Cable laying: in the ground or in the air — which one to choose?

Introduction: Two Main Laying Methods

A cable line from source to consumer can run either underground or above ground. Each method has its own standards, materials, and constraints. Underground laying reliably protects the cable from wind, ice, and vandalism, but requires excavation and protection against groundwater. Overhead lines are cheaper to install and faster to repair, yet are susceptible to weather conditions and mechanical damage. The choice depends on distance, terrain type, voltage, and budget.

For underground laying, armoured cables are used in polyethylene pipes or simply in a trench with a sand bedding. Overhead, self-supporting insulated wire (SIP) is strung, or a regular cable is suspended on a steel tension wire. The PUE (Electrical Installation Rules) clearly regulate trench depth, distances to foundations and trees, sag, and clearance to ground. Detailed rules for power circuits are given in the article power cable laying. Specifics for fibre optics are covered in optical cable laying, and for low-current UTP in UTP cable laying. Overhead SIP installation is detailed in the guide SIP laying.

Comparison Table: Underground vs. Overhead Laying

Regulatory Requirements: PUE and GOST Standards

The 7th edition of PUE sets the minimum trench depth for cables up to 10 kV at 0.7 m, and at road crossings – 1 m. Distance from building foundations – at least 0.6 m, from trees – 1–2 m. A sand bedding of 10–15 cm is placed at the trench bottom; after laying, the cable is covered with sand, then soil. For overhead lines up to 1 kV, clearance above pedestrian areas is 4 m, above roadways – 6 m, to balconies and windows – at least 1 m. Sag is calculated based on temperature and wire weight. All these are strictly regulated, and deviations incur administrative liability.

Cable grades for underground must have armour or a protective sheath resistant to ground pressure. Overhead lines may only use wires designed for that purpose: SIP with weather‑resistant insulation, or a supporting tension wire with a suspended cable. Ordinary VVG without protection is prohibited underground, and overhead without a tension wire it will sag and break. Therefore, design begins with choosing the method, and then selecting the appropriate cable grade.

Underground Laying: Depth, Protection, Cable Grades

For underground 0.4 kV from substation to house, armoured cable with copper or aluminium conductors is used, e.g., VBbShv 3×10 or AVBbShv 4×16. The steel tape armour protects against soil compression and accidental shovel strikes. If budget is limited, non‑armoured VVGng can be laid in an HDPE pipe of 50–110 mm diameter, but such a line is less reliable with ground movements. A depth of 0.7 m is mandatory, but on arable land it is increased to 1 m, and under roads to 1.2 m. The cable is backfilled with sand, and a warning tape “Caution, cable” is placed above.

For underground laying, heating cables for pipe tracing, armoured optical fibre, and outdoor UTP in corrugated conduit are also used. Each type has its own protection requirements. Low‑current lines are often placed in DKS or HDPE corrugated tubes to facilitate replacement. Ground temperature at 0.7 m is relatively stable, which positively affects current ratings – the cable can be used at nominal load year‑round. Limitations include rocky soils, permafrost, and high groundwater levels, which require additional measures.

Overhead Laying: Tension Wire, SIP, Self‑supporting Structures

Overhead laying saves on excavation but imposes requirements on the cable’s load‑bearing capacity. Self‑supporting insulated wire (SIP) has an integrated neutral or steel core, allowing it to be tensioned between poles without a separate tension wire. For ordinary cables like VVG or outdoor UTP, a steel tension wire is needed, to which the cable is attached with hangers every 0.5–1 m. For outdoor fibre optics, self‑supporting optical cables with Kevlar yarns or steel elements are used. Flexible metal conduit with a tension wire is also used for street lighting.

The main enemy of overhead lines is ice accumulation. Ice and snow can multiply the weight several times, causing wire breaks or sagging beyond permissible limits. Therefore, in regions with severe icing, wires with enhanced mechanical strength are chosen, or underground entry is adopted. Wind load also loosens fastenings and can cause conductor clashing. For SIP, clashing is not dangerous due to insulation, but bare overhead conductors clashing is an accident. That is why modern 0.4 kV overhead lines are built with SIP, gradually phasing out bare aluminium.

Economics and Labour Costs

For short distances up to 50 metres, an overhead line is cheaper: no excavation, no expensive armoured cable. The cost of SIP 4×16 and a few anchor brackets is lower than a trench with VBbShv and HDPE pipe. However, for longer distances, intermediate poles are required, each adding cost. In dense urban environments, underground laying may be simpler because there is no need to coordinate overhead crossings. In swampy areas, underground lines require expensive drainage, making overhead more economical.

Operational costs for underground lines are lower: no wind‑related breaks, no tree trimming, and no lightning surge problems if armour is properly earthed. Overhead lines require regular inspections, tightening of sagging sections, and ice removal. However, repairing an underground cable is more expensive: fault location with a reflectometer, excavation, and splicing. An overhead section can be replaced in an hour, provided there are no landslides or fallen trees.

Reliability, Maintainability, and Environmental Impact

An underground cable line lasts longer if protected from stray currents and corrosion. Galvanised steel armour can degrade in acidic soils within 10–15 years, so cables with an additional polyethylene sheath over the armour are sometimes used. The temperature regime underground is stable, reducing thermal ageing of insulation. Overhead lines age faster: UV radiation degrades SIP polyethylene, but cross‑linked polyethylene is resistant, whereas ordinary PVC cracks on sunlight within 2–3 years. Therefore, only weather‑resistant grades are used overhead.

Maintainability of overhead lines is higher: damage is visible to the naked eye, no digging required. But the frequency of failures is also higher – branches, cranes, tall vehicles, vandalism. In forest areas, underground laying is definitely preferable: no need to cut clearings. In rocky terrain, overhead is simpler: no blasting needed. Thus, reliability is determined not by the method itself, but by how well it matches the conditions.

When to Choose Underground Laying

• Areas with dense urban development where overhead lines are prohibited by architectural regulations.
• Forest areas, parks, gardening communities – eliminates breakage by trees and simplifies maintenance.
• Long distances in confined spaces without the possibility of installing poles.
• Areas with high landscape aesthetic requirements – underground entry does not spoil the view.
• Facilities with increased lightning hazard, where armoured cable in the ground is safer than overhead.

When to Choose Overhead Laying

• Rural areas with low building density and large plots.
• Short service drops from pole to house where excavation is not cost‑effective.
• Rocky ground, permafrost, swamps – trenching is impossible or extremely expensive.
• Temporary power supply for construction sites – overhead line is quick to install and dismantle.
• Facilities where frequent reconfiguration or cable replacement is needed – overhead is more flexible.

Typical Mistakes in Choosing the Method

• Laying non‑armoured VVG underground without a pipe. Within a year, the sheath is damaged by stones and the cable is punctured.
• Suspending a regular cable on a tension wire without considering its weight. The wire sags, the cable stretches, possibly breaking.
• Ignoring trench depth: a cable at 0.3 m depth is easily damaged by garden tools.
• Overhead line below the required clearance: tall vehicles tear down the wires.
• No warning tape in the ground: excavators cut the cable during digging.
• Using SIP with aluminium conductors for underground laying – it is not designed for ground contact.

Frequently Asked Questions

Which cable should be used for underground laying?

Armoured VBbShv, AVBbShv, or PvBbShv, or non‑armoured VVGng in an HDPE pipe. Depth not less than 0.7 m, sand bedding is mandatory.

Can VVG be run overhead?

Only on a supporting tension wire with hangers. Without a tension wire, VVG has no self‑supporting capacity and will sag. For overhead, it is better to use SIP or a dedicated self‑supporting cable.

What is the trench depth for a 220 V cable?

According to PUE – at least 0.7 m for cables up to 10 kV. Under roads – 1 m, on arable land – 1 m.

Which method is more reliable during thunderstorms?

An underground cable with earthed armour is less susceptible to lightning surges than an overhead line.

Can optical fibre be laid underground without armour?

In an HDPE pipe it is acceptable, but it is better to use armoured optical cable. The pipe protects against mechanical damage, and the armour against rodents and pressure.

What is the maximum span of an overhead line?

For SIP‑4 4×16, the maximum span is about 40–50 metres. For SIP‑2 with a supporting core – up to 90 metres. Exact figures depend on icing and wind zone.

Conclusion

The method of cable laying – underground or overhead – is chosen not by habit, but by calculation. Underground offers concealment, durability, and weather protection, but is more expensive to install and harder to repair. Overhead is quicker and cheaper upfront, but is vulnerable to the elements and requires maintenance. In each case, one must analyse soil, climate, terrain, development, and budget. A properly chosen method in compliance with PUE and GOST ensures decades of trouble‑free operation of the cable line.