Armoured vs unarmoured cable: burial, ducts and trays

What armour adds

An unarmoured power cable has conductor, insulation, fillers and a sheath. The 0.6/1 kV Cu/XLPE/PVC reference on the LV power cable line is a typical example. An armoured cable adds an inner sheath (bedding) over the laid-up cores, a metal armour layer and an outer sheath over the armour. The armour may be steel tape (STA), steel wire (SWA) or, on single-core AC cables, aluminium wire (AWA). The Chinese model code shows it with a number suffix: YJV is unarmoured, YJV22 is steel tape armoured.

The armour does mechanical work: it spreads crushing loads, resists impact from stones and tools, and makes accidental damage from digging less likely to reach the cores. On steel wire constructions it also gives tensile strength for vertical runs. It does not make the cable waterproof, change its fire behaviour or make it immune to damage; it raises the force needed to cause damage.

Armour also changes how the cable is handled and terminated. Armoured cables need glands that clamp the armour and, where the armour is earthed, a bonding connection at each end. They are stiffer, so the minimum bending radius on the datasheet is larger, and they need more space at entries into switchboards. Installers often prefer unarmoured cable inside buildings for exactly these reasons, and keep armour for the sections of the route where mechanical protection is really needed. A mixed design, armoured outside and unarmoured inside with a joint or a distribution board at the transition, is common on industrial and commercial sites.

Construction compared

Reference constructions from a partner plant catalogue, 0.6/1 kV Cu/XLPE with PVC sheaths. The two sizes differ, so read the table as a layer comparison rather than a like-for-like weight comparison.

Even allowing for the larger conductor, the armoured cable is visibly bigger and heavier. The bedding, armour and second sheath add several millimetres to the diameter, which changes gland sizes, bending radius, drum size and the number of metres that fit on a drum. Medium voltage cables follow the same pattern with more layers underneath, as described on the armoured cables line.

Burial, ducts and trays

The installation method drives the choice. Common practice, which the project specification and local wiring rules override:

• Direct burial: armoured cable is the usual choice, laid on sand with marker tape above. Steel tape armour resists crushing from soil and stones, but it is not designed for heavy pulling, so tape armoured cable is laid into an open trench rather than winched through long runs.
• Ducts and conduit: unarmoured cable is common, because the duct provides the mechanical protection. Where the duct can be crushed or dug up, many specifications still call for armour.
• Cable trays and ladders indoors: unarmoured cable is normal in protected areas; armoured cable is used in industrial plants where forklifts, falling objects or vibration are a risk.
• Vertical runs and shafts: steel wire armour gives tensile strength to carry the cable's own weight between supports.
• Outdoor exposed runs: armoured cable with a UV-resistant sheath where the cable is surface-mounted and exposed to impact.

The current rating also changes with installation method, burial depth, soil and grouping, so the project engineer sizes the cable for each case under IEC 60364-5-52, BS 7671 or the national rules. Armour itself does not set a current figure.

Which armour, if any

If armour is needed, the next question is the type. Steel tape (STA) is common in IEC and Chinese practice and is one of the armour forms allowed under IEC 60502-1. Steel wire (SWA) is the UK and Middle East standard for multicore cables, made to BS 5467 with PVC sheaths or BS 6724 with LSZH sheaths. Single-core cables in AC circuits use aluminium wire (AWA), because a magnetic steel armour around a single conductor carrying alternating current causes eddy current losses and heating. The full comparison is in SWA vs STA armour and AWA armour on single-core cables.

Where the armour is used as a protective conductor or earth path, its resistance becomes an electrical requirement as well as a mechanical one. BASEC has warned about SWA cables whose effective conductor cross-section was up to two sizes smaller than nominal, with conductor and armour resistance both out of limits. That is why the desk checks conductor resistance and, on SWA, armour resistance per drum before shipment.

Cost, lead time and sourcing

Armour adds steel, a second extrusion, an armouring pass and more drum capacity per metre, so armoured cable costs more per metre and takes more production steps than the same conductor unarmoured. Weigh that against the alternative: a duct system or protected route can make unarmoured cable the better value overall. Points to settle before quoting:

1. Mark each run on the schedule as burial, duct, tray or exposed.
2. For armoured runs, state the armour type and standard: STA to IEC 60502-1, SWA to BS 5467 or BS 6724, AWA for single cores.
3. Choose outer sheath compound: PVC, PE for wet burial, or LSZH where the route enters buildings.
4. Confirm drum lengths from the cable's weight and diameter; wooden drums take two to four weeks to make.
5. Agree test records: conductor resistance, voltage test, armour resistance where relevant, dimensions and print.

Steel tape armoured cable is covered by the partner plant catalogue behind the reference above; SWA and AWA constructions are matched to plants that run those standards, often in the Yixing, Ningjin and Wuwei clusters. LV and armoured power cable MOQs are commonly around 500 m per size, and armoured cables take more production steps; all terms are confirmed in writing for each order, as set out on the MOQ and lead time page. Send the run schedule through the quote form.