Cable failures rarely occur mid-span—they almost always happen at the ends. The termination is the most vulnerable point in any Medium Voltage (MV) network. While manufacturers provide instruction manuals, these often lack the underlying physics that explain why each step matters. Without this understanding, technicians may take shortcuts, leading to defects that can remain hidden for years.
To properly terminate, you must understand the specific layers involved—from the conductor screen to the outer jacket. For a detailed breakdown, see our guide: Exploring Medium Voltage Cable. Remember: you cannot terminate what you do not fully understand.
Below, I’ve grouped the 10 most common failures into three categories: Mechanical Integrity Errors, Geometric & Stress Control Errors, and Hygiene & Environmental Errors.
Mechanical Integrity Errors
Before considering the electric field, start with the physical construction. Mechanical mistakes compromise the foundation of a reliable termination. For example, if you’re working with specific cable types—such as 5kV cable or 15kV cable—it’s essential to understand the unique handling and termination requirements for each voltage class.
1. Damage to the XLPE Insulation During Stripping
The interface between the insulation and the semiconductive screen is critical, yet the most fatal error I see is the use of incorrect tools. Technicians often use standard hook blades to score the bonded semi-con layer, inadvertently cutting into the underlying XLPE insulation. While a scratch is negligible in Low Voltage systems, in Medium Voltage, it acts as a stress riser. The electric field concentrates at the bottom of this “V” notch, initiating “electrical treeing”—a carbonized path that grows through the insulation until it bridges the phase-to-ground potential.
How to do it correctly
- Prohibit knives for screen removal.
- Use adjustable scoring tools set to 90% of the screen thickness, then peel the remainder so it tears naturally.
- If you see a score line on the insulation, polish it out with aluminum oxide cloth or reject the work.
2. Lug Compression Failure (Thermal Runaway)
While the insulation controls voltage, the connector controls current, making this a thermal failure rather than a dielectric one. Using incorrect die sizes, mixing metals without bi-metallic washers, or “shaving down” conductors to fit undersized lugs creates high electrical resistance. This generates massive heat that travels down the conductor, softening the XLPE insulation until the termination seal breaks and the unit blows out.
| Crimping Method | Connection Resistance (mΩ) | Temperature Rise at 100A (°C) | Result |
| Correct Die/Indent | 0.15 | 15 | Pass ✅ |
| Incorrect Die | 0.45 | 45 | Fail ❌ |
| Shaved Conductor | 0.60 | 60 | Fail ❌ |
How to do it correctly
Ensure the die code matches the lug code exactly. For MV applications, deep indent crimping is often preferred over hex crimping for compacted conductors to ensure gas-tightness.
3. Compromised Earth Path (Screen Grounding)
The metallic shield carries capacitive current and, crucially, fault current, yet I frequently see earth braids soldered to copper tape or loosely attached. Soldering risks melting the underlying insulation and creating rigid points that crack under thermal expansion, while loose springs create resistance that can generate enough heat during a fault to ignite the termination before protection relays trip.
How to do it correctly
- Collect screen wires neatly and braid or compress them before connecting to an earthing lug.
- Use constant-force springs or dedicated clamps to maintain stable pressure on screen connections over the long term.
- Size earthing conductors based on short-circuit current and duration, not just convenience.
- Follow the designed earthing strategy (single-ended, both-ended, cross-bonded) and document where each screen is connected.
Geometric & Stress Control Errors
In a healthy cable, the electric field is uniform. When we cut the screen, the field lines crowd together. These errors occur when we fail to manage that geometry.
4. Poor Cutback Geometry
The transition point where the semi-conductive screen ends is the area of highest electrical stress. Leaving a jagged, uneven edge or a sharp “step” creates focal points for discharge. If the transition isn’t smooth, the stress control tube may not fill the gap perfectly, leaving microscopic air voids that lead to ionization and failure.
How to do it correctly
Use a tool that creates a spiral cut. While not always required for modern cold shrink, it is best practice to lightly sand (chamfer) the edge of the semi-con to create a smooth ramp rather than a cliff.
5. Misalignment of Stress Control Components
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.
How to do it correctly
Mark, don’t guess. Use white electrical tape (not pencil, which is conductive) to mark the exact reference measurements on the cable before sliding on the components.
- For cold-shrink accessories, align the component carefully before removing the inner core and let it fully recover.
- For heat-shrink products, heat evenly from the center toward the ends and confirm good contact with the underlying surface.
6. Trapped Air Voids
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.
How to do it correctly
When applying yellow void-filling mastic, stretch it until it turns translucent; this tension forces it into voids. Always shrink tubes from the bottom (screen end) upwards to chase the air out.
Hygiene & Environmental Errors
The most preventable failures are caused by simple contamination.
7. Surface Contamination
Cleaning insulation with dirty rags or wiping back-and-forth is a recipe for disaster. This smears conductive particles (carbon from the screen or dirt from the trench) onto the white insulation, creating a path for surface tracking. These conductive residues allow currents to creep along the interface, eventually causing a flashover.
How to do it correctly
Always wipe from the conductor (high voltage) down towards the semi-con screen (ground). Use only approved non-residue solvents (high-purity Isopropyl Alcohol) and change gloves immediately before handling the clean insulation.
8. Semiconductive Residue
Even after stripping, bonded screens often leave invisible microscopic carbon residue on the insulation surface. Failing to polish this away effectively reduces the insulation thickness and provides a tracking path, compromising the termination’s voltage rating.
How to do it correctly
Use non-conductive aluminum oxide abrasive cloth (typically 120 or 240 grit) to polish the insulation until it is pristine. Never use emery cloth, as it contains conductive metallic particles.
9. Inadequate Weather Sealing
MV cables “breathe” during thermal cycling; they expand with heat and create a vacuum when cooling. If the top (lug) or bottom (breakout) seals are weak, this vacuum sucks moisture into the conductor strands, leading to corrosion and steam pressure explosions.
How to do it correctly
Ensure mastic sealant is applied over the barrel of the lug and the insulation before shrinking the outer tube. For outdoor terminations, ensure rain skirts are oriented correctly to shed water.
10.Environmental Neglect
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.
How to do it correctly
Jointers must have the authority to halt work. Always use a habitat or tent. In high humidity, use a heat gun to gently warm the cable surface (keeping it above the dew point) before applying components.
In my experience, relying solely on visual inspection is insufficient because you cannot see inside the completed layers. Once the termination is complete, we must verify the work before energization using advanced diagnostics. For detailed procedures, please refer to: Medium Voltage Cable Testing and Certification.
Cable termination is a craft that tolerates zero error. The ten mistakes above appear again and again in the field, but the good news is that every one of them is preventable with the right tools, procedures and testing strategy. By mitigating these risks, you transform potential weak points into reliable assets that align with the projected Medium Voltage Cable Life Expectancy, ensuring your system delivers uninterrupted performance throughout its 30-year design life.
If you are sourcing materials for your next project, ensure the cable quality matches the installation effort. I encourage you to review our insights on [Global Leading Medium Voltage Cable Manufacturers] to select products that meet the rigorous specifications.
Frequently Asked Questions (FAQ)
Q: Is it better to use Heat Shrink or Cold Shrink to avoid these mistakes?
A: Cold Shrink is generally more “mistake-proof” regarding air voids and sealing because it exerts constant radial pressure and doesn’t rely on the jointer’s torching skill. However, Heat Shrink is excellent if installed by a skilled technician. The choice often comes down to environment and budget.
Q: Can I fix a “Knife Score” if I catch it before terminating?
A: If the score is very shallow (superficial scratch), it can be polished out using aluminum oxide cloth until the surface is smooth. However, if the cut has any depth that reduces the insulation wall thickness below tolerance, the cable must be cut back, and you must start over. Do not risk it.
Q: Why do terminations fail mostly in the first 3 years?
A: This is the “Infant Mortality” period. Failures here are almost exclusively due to the workmanship errors listed above (voids, dirt, knife cuts). Failures after 20 years are usually due to natural aging or water treeing.
