24kV/35kV medium voltage underground cables: Construction, classification, and installation guide

A medium voltage underground cable is built, from the inside out, of six main layers:

2.2 Conductor shield

The conductor shield is a thin extruded semi-conductive layer (about 0.25–0.5mm) compatible with the primary insulation material, applied directly over the conductor. Its role is to eliminate the air gap between the conductor and the insulation — the location most prone to ionization. Together with the insulation shield on the outside, these two layers act like the plates of a capacitor, helping the electric field distribute evenly and symmetrically around the insulation rather than concentrating locally.

2.4 Insulation shield

Beyond creating a grounded surface around the insulation to distribute the electric field symmetrically, this shield layer serves three lesser-known roles: reducing radio-frequency interference generated by the cable, enabling insulation testing of individual conductors, and, when properly grounded, reducing the risk of electric shock to personnel working near the cable.

Structurally, the shield consists of two parts: the extruded semi-conductive insulation shield (common in modern cables) and the overlying metallic shield. The metallic shield is typically helically applied overlapping copper/brass tape, or concentric copper wires around the insulation; another variant ("Unishield") embeds flat or corrugated wires directly in the extruded semi-conductive jacket instead of using a separate tape wrap.

2.5 Metallic shield

For the metallic shield, the distribution industry has long favored wire-type designs — flat strap and round wire — with various neutral conductor cross-sections (sized as a ratio to the phase conductor) to balance neutral/fault current carrying capacity against the desire to reduce cost and cable weight.

A helically applied copper tape design, with a nominal thickness of about 0.125mm, is also still used. Under normal operating conditions, both wire and tape designs perform the two core functions — discharging accumulated charge from the semi-conductive layer and carrying sufficient fault current. The difference between the two lies mainly in cost, weight, and whether an outer sheath is still needed, not in operational safety.

One additional point: two cables with different Uo/U ratings can still be used interchangeably if they share the same Um value — because Um is the parameter that determines whether a cable is suitable for a given grid, while Uo/U simply reflects the specific connection configuration. For example, both 12/20(24)kV cable and 12.7/22(24)kV cable can be used on a system not exceeding 24kV, since both share Um = 24kV — which is also why some catalogs list both ratings side by side for the same grid voltage class.

For example, HELU’s medium-voltage cable catalog currently uses this standard notation: 6/10 kV, 12/20 kV, 18/30 kV—where 12/20 kV corresponds to “24 kV” and 18/30 kV corresponds to “35 kV” in common usage.

Despite having multiple protective layers — especially the durable XLPE insulation — this insulation can still degrade through two common mechanisms seen in underground cables:

• Water treeing: forms when moisture penetrates the insulation through microscopic defects, creating a branching, tree-like network of water-filled channels (each droplet only about 0.1–1 micrometer) under the effect of a sustained electric field. Cables rated 6kV and above are considered especially susceptible to this phenomenon. This is why the outer jacket — and especially cable joints and terminations — must be completely watertight, since most underground cable failures originate at connection points rather than in the cable body itself.
• Electrical treeing: develops from partial discharge at defect sites or impurities within the insulation, gradually forming branching conductive channels that can lead to complete breakdown if not detected early.

These degradation mechanisms are not solely the result of natural aging of the insulation material — in large part, they originate from the installation process itself.

5.3 Burial depth and installation methods

Direct burial cable is considered to have adequate mechanical protection without any additional protective layer, at a minimum depth of 0.6m in ordinary ground and 0.8m under roadways, per manufacturer recommendation. If the actual depth is shallower than this, corresponding additional protection must be added.

Besides direct burial, cable can also be installed by cable ploughing or pulled through a pre-formed duct created by horizontal directional drilling (HDD). When installed in a duct or formed conduit, the duct's internal diameter must be at least 1.5 times the cable's outer diameter, and measures should be taken to prevent silt buildup inside the duct.

5.4 Fixation of the cable

The maximum horizontal spacing between fixing clamps is 20 times the cable diameter (up to 80cm maximum); the maximum vertical spacing is 150cm. Single-core cables can be installed individually or bundled together — if bundled, they must be treated as multi-core cable for fixing purposes.

When installing single-core cable, plastic clamps or non-magnetic metal clamps should be used. Steel clamps are only permitted if the magnetic loop formed by the clamp does not enclose each individual cable core separately, to avoid localized heating from induction effects. Clamps must be tightened securely while still allowing for thermal expansion of the cable without causing damage.

5.5 Tensile force

The maximum tensile force is determined based on three cases:

• Conductor with pulling eye: maximum tension = total conductor cross-section × allowable tension per unit cross-section (copper: 50 N/mm², aluminum: 30 N/mm²). Note that only the conductor cross-section is counted — not the cross-section of the metallic shield, concentric neutral wires, or armor layer.
• Cable Grip with effect on the conductor: applies when pulling cable with thermoplastic insulation/jacket and no outer metallic covering, using the same formula and values as above.
• Cable Grip with effect on the sheath: maximum tension P = 3D² (N), where D is the cable's outer diameter in millimeters.

6.4 TR-XLPE insulated medium-voltage cable with LLDPE jacket

6.6 Medium voltage cable terminations and joints

A termination and a joint are the points connecting cable to switchgear or connecting two cable sections together — also the most sensitive points along the entire cable route.

Joints are common throughout medium voltage distribution networks because, in practice, a single cable route's length usually exceeds what a standard cable drum can hold, requiring multiple cable sections to be joined to complete the route. The joint's main role is to ensure electrical and mechanical continuity at the connection point, while also controlling the electric field at the joint (stress grading) just as rigorously as at a termination. There are two types of joints for medium voltage cable: heat-shrink and cold-shrink.

HELU's medium voltage cable accessories range includes:

• Plug-in terminations (T-connectors): used to terminate a medium voltage cable end safely and in a controlled manner, connecting the cable to medium voltage switchgear, transformers, busbars, or outdoor connection points. See HELU's product (Part no. 905287).
• Heat-shrink joints: use heat to shrink the tubing tightly around the cable, creating a uniform, moisture-tight insulation layer and solid mechanical protection.
• Cold-shrink joints: after positioning on the cable, the installer simply removes the core support — the elastomeric material shrinks and grips the cable tightly on its own, with no heat required.