MV Reeling Cable (3.6/6kV–6/10kV) — Medium Voltage Drum-Wound Power Cable with Semiconductive Screens for Large Mining, Crane, and Heavy Industrial Reeling

Medium Voltage Cable Construction — Three Critical Layers

At voltages above 1 kV, the electric field stress in the cable insulation becomes the dominant design consideration. A 0.6/1 kV cable has a simple construction: conductor + insulation + jacket. This works because the electric field at 600 V is low enough that minor imperfections in the conductor surface or insulation do not cause problems.

At 6 kV, the electric field is 10× stronger. Microscopic air gaps between the conductor and insulation become sites of partial discharge — tiny electrical arcs that progressively erode the insulation, leading to eventual failure. Three additional layers prevent this:

Layer 1: Conductor Screen (Semiconductive)

An extruded semiconductive compound applied directly over the copper conductor. This layer:

• Fills the interstitial spaces between individual conductor strands — eliminating air gaps
• Creates a smooth, uniform electrical surface — no protruding strands that concentrate the electric field
• Is bonded to the conductor — cannot separate during bending

Layer 2: EPR Insulation (Dielectric)

The primary electrical insulation layer. Thickness per IEC 60502-2:

• 3.6/6 kV: 2.5 mm
• 6/10 kV: 3.4 mm
• 8.7/15 kV: 4.5 mm

EPR is specified (rather than XLPE) for reeling applications because its rubber-like flexibility withstands repeated bending better than the stiffer, semi-crystalline XLPE.

Layer 3: Insulation Screen (Semiconductive)

An extruded semiconductive compound over the EPR insulation. This layer:

• Provides a smooth outer electrical boundary for the insulation — defines the ground reference surface
• Is strippable — can be cleanly removed during termination without damaging the EPR insulation
• Prevents partial discharge between the insulation outer surface and the metallic screen

These three layers are applied in a single triple-extrusion process — all three extruded simultaneously and cross-linked together. This ensures perfect bonding between layers with no contamination or air at the interfaces — the most critical quality requirement for MV cable.

Copper Tape Metallic Screen — Ground Fault and Electric Field Control

Each insulated core has an individual metallic screen — typically helically applied copper tape. The screen serves two functions:

1. Ground fault current path: In the event of an insulation fault, the screen carries the fault current to ground, enabling the protection system to detect and clear the fault. The screen cross-section (≥16 mm² copper) is sized to carry the system's prospective ground fault current without overheating
2. Electric field containment: The screen creates a grounded equipotential surface around the insulation. All electric field lines terminate at the screen — no external electric field exists beyond it. This allows the three screened cores to be in direct contact during stranding without risk of phase-to-phase discharge

The copper tape is applied with an overlap (typically 15–20%) to ensure complete coverage during bending. For reeling applications, a tinned copper braid screen may be specified for improved flexibility.

Triple Extrusion — The Manufacturing Difference

The conductor screen, EPR insulation, and insulation screen are extruded in one pass through a triple-head extruder. All three layers exit the die simultaneously and enter a continuous vulcanization (CV) tube where they are cross-linked together.

Why this matters:

• No contamination between layers: Because all three layers are applied in one continuous process, there is no opportunity for dust, moisture, or handling contamination to enter at the layer interfaces — the most common cause of MV cable failure
• Molecular-level bonding: The EPR cross-links to the semiconductive screens, creating a seamless transition with no mechanical interface that could separate during bending
• Uniform thickness: The triple-head die precisely controls all three layer thicknesses — critical because an undersized insulation section at any point creates a weak spot

This is the same manufacturing technology used for utility-grade MV underground power cable — adapted for flexible reeling applications through EPR formulation and stranding design.

Why MV Reeling Requires Conservative D/d Ratios

The minimum bend radius for MV cable (15:1 D/d) is larger than for LV reeling cable (10:1 D/d). Two reasons:

1. Thicker insulation: MV cable has 2.5–4.5 mm of EPR insulation plus two semiconductive screens. The total insulation system is 3–6 times thicker than LV cable (0.6–1.2 mm PVC/EPR). Thicker layers experience higher bending strain at the inner and outer radii
2. Triple-extruded layer integrity: The conductor screen-EPR-insulation screen bond must survive millions of bending cycles without delamination. A larger bend radius reduces the shear stress at these bonded interfaces

D/d practical rule: 20:1 for >100 000 cycle life; 25:1 for maximum service life. Do not operate MV reeling cable below 15:1 D/d — partial discharge at the screen interfaces is likely below this threshold.

Why Choose Yichi Cable MV Reeling?

• Triple-extrusion quality: Our MV cables are manufactured on a triple-head CV extrusion line with in-line diameter and eccentricity monitoring — every millimeter of cable is verified for uniform insulation thickness
• 100% PD tested: Every meter of MV reeling cable is partial discharge tested at 2 U₀ (<5 pC acceptance) — not batch-sampled. A cable with PD at 2 U₀ will fail in service
• Tan delta tested: Dielectric loss measurement verifies that the EPR insulation compound is properly cross-linked and free from contaminants — tested on every production length
• Strippable insulation screen: Our screen compound is formulated for clean, easy stripping during termination — reducing installation time and the risk of insulation damage at the most critical point
• Application review: Send us your system voltage, prospective fault current, and drum dimensions — we verify conductor size, screen cross-section, and D/d ratio compliance