on XLPE (or the reverse), or an 11kV kit on a 22kV cable, will not control stress properly.
Prevention: match voltage, cable type, cores and size band before ordering. Use our kit selection chart to find the exact kit code.
10. Indoor Kits Used Outdoors
Indoor terminations do not have rain sheds or the same UV and tracking resistance. Outdoors, rain and pollution cause surface tracking and flashover.
Prevention: always use outdoor terminations (with sheds) for pole tops, outdoor transformer bushings and switchyards.
11. Poor Screen and Armour Earthing
If the metallic screen or armour is not continuously and firmly earthed across a joint, or the earth braid is loose or corroded, circulating currents, heating and dangerous touch voltages can occur, and the joint fails under fault conditions.
Prevention: use the earthing braid and constant-force springs or clamps supplied, make solid connections to the screen wires and armour, and connect the termination earth to the station earth.
12. Mechanical Stress and Ground Movement
Sharp bends close to the joint, unsupported terminations, heavy vehicles above a shallow trench, or ground settlement pull on the joint and crack the insulation or break the seals.
Prevention: respect the minimum bending radius, support cable ends and terminations with proper clamps, lay joints on a level, firm bed, and protect them with a joint box or bricks where required.
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Cause, Warning Sign and Prevention at a Glance
| Cause | Typical Warning Sign | How to Prevent |
|---|---|---|
| Moisture ingress | Falling insulation resistance, tracking marks | Cap cable ends, dry jointing, full adhesive flow |
| Insulation cuts | Partial discharge activity, early breakdown | Proper semicon tools, no knife towards insulation |
| Rough screen cut | Burn marks at screen end | Clean chamfered cut, remove semicon residue |
| Stress tube misplaced | Failure at screen end within months | Follow kit dimension sheet exactly |
| Poor crimping | Hot spot on thermal camera, discoloured tubes | Correct die and crimps, bimetallic or shear-bolt lugs |
| Voids / air gaps | Partial discharge, slow internal erosion | Void-filling mastic, shrink from centre outwards |
| Over / under heating | Scorched or loose tubes, no adhesive at ends | Soft flame, even heating |
| Contamination | Surface tracking, flashover | Solvent-clean just before shrinking |
| Wrong kit | Loose fit or impossible installation | Match voltage, type, cores and size band |
| Indoor kit outdoors | Tracking and flashover in rain | Use outdoor terminations with sheds |
| Poor earthing | Heating at braid, sheath currents | Firm braid connections, station earth |
| Mechanical stress | Cracked tubes, broken seals | Bending radius, support, level bed |
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How to Check a Joint or Termination Before and After Energising
Insulation resistance (IR) test: measure with a suitable insulation tester before energising and compare with previous values and the other phases. A low or unbalanced reading points to moisture or contamination.
High-voltage withstand test: after jointing, many utilities specify a VLF (very low frequency) or DC withstand test as per their specification before commissioning.
Partial discharge (PD) test: on critical feeders, an online or offline PD test finds voids, cuts and stress-control problems before they cause a fault.
Thermal imaging: a thermal camera scan of terminations under load finds loose or under-crimped connections as hot spots.
Visual inspection of outdoor terminations: look for tracking marks, cracked sheds, birds' nests and heavy pollution, especially before the monsoon.
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Prevention Checklist for Site Engineers
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Why Heat Shrink Kits Are Reliable When Installed Correctly
Heat shrink cable accessories made from cross-linked polymers recover tightly onto the cable, seal against moisture with hot-melt adhesive, and grade electrical stress with stress-control tubing. When the kit is correctly selected and installed, heat shrink joints and terminations give long service life across Indian conditions — high heat, monsoon humidity and coastal pollution. For a comparison with other technologies, read heat shrink vs cold shrink cable accessories, and for standards read our IS 13573 guide.
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Frequently Asked Questions
Q: What is the most common cause of cable joint failure? The most common cause of cable joint failure is poor installation workmanship, especially moisture ingress, damage to the insulation during cable preparation, and incorrect positioning of the stress-control tube at the semiconductor screen end.
Q: Why do cable terminations fail at the screen end? The end of the semiconductor screen is where electrical stress is highest. If the screen cut is rough, semicon residue is left on the insulation, or the stress-control tube does not overlap the screen correctly, the electric field concentrates there and the insulation breaks down.
Q: How long should a heat shrink cable joint last? A correctly selected and installed IS 13573 certified heat shrink joint is designed to last as long as the cable it is installed on. Early failures within months usually point to installation or kit selection errors rather than ageing.
Q: How can I tell if a cable joint is failing before it faults? Warning signs include falling or unbalanced insulation resistance, partial discharge activity, hot spots on a thermal camera, and visible tracking, cracking or burn marks on outdoor terminations.
Q: Can moisture in a cable cause joint failure? Yes. Moisture inside the cable or joint causes tracking, conductor and screen corrosion and water treeing in XLPE insulation, which leads to breakdown. Cable ends must be kept sealed and dry until jointing.
Q: Does using the wrong size kit cause failure? Yes. A kit outside the cable's size band will not shrink tightly or seal properly, and a kit for the wrong voltage or cable type will not control electrical stress correctly. Always match the kit code to voltage, cable type, cores and conductor size.
Q: Where can I buy reliable heat shrink cable jointing kits in India? VIKCHEM (Vikram Power Technologies Pvt. Ltd.), Yamuna Nagar, Haryana, manufactures IS 13573 certified heat shrink termination and jointing kits from 1.1kV to 33kV and supplies DISCOMs, Railways, EPC contractors and industry across India. Contact sales@vptpl.com or +91-99966-30201.
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Get Expert Help With Your Cable Jointing Project
Facing repeated joint failures on a feeder, or planning a new HT cable project? Send us the cable details and site conditions — VIKCHEM's engineers will recommend the right kit and share installation guidance.
Related reading: Kit Selection Chart by Voltage and Cable Size | XLPE Cable Jointing Procedure — Step by Step | Heat Shrink Cable Accessories for Indian DISCOMs