Heat Shrink Cable Accessories Failure Analysis: Why MV Accessories Fail — and How to Stop It

Mr. Xiao — Senior Technical Advisor, Zhizheng

Over 20 years in heat shrink & cold shrink cable accessory R&D, failure analysis, and global project support. Based in Huangshi, Hubei, China.

I want to start with something that gets said quietly at industry conferences but rarely appears in supplier catalogues: a disturbing proportion of medium-voltage cable accessories fail — not in service after decades of operation, but during or shortly after installation. The data behind this is not speculative. It comes from laboratory testing programs involving over a thousand cable accessories, and it should reshape how reliability engineers and maintenance managers think about what they specify and from whom they buy.

This article is a technical post-mortem. I'll walk through what the failure data actually shows, break down the dominant failure mechanisms — both material-side and installation-side — and explain what responsible manufacturing looks like when you're trying to solve the problem at the source rather than shift the blame to field crews.

Disclosure: I work at Zhizheng, and we make heat shrink cable accessories. I'll explain what we do differently and why. But the failure analysis comes first, because if you don't understand why things fail, you can't evaluate whether any supplier's quality claims are genuine or cosmetic.

The KEMA Labs Finding That the Industry Would Rather Not Discuss

KEMA Labs in the Netherlands has accumulated years of experience testing cables and accessories for medium and high voltage power networks. Their testing database is one of the most authoritative in the world for understanding real-world product quality. And the numbers are sobering.

Based on more than 1,000 cables and accessories tested, the initial failure rate when entering a type test program is approximately 25%. Let that settle for a moment. The outcome of this analysis showed an initial failure rate of approximately 25%, independent of the type of equipment or component being tested — in other words, every 4th type test sequence fails, which is a high proportion compared to other industries.

A survey by KEMA Labs found that the failure rate of medium-voltage cable accessories is significantly higher than that of low-voltage and high-voltage cable accessories. And critically, the leading cause is improper material handling during the heat shrink process.

The linear trend lines in failure rate data over time have a positive inclination — meaning the initial failure rate is actually increasing over time, not improving. That's an industry-wide quality trajectory that should alarm anyone responsible for system reliability.

The practical consequence is stark: components for power networks cannot be treated as "catalogue items" purchased at the lowest price. Yet that is exactly how cable accessories are procured on many projects — last-minute line items, lowest-cost compliant bid, no factory audit.

Why Medium Voltage Is the Most Dangerous Tier

There is a counterintuitive phenomenon at work in this data. You might expect that higher-voltage accessories — operating under greater electrical stress — would fail more often. In some data sets they do. But for accessories specifically, the MV tier (approximately 6kV–35kV) is consistently identified as the critical risk zone. Here is why.

Medium voltage terminations and joints show a high failure rate, due mainly to improper materials used in the heat shrink technology. The physics at medium voltage create a particularly unforgiving interface. The insulation is thick enough that geometric stress concentration at the shield cutback is significant — requiring precision stress control tubing — but the voltage is not so extreme that manufacturers invest in the same level of design rigour applied to EHV accessories.

Cable accessories are often the most prone to failure of any part of the cable system. Accessories include terminations and joints, also called splices. And within those accessories, the termination — the interface between the cable insulation and the outside world — is where the most variables converge.

Medium-voltage cable terminations fail more often than the cables they connect. In field assessments across 75+ industrial substations operating at 6.6–35 kV, improper stress cone installation accounts for approximately 40% of premature termination failures leading to partial discharge activity.

The Italian distribution network data tells a similar story: 84% of faults in medium voltage networks of two cities in Northern Italy were associated with joint failures. That's not a coincidence. That's a systemic problem rooted in both material quality and installation practice.

Failure Mode Anatomy: The Six Root Causes

When we pull apart MV cable accessory failures — from our own quality audits, from published post-mortem reports, and from the engineering literature — six root causes emerge consistently. I'll take them one at a time.

Root Cause 1: Improper Heat Application During Shrinking

Improper heat shrinking during installation is a major cause of MV cable failures. If the heat shrink tubing is not evenly applied or fails to adhere properly, it leaves gaps that allow moisture and contaminants to enter. These gaps compromise the insulation and increase the likelihood of failure.

Specifically, a heat-shrink termination can experience breakdown between the cable lug and the insulation screen. Electrical tracks and splits in the heat-shrink sleeve indicate arcing. The root cause is under-shrinking of the heat-shrink tubing, which allows moisture to enter at the conductor lug.

Overheating creates an equal and opposite problem. Excess heat can degrade the shrink tubing or stress-control components. The working window between under-application and degradation is narrower than most field crews appreciate — which is why material quality and consistent shrink ratio are not optional engineering specifications. They are failure-prevention parameters.

Root Cause 2: Inadequate Cable Surface Preparation

Inadequate removal or radial cutback of the semi-conductive layer may result in uneven stress control and void formation. Rough or damaged XLPE insulation surfaces can impair adhesion and lead to degradation.

Air has a much lower dielectric strength than XLPE, making it the enemy of medium voltage. Careless application of void-filling mastic or shrinking heat shrink tubes from the top down traps air pockets — typically near the semi-con step or lug. The voltage across these pockets causes the air to break down (Partial Discharge), creating ozone that eats the insulation from the inside out.

Root Cause 3: Stress Cone Misalignment

The stress control tube is the heart of the termination, designed to refract voltage lines. A common error is misinterpreting the template and positioning the tube too high or low. The material must overlap the semi-con screen by a precise distance (usually 10–20mm) to "pick up" the earth potential. Missing this overlap leaves stress uncontrolled, leading to rapid air ionization and breakdown.

Root Cause 4: Moisture Ingress from Poor Sealing

Loose or improperly sealed terminations can cause arcing, short circuits, or open circuits. Poor sealing allows water or contaminants to penetrate, leading to corrosion.

If seal tapes are incorrectly applied, moisture or air can enter, causing electrical tracking. On outdoor MV terminations in particular — which is the most common installation scenario for power distribution — sealing integrity at the conductor lug entry and at the cable sheath transition is the single most critical quality attribute of the finished installation.

Root Cause 5: Environmental Contamination During Installation

Proceeding with work in high humidity, rain, or dust storms is a logistical error with technical consequences. Trapping airborne moisture or dust inside the layers of a termination is fatal, as humidity above 80% significantly increases the risk of surface tracking.

Severely defective terminations with voids larger than 1 mm or gross contamination can fail within weeks. Marginal defects typically require 2–5 years to progress through the full degradation sequence, with the actual timeline depending on operating voltage relative to PDIV and environmental conditions.

Root Cause 6: Substandard Material Quality

This is the root cause that suppliers are least eager to discuss openly. Field installation errors are easy to attribute to the contractor. Material failure requires the manufacturer to look inward.

A survey by KEMA Labs found that the failure rate of medium-voltage cable accessories is significantly higher than that of low-voltage and high-voltage cable accessories, with the leading cause being improper material handling during the heat shrink process. Specifically, this includes inadequate cross-linking density, incorrect shrink ratios, and stress control compounds that fail to maintain their dielectric properties under sustained thermal cycling.

Frequent temperature changes can weaken the materials used in cable terminations. Expansion and contraction caused by heat and cold cycles create mechanical stress, which can lead to cracks or gaps in the insulation. Over time, these small defects allow moisture or contaminants to enter, further accelerating degradation.

The "Infant Mortality" Problem — and What It Tells Us About Procurement

Here is an observation that I find more useful than almost any other data point when I'm talking to maintenance managers: most accessory failures don't happen after decades of service. They happen early.

Terminations fail mostly in the first three years. This is the "Infant Mortality" period. Failures here are almost exclusively due to workmanship errors — voids, dirt, knife cuts.

What does this mean practically? It means that if you have a cable circuit that has run without problems for 15 years, the accessory failure risk you're managing is very different from the risk at commissioning of a new installation. Early-life failures are primarily a product of two interacting variables: the quality of what came out of the factory, and the quality of what happened on site.

You can't control site workmanship after the fact. But you can — before procurement — control the material quality that the field crew is working with. A heat shrink kit that shrinks predictably, seals completely, and provides consistent stress relief geometry makes a skilled installer's job straightforward and gives an average installer a much narrower window for error.

A field report analyzing 6,214 cable failures over 14 years revealed that 23% of failures stemmed from cable joint issues, often caused by improper installation. Additionally, 20% to 50% of accessory-to-cable transitions result in failure, highlighting the importance of proper installation and testing. That 20–50% transition failure range is enormous — and much of that variance comes down to the quality of the accessory itself.

How Irradiation Cross-Linking Addresses Material-Side Failure at the Source

When I explain what differentiates Zhizheng's manufacturing process to engineers who haven't visited our facility, the conversation almost always comes back to one process: electron beam irradiation cross-linking.

Cross-linking is the chemistry that transforms a thermoplastic polymer — which would melt, deform, and lose its electrical properties under operating temperatures — into a thermoset structure with genuine dimensional stability, reliable shrink memory, and long-term dielectric performance. There are two ways to cross-link polyolefin: chemically (peroxide-based) and physically (electron beam irradiation).

Irradiation cross-linking, applied correctly, produces a more uniform and controllable cross-link density throughout the tube wall than chemical cross-linking. This matters for heat shrink accessories because:

• Uniform shrink ratio across the tube wall — prevents the differential shrinkage that creates voids at interfaces

• Consistent stress control compound dielectric properties — the permittivity value that gradient-control tubes rely on is a direct function of cross-link uniformity

• Predictable recovery temperature range — allows field installers to apply heat within a clearly defined working window, reducing under- and over-application errors

• Retention of mechanical properties at elevated temperatures — critical for accessories on cables under sustained load, where conductor temperatures regularly exceed 70°C

Our 40,000 m² integrated manufacturing facility processes the full chain in-house: extrusion of the base compound, irradiation, expansion to the required pre-shrink diameter, and final assembly. In-house irradiation control means we can verify and record cross-link dose for every production batch — a verification step that is simply not possible when irradiation is outsourced to a third-party facility with a queue of unrelated products.

Over time, small defects allow moisture or contaminants to enter, further accelerating degradation. To combat this, you should use materials designed to withstand extreme temperature variations, such as heat shrinkable polyolefin tubes or cold shrink solutions. The critical word there is "designed." A tube that merely claims polyolefin construction without documented cross-link verification is not a product you want on a 20-year service life medium-voltage circuit.

Zhizheng's Quality Control Architecture: Beyond Certification Wallpaper

Most supplier quality sections in blog posts read like a certificate list. I'll try to be more specific, because the devil is in the process, not the plaque on the wall.

Type Test Validation — 35kV Cold Shrink Accessories

We hold a Type Test Report for our 35kV cold shrink cable accessories. Type testing verifies a cable or cable system's design and manufacturing. A cable system includes the cable as well as accessories such as joints, open-air terminations, and SF6 or oil plug-in terminations. A cable or cable system is tested in the laboratory with various electrical tests followed by heat cycle tests to simulate ageing and loading.

Obtaining and passing a type test — not merely claiming compliance — is the minimum credible evidence that a medium-voltage accessory design actually works under realistic stress conditions. Given that manufacturers come to test laboratories such as KEMA for a formal type test once they believe their products meet all the requirements set out in the standards, and given that 25% of them fail on the first attempt, holding a passing type test certificate represents a genuine technical validation, not a procedural formality.

ISO 9001:2015 — Process Control, Not Just Documentation

Our ISO 9001:2015 certification covers the full production process. For heat shrink accessories, the relevant process control points include incoming raw material qualification (compound specification verification before any batch enters extrusion), in-process dimensional verification at extrusion and post-irradiation, and outgoing quality inspection of shrink ratio, wall thickness, and dielectric withstand performance. In order to offer good premises for distribution networks with high reliability and long lifetime, the commissioning and installation quality is essential. That quality engineering starts upstream — at the supplier's process controls.

IATF 16949 — Automotive-Grade Production Discipline Applied to Power Products

We obtained IATF 16949 certification (formerly TS16949) for our automotive wiring harness products. The significance of this for power cable accessory buyers is process discipline. Automotive qualification requires statistical process control, FMEA (Failure Mode and Effects Analysis), and measurement system analysis at a rigour level that exceeds what most power cable accessory manufacturers apply. The production habits and documentation culture built under IATF 16949 carry across our facility.

UL, REACH 2025, and RoHS Compliance

Our UL certificate provides third-party verification of electrical safety performance to North American standards. REACH 2025 and RoHS certifications (current, HST-certified) confirm material compliance for shipments to European markets. These are not optional on projects subject to EU procurement rules or development finance institution requirements.

Partial Discharge and Dielectric Withstand Testing

Our production testing protocols include partial discharge measurement on representative units from each production batch. PD testing is the most sensitive available method for detecting the void structures and interface contamination that cause the early-life failures described above. By addressing partial discharge activity early, you can extend the lifespan of your cable systems and reduce the likelihood of unexpected failures. Combining regular inspections with partial discharge testing provides a comprehensive approach to maintaining system reliability. We embed PD verification at the production stage — not only at the end-user commissioning stage.

→ View our full certification documentation: Zhizheng Certifications — ISO, UL, IATF 16949 & More

→ Explore our 10kV and 35kV heat shrink cable termination kits: RSY Series — Heat Shrink Cable Termination Kits

A Failure-Prevention Checklist for Reliability Engineers

This checklist is designed for reliability engineers and maintenance managers reviewing their accessory specification and procurement processes. It is not a product pitch — it applies regardless of which supplier you use.

At the Specification Stage

• Confirm voltage class: specify 10kV or 35kV kits explicitly — do not accept "MV compatible" without voltage confirmation

• Specify cable insulation type (XLPE, EPR, PVC) — the stress relief geometry must match

• Specify indoor vs. outdoor installation — outdoor terminations require anti-tracking outer tubes and correct creepage distance sheds

• Request type test report (IEC 60502-4 or equivalent) as a mandatory submission document

• Request gel content data (cross-link density) for stress control and outer insulation tubes

At the Procurement / Supplier Qualification Stage

• Components for power networks cannot be treated as "catalogue items" purchased at the lowest price — include quality audit criteria, not just price in the evaluation

• Verify that irradiation cross-linking is performed in-house with batch-level dose documentation

• Confirm ISO 9001 scope covers heat shrink accessory production specifically

• Check REACH and RoHS current certificate dates — materials compliance is date-specific

At the Installation Stage

• Keep relative humidity below 70% at the work location — use portable dehumidifiers or temporary enclosures when necessary. Establish a defined clean zone with controlled access.

• For heat shrink products, heat evenly from the center toward the ends and confirm good contact with the underlying surface.

• Always shrink tubes from the bottom (screen end) upwards to chase the air out.

• Certified installers reduce defect rates by 60–80% compared to uncertified personnel. Use certified jointers for all MV accessory installations

• Perform post-installation partial discharge testing before energisation

Frequently Asked Questions