What is a cable duct?
The buried pipe that carries AGL cables under pavements and into pits — laid from the design, with gentle bends, smooth sealed ends, a draw rope and spare ways, so cable can be pulled in, and replaced, without breaking the surface again.
A cable duct is a buried pipe or conduit that carries AGL cables, usually where they cross pavements or run into pits and bases. It is laid from the design first, with gentle bends and smooth, sealed ends, and the cable is pulled through afterwards, so it is protected and can be replaced without digging up the surface.
A duct is only cheap to get right while the trench is open. Once it is buried under a pavement, a wrong route, a sharp bend or a collapsed section means breaking the surface again or a cable that cannot be pulled. Leave a draw rope in every duct, cap the spares, place a warning tape above and record the as-built route. Locate existing services before digging, treat any cable as live until the circuit has been isolated, locked out and proved dead, keep debris off the airfield surface and work to the airport’s permit-to-work.
- Route. Set out the run from the design, clear of existing services.
- Trench. Excavate to the design depth and width and form a firm, clean bed.
- Lay and join. Lay the duct to a fall towards a drained pit and seal the joints.
- Form the bends. Keep them gentle, with no kinks or sharp edges for the cable.
- Finish the ends. Bring them to the pit positions, bushed and sealed as specified.
- Rope, mark and record. Leave a draw rope, cap spares, lay a warning tape and record the route.
Frequently Asked Questions
Q1What is a cable duct in airfield ground lighting?
A cable duct is a buried pipe or conduit that carries AGL cables, usually where they cross pavements or run into pits and bases. The duct is laid first and the cable is pulled through afterwards, so the cable is protected and can be replaced later without digging up the pavement.
Q2Why use a duct instead of burying the cable directly?
Under pavements and at crossings, digging up a direct-buried cable means closing and breaking the surface. A duct protects the cable from loads and ground movement and lets a damaged cable be pulled out and replaced. Direct burial remains common in open ground, as the design specifies.
Q3Why do ducts need gentle bends and smooth ends?
A cable is pulled through the duct, so sharp bends and rough edges raise the pulling load and can damage the cable sheath. Gentle bends and smooth, bushed ends let the cable pass without scuffing and avoid forcing it below its minimum bending radius.
Q4Why leave a draw rope and spare ducts?
A draw rope left in each duct means the cable can be pulled in later without having to rod the duct. Spare ducts allow future circuits or replacement cables to be added without breaking the pavement again. Whether and how many spares to provide is a design decision.
Q5How is water kept from collecting in a duct?
The duct is laid to a fall towards a pit where the design shows a drain, the duct ends are sealed around the cable as specified after pulling, and the pits themselves drain. A duct that holds standing water can damage the cable over time and is hard to inspect.
Q6What should be checked before the trench is backfilled?
Check the route and depth against the design, a firm bed, joints made and sealed, gentle bends, duct ends at the pit positions, a draw rope in every duct, spare ducts capped, and a warning tape placed above. Record the as-built route, because buried ducts are easy to hit later. Locate existing services first, treat any cable as live until proved dead and work to the permit-to-work.
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