From design to acceptance — the stages of building an MV cable line

Stage 1: inputs and the base map for design

The starting point is an investment need: rebuilding a fault-prone line, undergrounding an overhead section, supplying a new area, connecting a customer or a renewable generator, or removing a conflict with a planned road. For connections, the basis is the connection conditions issued by the operator; for the DSO's own investments, its network development plans and internal assumptions.

The designer begins by ordering a base map for design purposes — an up-to-date topographic map showing existing utilities, prepared by a licensed surveyor. From it, the designer analyses route options, looks for conflicts with other networks, identifies obstacles (roads, watercourses, railways, forests) and the owners of the plots the line will cross.

Decisions taken at this stage have a big effect on construction cost and time. A route along public roads usually makes consents easier to obtain but requires agreements with road authorities and often more drilled crossings. A route across private land may be shorter but needs consent from many owners. A good designer looks for a compromise, also considering future access to the line for operation and repairs.

Stage 2: design and approvals

The design of an MV cable line specifies the route, cable type and cross-section, installation method, the locations of junction boxes, joints and substations, drilled crossings and connection points to the existing network. The cable and accessory types must comply with the technical standards of the operator who will take the line over.

During design, a number of agreements and consents are obtained, in particular:

• landowners' consent for the line to cross their land — often in the form of transmission easement agreements;
• agreements with road authorities on the cable's position in the road corridor and how carriageways will be crossed;
• agreements with other utility owners — water, sewerage, gas, telecoms;
• coordination of the network's location at a coordination meeting held by the district authority (still colloquially called “ZUD”) — to the extent and in the manner required by current regulations;
• where necessary, agreements with the authorities responsible for watercourses, railways and forests, and with nature conservation or heritage authorities.

Depending on the nature of the project, a cable line requires either a building permit or a notification of works — the designer determines which on the basis of current building regulations.

Stage 3: site preparation

Once the documentation and decisions are in place, the contractor prepares the site. This stage is often underestimated, yet the smooth running of everything that follows depends on it. It usually covers:

• reviewing the design and bill of quantities and checking the route on the ground;
• a setting-out survey of the route, drilled crossings and equipment locations;
• obtaining road occupation permits and approval of a traffic management plan for the works;
• notifying the start of works and informing landowners and utility owners;
• ordering and delivering cables, accessories, ducts, junction boxes and substations — taking drum lengths into account to minimise the number of joints;
• organising site facilities, transport and plant.

At this stage it is also worth agreeing communication rules with the investor and the operator: who approves route changes arising from site conditions, how outage requests are submitted, how progress is reported and what documents will be required at interim acceptances. Clear rules from the outset help avoid stoppages during construction.

Stage 4: earthworks and drilling

Construction proper begins with earthworks. Wherever possible, the cable is laid in an open trench, dug with an excavator or — in confined spaces and near existing utilities — with a mini excavator and by hand. Trial holes are dug at crossings with other networks to confirm their position.

Under roads, railways, watercourses, driveways and in woodland, directional drilling or pipe jacking is carried out and protective ducts are pulled in. Their length and position follow from the design but are often refined in agreement with landowners and road authorities. How directional drilling works is described in detail in How directional drilling works.

The trench bottom is levelled and a bedding layer of sand or suitable soil is placed to protect the cable from stones.

Trenches are dug in line with safety rules: suitable support of the sides or battered slopes, signage and barriers, and — in the road corridor — in accordance with the approved traffic management plan. Excavation near existing utilities is done carefully, often by hand, because damaging a live cable or a gas main poses a serious hazard.

Stage 5: laying the cables

MV cables arrive on drums, usually by lorries with an HDS crane that allows them to be unloaded and set on drum stands. The cable can be pulled out by hand, over guide rollers, or pulled mechanically with a winch — especially in protective ducts and on longer sections. During pulling, the tension and bending radius are monitored so as not to exceed the manufacturer's limits.

The three single-core cables of an MV line are laid as designed — most often bundled in trefoil. Once laid, the cables are covered with a layer of sand, a warning tape is placed and the trench is backfilled and compacted in layers. Before backfilling, a surveyor records the cable position for the as-built survey.

Finally, surfaces are reinstated — lawns, footways, driveways, verges — to the condition agreed with owners and authorities.

Stage 6: accessories, junction boxes and substations

Alongside or after cable laying, the accessories and equipment are installed:

• cable joints where cable lengths are connected, and transition joints where the line connects to existing older-type cables;
• MV cable junction boxes (ZKSN) at branches and sectionalising points;
• MV/LV transformer substations, if they are in scope;
• cable terminations — connector terminations in junction boxes and substations, indoor terminations in switching stations, outdoor terminations on poles where the cable line meets an overhead line (together with surge arresters).

Installing accessories requires cleanliness, precision and strict adherence to the manufacturer's instructions. It is done by suitably qualified jointers, and joint positions are documented and surveyed.

Stage 7: acceptance testing and switching

Before the line is energised, acceptance tests are carried out to the extent required by the design, the operator and applicable standards. They typically include:

Test	Purpose
Continuity of conductors and screens, phase identification	confirming correct connections along the entire line
Insulation resistance measurement	detecting moisture or insulation damage
Insulation voltage test	testing cable and accessory insulation at an elevated test voltage
Sheath voltage test	detecting damage to the outer sheath caused during laying and backfilling
Earthing measurements	confirming the required earth resistance at junction boxes, substations and poles

The results are recorded in test reports. After successful tests, the operator — following its procedures and at agreed outage times — switches the supply to the new line. If the project involves undergrounding, this is when the old overhead line is dismantled: conductors, insulators, poles and pole-mounted substations.

Stage 8: as-built survey, documentation and acceptance

The last stage formally closes the project. The contractor hands the investor the as-built documentation, which usually includes:

• the design marked up with changes made during construction;
• the as-built survey — a survey report lodged with the national geodetic records, showing the position of cables, joints, junction boxes and drilled crossings;
• test reports;
• declarations, certificates and data sheets for the products used;
• documents confirming reinstatement of the land and settlement of the road occupation;
• the site manager's statements and the site log, where one was kept.

Based on the documentation and a site inspection, final acceptance takes place, after which the line is handed over for operation. The warranty and statutory liability period set out in the contract also starts from this point.

For lines built by an investor and transferred to the operator, acceptance also has a technical dimension on the DSO's side: the operator's staff check that the work complies with the design and standards, and review the completeness of the documentation and the test results. Gaps in documentation are one of the more common reasons for drawn-out acceptance, so it pays to compile it as you go.

How long it takes and what affects the schedule

The duration of each stage varies widely. Design and approvals usually take longest — especially when the route crosses many private plots. Construction itself can be relatively short: for example, we built the 2,350 m MV 15 kV cable line in Bolewice between 25 November and 12 December 2024 (see the project), and the 15 kV line undergrounding in Toruń, with a 5,725 m route and about 600 m of drilling, between 30 September and 25 November 2024 (see the project).

The construction schedule is affected by, among other things: the number of drilled crossings and ground conditions, the weather, outage dates agreed with the operator, material availability, restrictions in the road corridor and the extent of old-network removal. A well-prepared site — with all approvals in place, the route set out and materials ordered — helps limit these risks.

Earthworks and cable laying can be carried out for most of the year, but in winter frozen ground and the cable's minimum installation temperature have to be taken into account. In the growing season, crops in the fields the route crosses can be an obstacle. An example of a project whose schedule ran through the winter months is our network rebuild for ENEA Operator, delivered between October 2025 and May 2026.

How we work at each of these stages is described on our How we work page, and our scope of services on the MV and LV cable lines page.