Steel Tape Armouring Guards Against Radial Load Not Tension
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Steel tape armouring protects a cable from the direction of force it will most likely meet underground — and buyers who treat "armoured" as one category choose wrongly. Two galvanised steel strips are wound over the bedding in opposite helices with controlled gap, closed under the oversheath; IEC 60502-1 construction records call the layer DSTA, double steel tape, and Chinese type coding marks it suffix 22 with PVC sheath or 23 with polyethylene. The defence is radial: soil weight, trench rock, ground settlement, rodent teeth and the case engineers design for — a spade striking a buried cable. A continuous steel strip offers no gap for a blade edge to enter, resists point compression better than separated wires, and adds less diameter and cost than wire armour. That is why direct-buried feeders are specified with tape. The limitation is axial: tape carries almost no pulling load; under tension the strips slip and the helix opens. Practice splits by load direction — mining assigns steel tape to roadways and gentle inclines, wire armour to shafts and steep runs; risers, submarine routes and long duct pulls take armour wire, which BS 5467 prescribes as a single layer of galvanised wire for multicore cable. Two electrical checks follow. Armour may serve as the earth-fault path only if its metallic section satisfies the adiabatic equation — a thin tape qualifies far less often than wire, and that is a calculation, not a catalogue claim. Steel around a single AC core forms a magnetic short-circuited circuit that heats, so tape belongs on multicore cable while single-core trefoil circuits use non-magnetic armour. At installation, respect the larger bending radius, clamp tape with fittings sized for it, and provide bonding — a gland that bites the tape without an earth connection has destroyed the protection it was fitted for.
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