Engineer's Notes: installing HV cables and substations
Most HV failures start on site
Factory-tested cables and switchgear rarely fail on their own. The usual causes are installation damage: a cable pulled too hard or bent too tight, a nick in the insulation during jointing, a stone in the bedding, a termination left untorqued. Good site supervision with clear hold points prevents most of them.
Pulling cable the right way
Calculate the tension before the pull, from both ends, and set the winch cut-out below the limit. Use rollers every 2 to 3 m and corner rollers at bends. Watch the sidewall pressure at bends, not only the total tension, and keep the bending radius above the maker value at all times.
Jointing is a skilled trade
Only trained jointers certified on the kit system should make HV joints and terminations. Work in a clean, dry tent, follow the kit dimensions exactly, and record each joint position. Most joint failures come from semicon damage, contamination, or wrong cut-back dimensions.
Lifting and earthing
Every transformer lift needs a lift plan: total mass, sling angle, leg tension, crane capacity at the radius and an exclusion zone. Earth electrodes go in before the slab where possible, and are tested with the fall-of-potential method before the equipment is energised.
Frequently asked questions
How do I calculate cable pulling tension?
For a straight horizontal section, tension rises by the friction coefficient times the cable weight per metre times the length. At each bend, tension is multiplied by e to the power of friction coefficient times the bend angle in radians. Work through the route section by section, from both ends, and pull from the end that gives the lower final tension.
What is sidewall pressure?
Sidewall pressure is the force per metre that a cable presses against the inside of a bend while it is being pulled. It equals the tension leaving the bend divided by the bend radius. Too much sidewall pressure crushes the insulation, so makers give a limit, often a few kN per metre for XLPE cables.
What is the jam ratio for cables in a duct?
For three single-core cables pulled into one duct, jam ratio is 1.05 times the duct inside diameter divided by the cable diameter. Between about 2.8 and 3.2 the cables can wedge side by side at bends and jam, so choose a duct size that avoids that range.
How deep should an 11 kV cable be laid?
It depends on the network operator and location. Around 0.9 to 1.0 m to the top of the cable is common for 11 kV in footpaths and service corridors, deeper under roads. Cables sit in bedding sand with protective tiles and warning tape above. Always follow the utility trench section.
Why is the fall-of-potential test probe placed at 62 percent?
For a small electrode and a far enough current probe, the potential probe reads the true earth resistance when it sits at about 61.8 percent of the distance to the current probe. Readings at 52 and 72 percent should agree within a few percent; if not, move the current probe further away.
Topics: HV cable installation · pulling tension · sidewall pressure · jam ratio · bending radius · trench section · depth of laying · cable tiles and warning tape · lifting plan · sling angle · 11 kV heat-shrink joint · terminations · earth rods BS 7430 · fall of potential · installation inspection checklist
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