Technical Insights from Shenghua Cable

Date: March 19, 2024

Cable insulation breakdown is one of the most serious risks in power transmission systems. Once insulation fails, it can lead to short circuits, equipment damage, large-scale outages, and even fire accidents.

Based on decades of operational and manufacturing experience, Shenghua Cable (Shanghai Shenghua Cable Technology Group Co., Ltd.) has continuously studied insulation failure mechanisms in power cables. By applying solid dielectric theory and combining real-world engineering cases, the company has summarized key causes and preventive strategies to improve cable safety and extend service life.

For technical specifications and certified cable solutions, visit:
https://www.shanpowercable.com/

I. Breakdown of Solid Insulation

In power cables, insulation materials act as solid dielectrics. When the applied voltage exceeds the critical electric field strength that the dielectric can withstand, a sudden surge of current occurs. The insulation loses its ability to block current — this is known as dielectric breakdown.

There are three main forms of solid insulation breakdown:

1. Electrical Breakdown

When voltage is high enough, free electrons inside the dielectric accelerate under a strong electric field. These electrons collide with atoms, creating ionization chains and more free charge carriers.

This avalanche effect eventually destroys the insulation structure.

Electrical breakdown typically occurs under:

• High overvoltage
• Strong localized electric fields
• Poor electric field distribution

2. Thermal Breakdown

When cables operate under voltage for long periods, dielectric losses generate heat inside the insulation.

If heat generation exceeds heat dissipation:

• Temperature continues to rise
• Insulation properties degrade
• Mechanical strength weakens
• Breakdown eventually occurs

Overload operation and poor heat dissipation are common contributors.

3. Electrochemical Breakdown

Over time, insulation materials are subjected to:

• Thermal stress
• Mechanical vibration
• Chemical corrosion
• Electrical stress

Aging causes gradual performance deterioration. If bubbles or air gaps exist, partial discharge or corona discharge may develop, leading to local carbonization and eventual breakdown.

II. Main Factors Affecting Insulation Breakdown

Through years of field experience, Shenghua Cable has identified several major influencing factors:

1. Insulation Thickness and Electric Field Strength

Electric field strength (E) is inversely proportional to insulation thickness (d):

E = V / d

Where:

• V = applied voltage
• d = insulation thickness

Thicker insulation reduces electric field intensity and increases breakdown voltage.

This is why Shenghua Cable carefully controls insulation thickness and uniformity during manufacturing, especially for medium- and high-voltage cables.

2. Impurities and Air Bubbles

Impurities and micro-bubbles inside insulation are extremely dangerous.

Because:

• Bubbles reduce effective dielectric thickness
• Electric field intensity concentrates in low dielectric constant areas
• Partial discharge forms inside voids

These weak points become starting points for breakdown.

Strict material drying and vacuum degassing processes are essential during production.

3. Poor Contact Between Conductor and Insulation

Gaps between the conductor and insulation layer create ionization zones similar to bubbles, lowering dielectric strength.

Precision extrusion and bonding control are critical for long-term performance.

4. Moisture Intrusion

Moisture dramatically reduces insulation strength by:

• Increasing conductivity
• Raising dielectric loss
• Accelerating thermal breakdown

Once insulation absorbs water, breakdown voltage drops sharply.

5. Uneven Electric Field Distribution

Insulation withstands higher voltage in a uniform electric field. In non-uniform fields, breakdown voltage drops significantly.

For cables rated 35kV and above, stress cones and grounding shield rings are used at terminations to optimize electric field distribution.

Shenghua Cable designs its high-voltage systems with controlled stress distribution to improve compressive strength and reliability.

III. Causes of Cable Insulation Breakdown in Practice

Based on operational cases, major failure causes include:

1. Mechanical Damage

Mechanical damage remains a leading cause of insulation failure.

Common examples:

• Heavy objects crushing cables
• Excavation accidents
• Excessive bending during installation
• Excessive pulling tension
• Soil settlement causing stress

Proper installation and mechanical protection are essential.

2. Terminal and Joint Failures

Cable terminations and intermediate joints are weak points.

Typical causes:

• Poor workmanship
• Improper sealing
• Low-quality materials
• Air gaps and moisture during installation

High-voltage terminations must be carefully constructed to avoid electric field concentration.

3. Insulation Dampness

Moisture intrusion occurs due to:

• Poor sealing of joints
• Damaged protective layers
• Corrosion of lead sheaths
• Long-term environmental exposure

Once moisture enters, dielectric strength declines rapidly.

4. Overvoltage

Overvoltage may result from:

• Lightning strikes (atmospheric overvoltage)
• Switching operations
• System faults

Power system overvoltage can cause simultaneous failure of multiple cables.

5. Insulation Aging

Long-term exposure to:

• Heat
• Overload
• Environmental stress

Leads to material brittleness and cracking.

Poor heat dissipation is often the root cause of thermal aging.

IV. Preventive Measures

To reduce insulation breakdown risks, Shenghua Cable recommends the following:

1. Prevent Mechanical Damage

• Protect overhead cables
• Install physical barriers where necessary
• Mark underground cable routes clearly
• Control excavation procedures

2. Improve Joint and Termination Quality

During installation:

• Ensure tight wrapping of insulation
• Avoid air gaps
• Dry epoxy resin and filler materials before use
• Apply stress cones and shielding rings for ≥35kV cables

Proper field control significantly improves dielectric strength.

3. Strengthen Outer Sheath Protection

To prevent corrosion and moisture intrusion:

• Inspect outer protective layers regularly
• Reapply protective coatings such as asphalt paint every 2–3 years
• Repair damaged sheathing immediately

4. Control Load and Temperature

• Avoid overload operation
• Ensure proper ventilation
• Monitor operating temperature
• Use cables rated appropriately for system capacity

Shenghua Cable: Committed to Long-Term Reliability

Cable insulation breakdown is not a single-factor issue — it is usually the result of design, material, installation, and operational influences combined.

With decades of manufacturing expertise, Shenghua Cable focuses on:

• Strict material quality control
• Advanced extrusion and insulation processes
• Optimized electric field design
• High-voltage stress control technology
• Comprehensive quality inspection

By combining solid engineering theory with real-world application experience, Shenghua Cable delivers reliable power cable solutions for energy, infrastructure, and industrial projects worldwide.

For product specifications and technical consultation, visit:
https://www.shanpowercable.com/