33kV Underground Cable Laying – Route Planning and Survey

Updated: 23 October 2026

What is a 33kV cable link and why is it laid underground?

A 33kV cable circuit carries power at sub-transmission voltage, usually from a 132/33 kV or 220/33 kV grid substation to a 33/11 kV substation that then feeds the local 11 kV network. In open country these links are often overhead lines. In cities, industrial estates and airports, there is rarely room for towers or poles, so the link goes underground.

Under Indian electricity regulations, 33 kV is the top of the high-voltage band; anything above it counts as extra-high voltage. That puts a 33kV underground cable between everyday 11 kV work and larger EHV projects. The cables are heavier, the circuits more critical and the routes longer, so planning carries more weight than on a feeder spur.

Which 33kv xlpe cable types are used?

Most new links use cross-linked polyethylene insulation with aluminium conductors. Two arrangements are common.

Three-core cable

One cable carries all three phases, with steel armour over the cores. Sizes such as 300 and 400 sq mm are frequent for moderate loads. Handling is simpler because there is one cable per circuit.

Single-core cables

Three separate cables, often laid touching in a triangle called trefoil, carry heavier loads with larger conductors. Armour, where used, is non-magnetic. Because each cable has its own metallic screen, the engineer must design how screens are bonded and earthed, so circulating currents do not overheat the cable.

Insulation grade follows the network's earthing: 19/33 kV for earthed systems and 33/33 kV for unearthed ones. The DISCOM or transmission utility specification fixes size, grade and construction, and it governs over any general description here.

How is a 33 kV underground cable route planned?

Route planning starts at both substations and works towards the middle. The designer looks for the shortest practical line that can be reached for repairs and that avoids the most congested ground.

• Corridor choice. Service roads, verges and dedicated utility corridors are preferred over main carriageways.
• Crossings. Highways, flyover approaches, rail lines, canals and large drains are listed early, since most will be bored.
• Joint bays. Positions follow drum lengths and must sit on ground with room for a wide, deep pit.
• Right of way. Permissions are needed from road owners, development authorities and, at crossings, railways or irrigation departments.
• Future circuits. A second link is often added later, so spare ducts at crossings save another bore.

The same planning logic carries over to an EHV cable route at 66 kV or 132 kV, though those cables need wider trenches and more complex bonding.

Trench or HDD: which method suits each part of a 33 kV route?

Most links combine both methods, and the split is decided stretch by stretch.

Open trench

Suits verges, service roads and open land where cutting is allowed and traffic can be managed. It is usually cheaper per metre, lets the crew see what it meets, and gives cables good heat dissipation when bedded properly.

Horizontal directional drilling

Suits highways, rail lines, canals, large drains and roads where cutting is refused. The cable goes in a duct pulled through the bore. Drilling avoids surface disruption, but the bore cannot be inspected while it is made, so knowing what lies beneath is essential.

Ducted open cut

A middle option: ducts laid in a short trench during a planned road closure, with spares for future circuits.

How is 33 kV cable laying carried out on site?

1. Survey and setting out. Existing services are located and marked; trench lines, bays and bore entry and exit points are pegged.
2. Trenching. Trenches are excavated to a typical cover of about 1.05 m, with stone-free bedding and protective covers. Widths increase for trefoil groups and parallel circuits.
3. Drilling at crossings. HDD places ducts under roads, canals and rail lines; the bore profile must clear every service found by the survey.
4. Pulling. Heavy drums sit on a drum trailer or stand while a winch draws the cable over rollers, with tension monitored and bending radius respected.
5. Jointing. Straight-through joints are made in prepared bays by trained jointers; link boxes are fitted where the bonding scheme needs them.
6. Testing and commissioning. Sheath and insulation tests set out in the specification are completed before the link is energised.

Good 33 kV cable laying is slow and deliberate. Rushing a pull or a joint saves hours and can cost years of cable life.

Which terms appear in 33 kV route documents?

• Trefoil: three single-core cables laid touching in a triangular group.
• Flat formation: single-core cables laid side by side with set spacing.
• Sheath bonding: the way metallic screens are connected and earthed to limit circulating currents.
• Link box: a buried or surface box giving access to screen bonding connections.
• Joint bay: the pit where two drum lengths are jointed.
• Entry and exit pits: the launch and receiving points of an HDD bore.
• Pulling tension: the force on the cable during installation, kept within the maker's limit.

When should the GPR route survey be done on a 33 kV link?

Ideally at design stage, before joint bays and bore profiles are fixed, and again as a refresh if months pass before excavation. A 33 kV route usually runs through the same ground as water mains, trunk sewers, gas lines, telecom ducts and older HT cables. Ground penetrating radar (GPR) scanning across the alignment locates these, and an EM cable locator confirms live conductors. The results show where the trench can run, where bays fit and how deep bores must go.

Crossings deserve special care. A bore designed without knowing what lies beneath a highway can clip a pipe far below the surface. Our HDD route survey scans past both ends of each planned bore, and an L-section survey adds a depth profile of detected services along the route, which helps set bore levels. Radar reach depends on soil and moisture, so the report states it stretch by stretch.

The HT/LT cable route GPR survey describes the full scope for trenched sections, and our power utilities page explains how DISCOM teams use survey records.

What records should a 33 kV link leave behind?

• As-laid alignment with offsets from fixed features, and measured cover at intervals.
• Joint bay and link box positions, with joint numbers and dates.
• Bore profiles at every crossing, including duct sizes and spares.
• The sheath bonding diagram as installed.
• The pre-construction survey report showing services found near the route.

Sub-transmission links are expensive to repair, and faults can take whole substations off supply. A complete record shortens fault location and protects the link from later digging. For the 11 kV feeders that leave the 33/11 kV substation, see our guide to 11 kV cable size and laying.

Planning a 33 kV link? Start with the route survey

Our route and crossing surveys are carried out by Mezux Touch staff from Faridabad, on radar units and locators the company owns outright. Standard scanning is billed at ₹24 a metre and L-section stretches at ₹32 a metre, with every order charged for at least 4 km and a ₹1,00,000 project minimum beyond Delhi NCR. GST is extra. Charges for getting crews to site, working at night, managing traffic and preparing drawings are added separately. Mezux is not affiliated with any DISCOM or transmission utility; our client is whichever contractor or owner orders the survey. Send the substation-to-substation route through the quote form below.