Overhead to Underground Cable Conversion – Planning the Survey

Updated: 23 October 2026

What is overhead to underground conversion?

Overhead to underground conversion is the replacement of a distribution network that hangs on poles with one buried in the ground, and it is among the largest changes a power distribution network can go through. In a typical Indian town that covers 11 kV feeders, LT distribution lines, street-light circuits and the service drops to individual buildings. Once the cables are in and commissioned, the old conductors and most poles are removed.

The equipment changes as well. A transformer that sat on a pole platform moves to a ground-mounted plinth or a compact substation. Overhead switches give way to ring main units, and LT distribution boxes become feeder pillars. Each house is then reconnected through an underground service cable. In effect, converting an overhead line to underground cable means rebuilding the local network rather than simply lowering wires.

Which parts of the network change in a conversion?

Scope varies by tender, but four layers of the network are usually involved, each with its own ground needs.

HT feeders

11 kV lines between the substation and transformers become XLPE cables in trenches or ducts, joined through ring main units that let supply be rerouted around a fault.

LT distributors

The 415 V lines along streets become LT cables running from each transformer to feeder pillars, which split supply towards groups of buildings.

Service connections

Each consumer's overhead drop becomes a small underground cable from a pillar or distribution box to the meter. In dense lanes this is the largest number of individual excavations.

Street lighting and others

Street-light circuits are often moved underground at the same time, and some projects add ducts for telecom operators that shared the poles.

Why are DISCOMs moving lines underground?

• Safety. Snapped conductors, low-hanging wires and contact with buildings cause accidents; buried cable removes most of that exposure.
• Storms and cyclones. Trees and wind bring down overhead lines. Coastal utilities in particular have put resilient underground networks into their plans.
• Fewer outages. Bird faults, kite strings and tree contact disappear, although a cable fault, when it happens, takes longer to find.
• Lower losses. Hooking onto bare overhead conductors becomes far harder, which supports the loss-reduction targets utilities work towards.
• Space and appearance. Pole removal frees footpaths and improves heritage, market and tourist areas.

Funding often comes through national distribution programmes. The current one is described in our guide to the RDSS scheme and underground cabling. State budgets and city projects pay for other conversions.

How does a UG cabling project run from start to finish?

Every DISCOM underground cabling package differs in detail, but the order of work is broadly similar.

1. Network study. The utility fixes the area, feeders and loads to be converted, and the voltage levels involved.
2. Survey. Existing underground services are located along proposed routes and at planned equipment sites.
3. Design. Cable routes, RMU and transformer plinths, feeder pillar positions and crossings are drawn on the surveyed base.
4. Permissions. Right of way and road-cutting consent are obtained from municipal bodies, development authorities and other road owners.
5. Civil and cable work. Trenches and bores are made, ducts and cables laid, and equipment foundations built.
6. Cutover. Consumers are moved from overhead to underground supply in planned shutdowns, area by area.
7. Dismantling and reinstatement. Old conductors and poles come down, and roads and footpaths are restored.

Which terms are used in conversion tenders?

• OH to UG: short form for overhead-to-underground work.
• Cutover: switching consumers from the old overhead supply to the new cable network.
• Shutdown: a planned outage needed for cutover, jointing or dismantling.
• Compact or package substation: a ground-mounted enclosure combining switchgear, transformer and LT panel.
• Service cable: the small LT cable from a feeder pillar or distribution box to a consumer's meter.
• Aerial bunched cable (ABC): insulated conductors twisted together on poles, an overhead alternative where full undergrounding is not taken up.
• Reinstatement: restoring road and footpath surfaces after excavation.

What makes conversion harder than a new cable route?

A new feeder can often pick a quiet verge. A conversion has no such freedom, because it must reach every pole position, every transformer and every house the overhead network served. That pushes trenches into old bazaars, narrow lanes and residential streets where water pipes, sewers, gas lines and telecom ducts were laid long before anyone planned for power cable.

Equipment needs ground space too. An RMU, compact substation or feeder pillar needs a plinth on land the utility can use, often a small corner of a footpath that is already full of services. Our guide to RMU and feeder pillar cables looks at those sites in detail. Lastly, cutover has to keep consumers on supply, so work is phased and sometimes squeezed into night hours.

When should the GPR survey happen on a conversion project?

Before design, wherever possible. If the cable route and plinth positions are drawn first and surveyed later, conflicts found on site force redesign while gangs wait. Ground penetrating radar (GPR) run along and across each lane picks up pipes, ducts, cables and other underground utilities as reflections, and an EM cable locator picks out live conductors. Results are marked on the ground and written into a report that designers use to place the new network.

On large packages, the survey is phased to stay ahead of the trench gangs, division by division. How deep the radar sees varies with soil type and wetness, which is why the report gives a limit for each stretch. Scope details are on the HT/LT cable route GPR survey page, the power utilities page covers what DISCOM teams expect, and our BOQ guide for DISCOM tenders explains how to size the survey quantity. We hold no DISCOM affiliation or utility approval of any kind; our crews work under the contract of whoever commissions the survey.

How are plinth sites and house connections handled in the survey?

Linear routes are only part of a conversion. Every proposed RMU, compact substation, transformer plinth and feeder pillar is a point site that needs its own close-spaced scan, because cables and pipes there run in several directions. Sites are best checked as candidates, so the designer can pick the clearest one.

Service connections are harder to survey individually, since there may be hundreds in a single ward. A practical approach is to scan the footpath strip along each street where service cables will branch off, then rely on marking and careful hand-digging for the short final runs to each building. Water service pipes and gas connections are the services most often met on those short runs, so crews should expect them at every property line.

How should records be kept after conversion?

Many towns gain their first true underground power map from a conversion project, and that record is worth protecting.

• Combine survey findings with as-laid cable routes, depths, joints and crossings.
• Tie every RMU, substation and feeder pillar to the feeder it belongs to.
• Record which consumers each pillar serves, so faults can be traced quickly.
• Keep the data in a format the utility's GIS can import.

Our CAD and GIS utility mapping service can prepare survey results in drawing or GIS form, quoted separately from the field survey.

Pricing a conversion package? Start with the survey quantity

Mezux Touch surveys with company-owned GPR systems and its own crews, mobilising from Faridabad. A conversion area is priced by the metre of lane and road scanned: ₹24 for standard survey and ₹32 where an L-section is called for. The minimum bill is for 4 km, projects beyond Delhi NCR carry a ₹1,00,000 floor, and large orders earn whole-order discounts. Mobilisation, night working, traffic management and drawings are quoted on top, and GST is extra. Send your conversion area map through the quote form below.