Armour and screens are different layers
Armour is a metallic layer of steel tape, steel wire or aluminium wire laid over an inner sheath to resist mechanical damage. A screen is a thinner metallic layer, usually copper tape, copper wires or a braid, with an electrical job: in medium voltage cable it carries charging and fault current and holds the insulation at earth potential; in instrumentation cable it keeps electrical noise away from low-level signals. Some armour also serves as a protective conductor, but that depends on its resistance and on the wiring rules, and is decided by the project engineer.
Steel tape, steel wire and aluminium wire armour
IEC 60502-1 allows several armour forms for 0.6/1 kV cable. Steel tape armour (STA) wraps steel tape over the inner sheath. The reference construction from the partner plant catalogue is a 0.6/1 kV Cu/XLPE/PVC/STA/PVC 4x35 mm2 cable (YJV22 type) with 0.9 mm XLPE, a 1.2 mm PVC inner sheath, 0.2 mm steel tape and a 1.8 mm outer sheath, about 30 mm overall and about 2,067 kg/km. Two tapes are common in the industry; the catalogue does not state the number, so the spec sheet fixes it. STA resists crushing and suits direct burial, but it is not designed to take large pulling loads.
Steel wire armour (SWA) uses a layer of round galvanised steel wires and is the usual form for multicore cable in the UK and many export markets, to BS 5467 with a PVC sheath or BS 6724 with an LSZH sheath. Steel wire handles tension better than tape. Single-core cable on AC circuits uses aluminium wire armour (AWA) instead, because steel around a single conductor carrying alternating current heats up through induced currents. SWA and AWA cables are matched to plants that run wire armouring lines, which the desk confirms before quoting; more in SWA vs STA armour and AWA on single-core cables.
| Armour | Typical use | Main strength | Watch point |
|---|---|---|---|
| STA, steel tape | Multicore LV, direct burial, trays | Crush and impact | Not for large pulling loads |
| SWA, steel wire | Multicore LV, BS 5467 / BS 6724 | Impact and tension | Armour resistance if used as earth |
| AWA, aluminium wire | AC single-core cable | Avoids induced heating | Needs plants with wire armouring |
| PSP or APL tape | Outdoor fibre, GYTS / GYTA | Moisture barrier and crush | Direct burial needs armoured type |
Screens in medium voltage and instrumentation cable
In the reference 8.7/15 kV Cu/XLPE/CTS/PVC 3x95 mm2 cable, each core carries a conductor screen, XLPE insulation, an insulation screen and a copper tape screen, with the three cores laid up, filled and bound under a 2.8 mm PVC sheath, about 61 mm overall and about 5,403 kg/km. The copper tape screen is part of the insulation system and must be earthed as the design requires; see MV semiconductive layers.
Instrumentation cable uses a screen to protect signals. The reference 450/750 V 4x1.5 mm2 overall screened cable (KVVP type) has PVC insulated cores, fillers, a polyester tape, a copper wire braid and a 1.2 mm PVC sheath, at most 12.2 mm overall and about 196 kg/km. Individually screened pairs, as in EN 50288-7 or PAS 5308 types, are a different construction and are matched to a separate plant. See instrumentation cables.
How armour and screens are tested
Armour and screens are checked by dimension and resistance. Inspection confirms the tape thickness or wire diameter, the number of tapes or wires, and coverage. Where armour is used as a protective conductor, its resistance must meet the design, and BASEC has warned of SWA cable where both conductor and armour resistance were out of limit because the cable was effectively two sizes small. MV screens are checked for continuity and cross-section, and the cable passes its routine voltage and partial discharge tests with the screen in place. Braid coverage is checked visually and against the datasheet.
What changes price, weight and lead time
- Weight and diameter: armour adds an inner sheath, metal and a thicker build, which increases drum size and freight weight.
- Process steps: armoured and MV cables pass through more lines than unarmoured cable, so slots take longer to align.
- Plant match: STA is widely available; SWA and AWA need specific lines, and MV above 8.7/15 kV needs a separately matched plant.
- Screens: braid and copper tape add copper, which moves with the metal price.
Lead time is confirmed in writing for each order; see MOQ and lead time or send your construction through the quote form.