HDPE Duct and OFC Cable Types – A Field Guide for Telecom Routes

Updated: 23 October 2026

What is an HDPE duct in telecom work?

An HDPE duct is a pipe of high-density polyethylene buried to carry and protect optical fibre cable. The duct goes in first, by trench or by bore; the cable is blown through it later, sometimes months afterwards and sometimes by a different team. Because the duct stays put, a damaged cable can be drawn out and replaced, and a spare duct leaves room for more fibre without another excavation.

Telecom duct in India normally arrives in long coils, which keeps the number of joints along a route low. Ordinary HDPE water or sewer pipe is a different product: it lacks the low-friction lining that telecom duct carries, so it should not be swapped in to save money.

What does PLB mean, and which duct sizes are used?

PLB stands for permanently lubricated. A PLB HDPE duct is co-extruded with a solid silicone-based lubricant layer bonded to its inner wall, so friction stays low for the life of the duct and cable can be blown over long sections between chambers. In India the requirements for this duct sit in a TEC generic requirement (TEC/GR/TX/CDS-008), and makers commonly list three sizes, written as outside/inside diameter:

Duct size has to suit the cable. Blowing works well only when the cable fills part of the bore, so a fat cable in a small duct stalls short of the next chamber. Micro-ducts, much narrower tubes, form a separate family used mainly for micro trenching and FTTH.

How does colour coding work on telecom ducts?

Manufacturers offer HDPE duct in many colours, among them orange, red, blue, green, yellow, grey, brown and violet, and some add lengthwise stripes. Which colour goes where is set by the operator or the project specification rather than by the duct itself. The aim is simple: where several ducts share a trench, a crew must be able to tell which belongs to which route, owner or purpose.

• Inside one trench – a working duct and a spare in different colours stop cable being blown into the wrong one.
• Between operators – shared corridors are easier to manage when each owner's ducts look different.
• At chambers – colour lets a splicing team pick the right duct quickly, years after laying.

Colour helps only when it is written down. A duct occupancy chart for each chamber, listing colours, sizes and the cable each holds, is what makes the code useful later.

Duct or direct burial – which suits OFC better?

Over the working life of a network the ducted route nearly always wins, because every repair and upgrade avoids digging. Direct burial survives mainly where the link is short, temporary or unlikely ever to need more fibre.

Which OFC cable types go into a duct?

Unarmoured duct cable

The standard choice for blowing: a loose-tube or central-tube design with a dielectric strength member and polyethylene sheath. Light and smooth, it travels far through PLB duct and relies on the duct for crush and rodent protection.

Armoured cable

Adds corrugated steel tape, or sometimes non-metallic armour, under the sheath. It resists crushing and rodent bites and is the usual pick for direct burial or rough ground. It is heavier and stiffer, so blowing sections are shorter, but steel armour gives an EM locator something to follow later.

Ribbon cable

Fibres sit in flat ribbons, often of 12, stacked inside the tube. High counts fit in a slim cable, and a whole ribbon can be mass fusion-spliced at once, which saves hours on large-count city and data centre links.

Micro cable

A very slim cable built for micro-ducts, blown over shorter sections on FTTH and micro-trenched routes. For what the letters OFC stand for and how single-mode and multimode fibre differ, see our OFC full form and laying guide.

Which telecom duct terms should a field team know?

• Coupler – a push-fit or screw joint that links two duct lengths airtight.
• End plug – a cap sealing an empty duct against water, silt and insects.
• Simplex or duplex plug – a seal fitted around a cable where it leaves the duct.
• Duct integrity test – an air-pressure hold, or a sponge or shuttle blown through, proving the duct is continuous and clear.
• Fill ratio – the share of the duct bore the cable occupies; too high and blowing stalls.
• Warning tape – printed tape laid above the duct to alert anyone digging later.

How does GPR find an existing HDPE duct?

Before a new duct is planned, the team must know where earlier ducts already run, and that is harder than it sounds. HDPE does not conduct, so an EM locator cannot follow it unless a steel-armoured cable or tracer wire lies inside. Many old ducts are empty, or hold unarmoured fibre with no metal at all.

Ground penetrating radar works on a different principle. The duct, and the air or water inside it, return radar energy differently from the soil around them, so a radar line crossing the duct records a small arch, or hyperbola. Crossing the corridor at regular intervals and joining those arches traces the duct's path, and a bundle shows as a cluster. Chambers, marker posts and old road cuts help confirm the line between scans.

Results depend on duct size, depth and ground; wet clay or saline soil shortens radar reach, and a sound report says where detection was weak. Our guide to GPR scanning for OFC routing explains the corridor method, and the telecom duct feasibility survey offers it as a service.

What duct records should a telecom route keep?

Ducts are cheap to lay and expensive to lose track of. A useful record set holds:

• duct count, size and colour for every section, shown on the as-built;
• occupancy at each chamber – which duct carries which cable, and which are spare;
• integrity test results taken before cable blowing;
• coordinates of chambers, bends and crossings, not just distances from kerbs;
• the pre-laying survey, showing other services near the duct.

Operators sharing corridors save the most from these records when another team plans work alongside. The utility mapping page for telecom operators shows how survey data feeds them; to have an existing duct corridor traced, send the route through the quote form below.