Steel Wire Armouring Transfers Load Through the Gland Not the Sheath
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Steel wire armouring is a single helical layer of hot-dip galvanised round wire laid over the bedding, and its capacity is arithmetic: wire diameter — commonly 0.9 to 3.15 mm — combined with wire count and helix angle sets the breaking force. BS 5467 and IEC 60502-1 define the construction; Chinese type coding marks fine round wire armour as suffix 32 and coarse wire as 42 with a PVC sheath, 43 where polyethylene replaces it. Selection runs in one order. Compute the tensile case first: a vertical riser loads the armour with the suspended weight of the cable per metre, and a duct pull adds capstan friction across every bend. That figure picks the armour class, and the class then fixes gland type and cleat spacing. Choosing armour from catalogue habit and hoping the pull works is how sheaths tear on site. The load path ends at a gland built for it. BS 6121-1 and BS EN 62444 govern armoured entries: a BW single-compression gland clamps armour and sheath without weather seals for indoor terminations, a CW adds sealing for outdoor exposure. In both, a tapered wedge cage must bite each individual wire so tension passes into the enclosure. Where the wedge compresses sheath instead of steel, the PVC carries the load, creeps under it, and the cable pulls out of its gland — a termination failure that presents as a cable fault. Two checks complete the duty. Bending follows the armoured limit, commonly twelve times overall diameter; installation guidance adds that if armour pull exceeds conductor pull by 50 per cent, the minimum bend radius rises by that same proportion. Electrically, SWA may act as a protective conductor only with both ends bonded and continuity measured, and never as a sleeve on a single AC core, where magnetic loss forces aluminium wire armour instead. Buy the armour for the force, then buy the gland for the armour.
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