A direct-buried cable depends as much on its trench as on its own construction. Insufficient depth or missing mechanical protection produces a cable that commissions correctly and is then damaged months later by other excavation. IS 1255, the Indian code of practice for installing power cables up to 33 kV, defines the requirements; the essentials are set out below.
Depth of laying
Depth is measured from ground level to the top of the cable and increases with voltage:
| Cable voltage | Typical minimum depth |
|---|---|
| LV, up to 1.1 kV | about 0.75 m |
| 3.3 kV to 11 kV | about 0.90 m |
| 22 kV to 33 kV | about 1.05 m |
| Under roads and crossings | Deeper, and drawn through a pipe or duct |
Bedding and backfill
The trench bottom is levelled and a bed of clean screened sand or soft riddled soil, about 75 mm, is laid before the cable. The cable is placed slack, with no drum tension remaining, and covered with a further 75 mm of the same material. Sand cushions the cable against stones and conducts heat away more effectively than lumpy backfill, which affects the current rating.
Mechanical protection and marking
A course of second-class bricks, or RCC or stoneware protective tiles, is placed over the sand so that any later excavation meets the protection before the cable. The trench is then backfilled and consolidated in layers. Route marker tape or markers are run at surface level, with marker posts at bends, joints and fixed intervals on long runs, so that the presence of a cable is evident before any digging reaches it.
Adequate separation from other services should be maintained, with crossings taken at an angle rather than in parallel, and the route recorded. Direct burial calls for an armoured construction such as LV XLPE armoured power cable, with the armour earthed, as the steel wire or strip is the final barrier between excavation and a live conductor. For the failure modes that follow poor burial, see the field guide to buried cable faults. These measures add little cost and largely determine whether a cable reaches its design life.