IEC 60794-3-11 and 3-10: Duct and Direct-Buried Fibre Cables — the Questions Engineers Ask

Most outdoor optical fibre cables go into the ground in one of two ways: pulled or blown into a duct, or laid straight in a trench. IEC 60794-3-10 and IEC 60794-3-11 are the specifications behind most of those duct and direct-buried fibre cables, and they appear in tenders from telecom operators, utilities and infrastructure projects worldwide. Yet the same questions come back in almost every enquiry: which part applies, does a duct cable need armour, can one design serve both duct and direct burial, and which tests prove it?

This article answers those questions. It is a practical summary, not a substitute for the standard itself.

Where do IEC 60794-3-10 and 3-11 sit in the IEC 60794 series?

IEC 60794 is organised by installation environment. Part 3 covers outdoor cables — in duct, directly buried and aerial. Inside Part 3, IEC 60794-3-10 is the family specification for duct, directly buried and lashed aerial telecommunication cables. IEC 60794-3-11 sits a level below it and fixes more of the requirements for single-mode cables of that family.

Both lean on the generic requirements and the test methods of Part 1. If you are new to that hierarchy, start with IEC 60794 explained: which part applies to your optical fibre cable.

Two neighbours are often confused with them:

• Microduct cables that are blown into small ducts belong to IEC 60794-5-10, with design priorities of their own — see IEC 60794-5-10 microduct cables.
• Self-supporting aerial cables belong to IEC 60794-3-20, and ADSS cables on power lines to IEC 60794-4-20 — see ADSS cable design. "Lashed aerial" in 3-10 means a cable lashed to a separate messenger wire, which is a different job from carrying its own span.

Which part should a tender reference: 3-10 or 3-11?

Customers quote either, and sometimes both. In practice:

• A reference to 3-10 tells you the family and the framework of tests, but leaves many values to the detail specification — usually the customer's own.
• A reference to 3-11 narrows things down for single-mode cables, so fewer values are left open.

Insight: neither reference describes a construction. Whichever part is quoted, the offer is defined by what the customer adds — fibre count, fibre type, tube and colour scheme, sheath, armour, permitted loads — and by the ratings you declare. Read the referenced part together with the customer specification, and flag every value that neither of them fixes.

Does the standard tell you how to build the cable?

No — and this is where engineers coming from power cables are most often surprised. The family specifications define which properties must be declared and how they are verified. The construction is the manufacturer's decision: multi loose tube or central tube, number of tubes and fibres per tube, strength member type, gel-filled or dry core, single or double sheath, with or without armour.

Two compliant 96-fibre duct cables can therefore differ noticeably in diameter, weight, stiffness and cost. When you compare offers, compare declared performance — tensile load during installation and in service, crush, impact, bend radius, temperature range, water tightness — not just the standard reference.

What changes between a duct cable and a direct-buried cable?

The installation, not the fibre, drives the design.

Duct cables

A duct protects the cable from the soil, but the cable has to survive installation. The design questions are about tension and friction: how far it will be pulled or blown, how many bends the route has, and what load the cable can take without straining the fibres. Light, smooth, non-armoured designs with a PE sheath and a central dielectric strength member are typical. In existing ducts, diameter can be the hard limit.

Direct-buried cables

A buried cable meets stones, backfill, soil movement, water and animals for its whole life. Crush and impact resistance matter more, and protection layers appear: a corrugated steel tape, steel wire armour, a second sheath, or glass yarns as a dielectric rodent barrier. Where termites are a concern, customers often ask for a polyamide outer layer.

Insight: "direct buried" is not one requirement. Rocky soil, rodent pressure and termites lead to different constructions. If the tender says only "suitable for direct burial", ask about the route.

Can one design serve both duct and direct burial?

Often, yes — many operators buy one armoured outdoor cable for both, which simplifies stock and logistics. The trade-off is that the duct sections then carry armour they do not need: a larger diameter, more weight, higher friction and shorter pulling or blowing lengths. In congested ducts this can rule the universal design out.

The reverse substitution — an unarmoured duct cable laid directly in the ground — is the risky one. It may pass the type tests, but it has not been designed for the crush, impact and rodent exposure of direct burial.

Metallic or all-dielectric protection?

Metallic armour gives strong crush and rodent protection, but brings electrical questions: lightning in high-risk regions, induced voltages near power lines and railways, and the need to bond and earth the armour at joints. All-dielectric designs avoid those questions, at the cost of different mechanical protection. The right answer depends on the route — and it should be agreed before quoting, because it changes the construction and the bill of materials.

Which tests prove a duct or buried fibre cable?

The family specification selects tests from the IEC 60794-1 methods and sets their severity, with several values left to agreement. Typical tests cover tensile performance, crush, impact, repeated bending, torsion, kink, temperature cycling and water penetration, plus tests on the sheath and the cable elements. The usual pass criterion is a limited change in attenuation and no visible damage.

Insight: severities follow declared ratings, and ratings cost material. A higher tensile rating means more strength member; a higher crush rating may mean armour or a thicker sheath. Customers sometimes specify loads beyond what their installation method needs — worth a polite question before you over-design.

Typical tender mistakes

• Copying a load from another project. A tensile rating set for long-distance duct pulling can be excessive for short urban runs.
• Assuming 3-10 compliance covers the customer's extras. Armour, double sheath, termite protection or a specific colour code are additions the customer must specify and the manufacturer must design.
• Ignoring duct size. A universal armoured design may not fit the existing duct, or may not reach the required pulling length.
• Mixing up duct and microduct cables. A microduct installation needs a cable built for blowing, not a standard duct cable.
• Offering a design the type tests do not cover. Changing tube count, strength member or armour can take the design outside existing test evidence.

A tender checklist for IEC 60794-3-10 / 3-11 cables

1. Installation: duct (pulled or blown), direct buried, lashed aerial, or a mix?
2. Referenced part (3-10, 3-11) and the customer's detail specification.
3. Fibre count, fibre type (G.652.D, G.657.A1 or A2), fibres per tube, colour code.
4. Permitted tensile load during installation and in service; crush, impact, minimum bend radius.
5. Operating, installation and storage temperature ranges.
6. Water blocking: gel-filled or dry core.
7. Sheath and protection: single or double sheath, corrugated steel tape, steel wire, glass yarn, polyamide for termites.
8. Metallic or all-dielectric — and, if metallic, bonding and earthing expectations.
9. Diameter and weight limits — duct size, drum size, pulling length.
10. Marking, drum length, end sealing and type test evidence for the offered design.

How CablePLM handles duct and buried fibre cables

The CablePLM fiber standards library includes IEC 60794 families 3-10 and 3-11 alongside 2-20, 2-50, 3-20, 4-20 and 5-10, for multi loose tube, central tube, micromodule and ribbon designs up to 3456 fibres with G.652.D and G.657.A1/A2 fibres.

Each family is a versioned calculation method. Enter the fibre count, cable element and protection layers, and CablePLM builds the construction from fibres and tubes through strength members, sheath and armour, calculates the bill of materials, and produces engineering and customer versions of the datasheet — every value marked as standard data or estimate. The AI Tender Agent pulls fibre type, colour code, loads and protection requirements out of tender documents, Tenders & Projects keeps the offer variants together, and Qualification & Quality shows whether the offered design is covered by your existing type tests.

Want to see your duct and buried cable range in CablePLM? Book a demo.