Inner Covering in Electrical Cable Manufacturing
What is an Inner Covering?
An Inner Covering (also called an Inner Sheath, Bedding, or Filler Covering) is a protective layer applied over the laid-up insulated cores of a multi-core electrical cable before armouring or the outer sheath is applied.
Its main purpose is to provide a smooth, round, and protective surface that shields the insulated cores from mechanical damage and prepares the cable for the next manufacturing stage.
Position in Cable Construction
The typical construction of a power cable is:
- Conductor
- Conductor Screen (for MV/HV cables)
- Insulation
- Insulation Screen (for MV/HV cables)
- Core Identification
- Laid-up Cores
- Inner Covering (Inner Sheath/Bedding)
- Armour (if required)
- Outer Sheath
Functions of Inner Covering
1. Protects Insulated Cores
- Prevents damage during armouring.
- Reduces abrasion between cable layers.
2. Provides a Round Shape
- Fills gaps between insulated cores.
- Produces a uniform cable diameter.
3. Cushion for Armour
- Acts as a bedding layer under steel wire or steel tape armour.
- Prevents armour from damaging insulation.
4. Improves Mechanical Strength
- Enhances impact resistance.
- Helps maintain cable integrity during installation.
5. Moisture Barrier (when suitable materials are used)
- Some compounds provide additional protection against moisture ingress.
Materials Used
Common inner covering materials include:
| Material | Application |
|---|---|
| PVC | Most common for LV cables |
| PE | Moisture-resistant cables |
| XLPE-compatible bedding | XLPE insulated cables |
| LSZH | Buildings, tunnels, public areas |
| Rubber compounds | Flexible cables |
Types of Inner Covering
Extruded Inner Sheath
- Applied by an extruder.
- Smooth and uniform.
- Used in modern cable manufacturing.
Wrapped Inner Covering
- Made from tapes or wrapped materials.
- Used in special cable constructions.
Manufacturing Process
- Insulated cores are manufactured.
- Cores are laid up using a laying-up machine.
- Fillers are inserted (if required).
- Inner covering compound is extruded.
- Cooling in a water trough.
- Diameter and surface inspection.
- Cable proceeds to armouring.
- Outer sheath is applied.
Thickness
The inner covering thickness depends on:
- Cable diameter
- Voltage grade
- Number of cores
- Relevant standards
Typical range:
- 0.8 mm to 2.5 mm for many low-voltage power cables (actual values depend on the applicable standard and cable design).
Quality Tests
Manufacturers typically perform:
- Visual inspection
- Thickness measurement
- Diameter measurement
- Spark test (where applicable)
- Tensile strength test
- Elongation test
- Adhesion check
- Surface smoothness inspection
Advantages
- Protects insulation
- Improves cable roundness
- Supports armouring
- Increases mechanical strength
- Enhances cable durability
- Simplifies installation
Common Defects
| Defect | Cause |
|---|---|
| Uneven thickness | Incorrect extrusion settings |
| Rough surface | Poor temperature control |
| Air bubbles | Moisture in compound |
| Eccentric covering | Improper die centering |
| Shrinkage | Incorrect cooling |
| Cracks | Poor compound quality or excessive cooling stress |
Typical Applications
- Low Voltage (LV) Power Cables
- Medium Voltage (MV) Power Cables
- Armoured Power Cables
- Control Cables
- Instrumentation Cables
- Industrial Power Cables
- Inner Covering in Electrical Cable
- Inner Sheath in Cable Manufacturing
- Cable Bedding Layer
- Power Cable Construction
- PVC Inner Sheath
- Armoured Cable Structure
- Multi-Core Cable Manufacturing
- Cable Extrusion Process
- Electrical Cable Layers
- Cable Manufacturing Guide
Summary
The inner covering is an essential protective layer placed over the laid-up insulated cores of a cable. It provides mechanical protection, creates a smooth and circular profile, supports the armour, and improves the cable's overall durability and reliability. In modern cable manufacturing, the inner covering is most commonly applied by extrusion using materials such as PVC, PE, or LSZH compounds, depending on the cable's intended application and performance requirements.
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