A fibre duct is a dedicated conduit system—typically plastic (HDPE, PVC) or composite—used to house, route, and protect optical fibre cables and microcables. It includes main ducts, sub-ducts, or microduct bundles, plus fittings (couplers, seals, manholes/handholes) that create a continuous, maintainable pathway from point to point (outside plant or inside buildings).

Why it’s used in ICT networks:

• Mechanical protection: Shields fibre from crush, impact, abrasion, and installation stresses; preserves bend radius and limits pulling tension to prevent microbends/macrobends and excess attenuation.
• Environmental protection: Mitigates water ingress (with sealed/pressurized ducts), soil movement, UV (for exposed runs), chemicals, and temperature swings; deters rodents.
• Pathway management: Provides organized, mapped routes for backbone, metro, access, and campus links; supports separation from power and compliance with codes (e.g., TIA-569, ISO/IEC 11801 pathways).
• Scalability and upgrades: Empty ducts and microducts allow later cable additions via pulling or air-blown installation without new civil works; supports high-density deployment and network growth.
• Installation efficiency: Enables long, continuous runs using jetting/blowing; simplifies maintenance, cable swaps, and fault localization via accessible chambers.
• Safety and reliability: Reduces accidental damage during excavation; identifiable colors/markers aid locate-and-avoid practices.

Types/contexts include underground direct-buried HDPE ducts, duct-in-duct with sub-ducts/microduct bundles, riser/plenum-rated indoor conduits, and aerial duct solutions.

• HDPE smoothwall: Most common for direct-bury/duct banks. Flexible, tough, long continuous lengths, good impact/chemical resistance, low friction (often silicone-lined). Not fire-rated; not for plenums unless LSZH variant.
• HDPE ribbed/lube-lined: Interior ribs or permanent lubricant to reduce pull/blow friction for longer cable runs.
• HDPE corrugated (flex): Highly flexible for short runs, handholes, and tight bends; lower crush strength than smoothwall.
• PVC (Schedule 40/80): Rigid sticks with couplings. Easy to align in duct banks, good crush resistance; economical. Less impact resistance in cold; not ideal for long directional drilling. Sch 80 for higher mechanical protection.
• CPVC/FR-PVC: Higher temperature and fire performance than PVC; used where heat/fire ratings are needed.
• LSZH halogen-free conduit: Low smoke, low toxicity for indoors, tunnels, stations; typically less flexible, costlier than HDPE.
• RTRC/FRP (fiberglass-reinforced plastic): Very high strength-to-weight, corrosion resistant, good fire/smoke options; premium cost.
• Steel rigid conduit: Maximum mechanical/EMI protection in high-risk areas; heavy, costly, corrosion-prone without coatings.
• Microduct (single): Small-diameter HDPE/LSZH tubes optimized for air-blown microcables; enables incremental capacity.
• Microduct bundles: Multiple microducts sheathed together; rapid deployment and segmentation; requires careful blowing practices.
• Innerduct/subduct: Smaller conduit installed inside a larger duct to partition capacity and protect each cable; available in smooth, ribbed, or fabric.
• Fabric/multi-cell textile innerduct: Collapsible woven cells pulled into existing ducts to create multiple pathways with minimal space loss.
• Direct-bury armored duct: Conduit with integrated crush/rodent protection for trenchless or shallow installs.
• Aerial figure-8 duct: Self-supporting conduit with integrated messenger for pole spans; UV and temperature rated.

Key differences: material/fire/LSZH rating, rigidity vs flexibility, crush/impact resistance, friction/blowability, installation method (trench, HDD, duct bank, aerial), environmental resistance, and cost.

• Determine present and future fibre counts, cable construction (loose tube, ribbon, micro), installation method (pull vs air-blown), and environment (access, distribution, backbone).
• Size by area fill: (Cable OD^2) / (Duct ID^2) ≤ 0.5 for pulling; ≤ 0.4 preferred for blowing. Keep single-cable duct fill 30–50%.
• Typical selections (duct OD/ID varies by spec):
  • Drop/access: 20/16–25/20 mm duct with 1× microcable (≤8–12 mm OD) or a 5–7-way microduct bundle (e.g., 7×10/8 mm).
  • Distribution up to ~144F (≈10–14 mm OD): 32 mm duct (ID ~26–28 mm) or 40 mm where long runs/blowing.
  • 288F (≈16–20 mm OD): 40 mm duct (ID ~32–34 mm); 50 mm if long, many bends, or future overblow.
  • 432–864F (≈20–28 mm OD): 50–63 mm duct.
  • Trunks with multiple cables or sub-ducts: 63–110 mm main with sub-ducts (3–7× 32/25 or microduct bundle).
• Future capacity strategies:
  • Install spare ducts: minimum 2-way in access (1 live, 1 spare); 3–4-way in distribution/backbone (1 live, 2+ spares); >4-way on major corridors.
  • Use sub-ducting: place a larger host duct (50–63 mm) and populate with multiple 10/8 or 12/10 mm microducts to overblow later.
  • Reserve 25–50% capacity for growth; plan 1–2 spare cables or microducts per route segment.
• Bends and installation:
  • Maintain cable bend radius ≥10× OD (dynamic) and ≥15× OD (static). Limit cumulative bends; prefer larger ducts if many bends.
  • Use low-friction, lined ducts for blowing; prove with mandrel and pull tape.
• Example quick picks:
  • 96F today, 288F future: 40 mm duct or 50 mm with 7×10/8 microducts.
  • 288F today, 864F future: 63 mm host with 3×32 mm sub-ducts or 12–19-way microduct bundle.

Trenching (open cut)

• Use where access is clear and depths are modest. Typical cover: 45–90 cm in verges, >1.0 m under roads (per local code).
• Maintain utility clearances; pothole to verify. Bedding with fine material; place detectable warning tape 30 cm above plant.
• Use sand/CLS backfill around ducts, compact in lifts, restore surface to permit specs. Install handholes at pull/blow intervals and slack loops.

Horizontal directional drilling (HDD)

• Select for crossings/obstacles and minimal surface impact. Design bore path with sufficient cover, gentle curvature (≥100× product OD).
• Pothole/locate utilities; monitor for frac-out. Use appropriate drilling fluid; ream to 1.5–2.0× product OD.
• Pull with swivel and breakaway link set below cable/duct tension limit; maintain steady rate; record pullback tensions.

Pulling (in ducts)

• Proof/mandrel and air-test ducts; dry/clean. Use oversized sheaves/rollers; control sidewall pressure per spec.
• Use approved lubricant; adhere to manufacturer max tension and minimum bend radius (dynamic ≥20× cable OD; static ≥10×).
• Use pulling grips with thimbles, rotating swivel, dynamometer. Cap/seal ends; avoid tight compound bends. Stagger pulls and use intermediate assist winches if long.

Blowing (jetting in ducts/microducts)

• Use clean, straight, dry, pressure-rated ducts; test and seal. Limit cumulative bend; avoid sharp bends and crushed ducts.
• Use calibrated compressor with dryer/filter; set speed/pressure per machine and cable. Employ intermediate boosters for long runs.
• Monitor push force and distance; stop if drag spikes; use correct size foam projectiles and seals.

General best practices

• Survey/permits/traffic and environmental controls; 811/locates and potholing.
• Use tracer wire with nonmetallic ducts; bond/ground metallic components.
• Maintain as-builts, depth, route, and test results. OTDR and insertion loss testing pre/post-install; inspect terminations.
• Provide slack management, markers, and closures; protect from moisture/UV/rodents; follow manufacturer specs. Safety and quality records throughout.

• Minimum bend radius (duct installation):

  • Under tension (during pull): ≥ 20 × the cable outer diameter (OD)
  • After installation (no tension): ≥ 10 × OD
  • Example: 12 mm OD cable → ≥ 240 mm during pull; ≥ 120 mm at rest
  • For steel-armored cables, some specs call for ≥ 15 × OD (under tension) and ≥ 10 × OD (at rest). Always confirm the specific cable’s datasheet.

• Allowable pulling tension (Maximum Allowable Tensile Load, MATL) for typical outside-plant loose-tube cable in ducts:

  • During installation: up to ~2700 N (≈ 600 lbf) when pulling via the strength members or a factory-installed pulling eye
  • Long-term (in service): ≤ ~600 N (≈ 135 lbf)
  • Lighter indoor/drop cables are often much lower (e.g., install ~220 N, long-term ~100 N). Use the manufacturer’s rating.

• Additional practical notes:

  • Use a swivel and pull only on the strength members; never on fiber elements or jacket alone.
  • Lubricate for long/curved duct pulls and observe sidewall pressure limits; increase bend radius if tension is high.
  • If microduct-blown fiber or specialty designs are used, follow the product-specific bend/tension limits from the datasheet.

Typical practice (verify locally):

• Footways/verges: 250–450 mm (10–18 in)
• Carriageways/vehicular areas: 450–900 mm (18–36 in)
• Under reinforced concrete/driveways: may allow 300–450 mm with encased/conduit and marker tape
• Crossings/rail/critical: often deeper and/or duct encased; as specified by asset owner

Key standards/codes (selection):

• United States:

  • National Electrical Safety Code (NESC), Rule 353/Table 353-1: minimum cover for communications cables/ducts (varies by location; ~12 in sidewalks, ~18 in residential/parking, ~24 in roadways; check latest edition).
  • TIA-758 (Customer-Owned Outside Plant), BICSI OSPDRM/TDMM: design/installation guidance.
  • Local DOT/municipal ROW, and 811 locating requirements; OSHA 29 CFR 1926 Subpart P for excavation safety.

• United Kingdom:

  • NJUG Volume 1: recommended burial depths (telecom typically 250–350 mm in footways, 450–600 mm in carriageways).
  • Street Works UK “Specification for the Reinstatement of Openings in Highways” (SROH).
  • Openreach/other operator works specs for ducts, marker tapes, and separation.

• European Union:

  • EN 50174-3 (Information technology cabling installation outside buildings).
  • National road authority/municipal permits.

• Canada:

  • CSA C22.1 (Canadian Electrical Code), local utility/municipal standards; typical ~450 mm residential, ~600 mm roadways.

• Australia:

  • AS/CA S009: minimum cover (commonly ~450 mm general, ~300 mm under slabs/paths with mechanical protection, ~600 mm under roads).
  • State road authority specifications; Dial Before You Dig.

• New Zealand:

  • NZUAG Code of Practice for Utility Access; typical ~300 mm footpaths, ~450 mm berms, ~600 mm carriageways.
  • AS/NZS cabling standards and network operator specs.

• India:

  • DoT/ROW guidelines and MoRTH/IRC specs; many authorities require ~1.2 m along highways and greater at crossings; operator (e.g., BharatNet/BSNL) standards apply.

Always maintain separation from other utilities, warning tape, detectable tracer, proper backfill, and as-built records.

• Design and materials

  • Use SDR-rated HDPE or PVC ducts with water-blocking microducts where feasible; specify watertight couplers with double O-rings.
  • Select gel-filled or water-swellable yarn/tape cables for outside plant.
  • Bed ducts in compacted sand, maintain minimum cover, avoid low points; slope 0.5–1% toward handholes/sumps for drainage.

• Entries, joints, and terminations

  • Seal building entries with UL-listed mechanical seals (link-seals), firestop putty/foam as required, and drip loops.
  • Use gasketed, bolted couplers; torque to spec. Test every joint.
  • Cap/plug all vacant ducts immediately; use reusable mechanical duct plugs with pressure/water ratings.

• Handholes/manholes

  • Install gasketed lids, cable restraints, and bell ends; include weep holes or sumps with pump in wet areas.
  • Elevate lids above grade in flood zones; avoid placing chambers in depressions.
  • Use corrosion-resistant racks and seal penetrations.

• Pressurization and testing

  • Pressure-test empty ducts (e.g., 5–10 psi for 15–30 min); soap-test joints.
  • Consider dry air/nitrogen low-pressure pressurization with desiccant in high-risk water tables; monitor gauges.

• Locating and records

  • Install detectable tracer wire or detectable pull tape; add RFID markers at changes of direction/handholes.
  • Place warning tape 12–18 in above duct.
  • Survey with GPS; maintain GIS/as-builts with depth, routes, spare occupancy, joint details, photos.

• Operation and maintenance

  • Mandrel and brush ducts before pulls; verify bend radius.
  • Use compatible cable lubricant; avoid over-tension; monitor with dynamometer.
  • Periodically rod/mandrel and CCTV inspect problem segments; clear silt with jet/vac as needed.
  • Inspect after major rain/flood; pump/dry chambers; replace compromised seals.
  • Keep spares: couplers, plugs, link-seals, gaskets, lubricant, desiccant.
  • Implement change control: document every new penetration, seal, and repair.