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SWA vs AWA vs STA Armoured Cable: Construction, Application, and Cost Under BS 5467 / BS 6724
Steel Wire Armour, Aluminium Wire Armour, or Steel Tape Armour—which is right for your project? Learn the mechanical protection levels, magnetic effects, and cost differences between SWA, AWA, and STA cables.
1. Introduction: The Hidden Cost of Wrong Armour Selection
Selecting the wrong armour type for a power distribution cable is not a minor specification error—it is a decision that compounds into installation delays, premature corrosion, and in AC systems, efficiency losses from induced eddy currents. For EPC contractors and infrastructure project managers, a cable that fails five years into a 40-year design life means costly unplanned replacement and reputational damage. SORIVO's power cable range covers all three armoured constructions with full third-party certification.
The three dominant armour constructions—Steel Wire Armour (SWA), Aluminium Wire Armour (AWA), and Steel Tape Armour (STA)—each serve distinct mechanical and electrical roles. Understanding their differences in construction, mechanical protection, weight, magnetic behaviour, and cost is essential for specifying the correct cable under BS 5467 (PVC) or BS 6724 (LSZH).
2. Construction Explained: SWA, AWA, and STA
All three armoured cable types share a common base structure—conductor, insulation, inner sheath—but differ in the armour layer itself. For a complete overview of SORIVO's cable manufacturing capabilities, visit the About Us page.
SWA — Steel Wire Armour
SWA consists of galvanised steel wires applied helically around the inner sheath. The steel provides the highest mechanical protection of the three types, resisting crushing, impact, and rodent damage. SWA is the standard choice for underground direct burial, industrial power distribution, and infrastructure projects.
Standard reference:
BS 5467 / BS 6724
Conductor:
IEC 60228 Class 2 (stranded circular or shaped)
Armour coverage:
Full helical layer, gap controlled
Bedding:
Extruded PVC or LSZH between inner sheath and armour
Typical application:
Direct burial, cable trenches, industrial plants
AWA — Aluminium Wire Armour
AWA uses aluminium wires instead of steel. Aluminium is non-magnetic, which eliminates the inductive heating losses that steel armour can cause in single-core AC circuits. AWA is significantly lighter than SWA—aluminium density is approximately one-third that of steel (2.7 g/cm³ vs 7.85 g/cm³)—making it preferable for vertical installations, long-span tray runs, and projects where weight is a structural concern.
Standard reference:
BS 5467 / BS 6724
Conductor:
IEC 60228 Class 2
Armour coverage:
Full helical layer of aluminium wires
Key advantage:
Non-magnetic—suitable for single-core AC circuits
Typical application:
Vertical risers, offshore platforms, high-current single-core feeds
STA — Steel Tape Armour
STA applies galvanised steel tape (or tapes) helically around the cable, often with overlapping layers. While the tape provides robust mechanical protection against crushing, it is less flexible than wire armour and offers moderate impact resistance compared to SWA. STA is commonly used in multicore pilot, control, and instrumentation cables where high crush resistance is needed but frequent bending is not required.
Standard reference:
BS 5467 / BS 6724, BS 5308 (instrumentation)
Armour:
One or two galvanised steel tapes, overlapped
Flexibility:
Lower than SWA—limited bending radius
Typical application:
Control cables, instrumentation, fixed installations
Comparison Table
| Parameter | SWA (Steel Wire) | AWA (Aluminium Wire) | STA (Steel Tape) |
|---|---|---|---|
| Material | Galvanised steel wire | Aluminium wire | Galvanised steel tape |
| Mechanical protection | Highest (impact + crush) | Moderate-high (slightly below SWA; non-magnetic) | High crush resistance, moderate impact resistance |
| Relative weight (per m)* | 1.0x (baseline) | ~0.6–0.7x | ~0.9x |
| Magnetic (AC suitable) | Yes (multicore); No (single-core) | Yes (all) | Yes (multicore); No (single-core) |
| Flexibility | Good | Good | Limited |
| Typical voltage (BS 5467/6724) | 0.6/1kV, 1.9/3.3kV | 0.6/1kV, 1.9/3.3kV | Low voltage / control |
| Cost (relative, approx)** | 1.0x (baseline) | ~1.2–1.4x | ~0.85–0.95x |
*Weight varies by conductor size and armour specification. Example: 185mm² single-core cable, SWA ≈ 2350 kg/km, AWA ≈ 1450 kg/km (≈38% lighter).
**Cost varies by market conditions, order quantity, and specification.
3. Standards and Certification
All three armoured constructions are governed by the same core British and international standards:
| Standard | Scope | Relevance |
|---|---|---|
| BS 5467 | PVC-sheathed armoured cables for power distribution | Covers SWA/AWA/STA with PVC bedding and outer sheath; rated 0.6/1kV and 1.9/3.3kV |
| BS 6724 | LSZH-sheathed armoured cables | The halogen-free equivalent of BS 5467; LSZH bedding and outer sheath; required in public buildings, tunnels, and ventilated spaces |
| IEC 60228 | Conductors of insulated cables | Class 2 stranded conductors for armoured power cables |
| IEC 60332-1-2 | Single vertical flame spread | Mandatory for all armoured cables per BS 5467/6724 |
| IEC 60332-3-24 | Bunched flame spread (Category C) | Required for grouped cable runs |
| IEC 60754-1/2 | Halogen gas evolution | <0.5% HCl content for LSZH cables per BS 6724 |
| BS 7671 | IET Wiring Regulations | Governs cable sizing, voltage drop, and installation methods in the UK |
BS 5467 vs BS 6724: BS 5467 covers PVC-sheathed armoured cables (PVC bedding + PVC outer sheath). BS 6724 covers LSZH (Low Smoke Zero Halogen) armoured cables (LSZH bedding + LSZH outer sheath). BS 6724 is required in public buildings, tunnels, and enclosed spaces where halogen gas release in a fire poses a health risk. Both standards cover the same voltage ratings:
0.6/1kV and 1.9/3.3kV.
Voltage rating clarification: BS 5467 and BS 6724 cover low-voltage armoured cables. For medium-voltage applications (6/10kV to 33kV), refer to BS 6622 (PVC) or BS 7835 (LSZH).
Third-party certification (BASEC, KEMA, LUL) provides independent verification that the cable meets these standards. Self-declared CE marking alone does not guarantee compliance with the underlying harmonised standards.
4. Application Suitability: Matching Armour to Environment
Magnetic Effects: The Critical Distinction
The single most important difference between SWA and AWA is magnetic behaviour in AC circuits:
Steel (SWA, STA) is magnetic — In single-core AC cables, steel armour acts as a magnetic core, inducing significant eddy current losses and heating. This is why SWA and STA are only suitable for multicore AC cables where the net magnetic field cancels (the magnetic fields from each phase conductor sum to near zero). For high-current single-core AC installations (typically above 100A or where thermal management is critical), SWA or STA should not be used.
Aluminium (AWA) is non-magnetic — AWA can be used for both single-core and multicore AC cables without inductive heating. For high-current single-core AC installations (transformer feeds, generator connections, UPS systems), AWA is the correct specification.
DC systems — Neither steel nor aluminium armour induces losses in DC circuits. Both are suitable.
Application Matrix
| Installation | Recommended Armour | Rationale |
|---|---|---|
| Direct burial (multicore) | SWA | Highest mechanical protection, cost-effective |
| Direct burial (single-core AC) | AWA | Non-magnetic, avoids eddy current heating |
| Vertical risers in buildings | AWA | Significantly lighter, reduces structural load |
| Offshore / marine | AWA (with LSZH) | Corrosion resistance + weight saving |
| Industrial control panels | STA | High crush resistance, lower cost |
| Cable trenches (multicore) | SWA | Standard, readily available |
| Public buildings (tunnels) | SWA/AWA + BS 6724 | LSZH required per BS 7671 |
| High-current gen-set connections | AWA | Single-core AC, non-magnetic essential |
| Solar PV DC feeders | SWA or AWA | Both suitable for DC; SWA more common for cost |
| Industrial manufacturing plants | SWA — Learn more | High mechanical protection in harsh environments |
Armour earthing requirement: The armour layer of all armoured cables must be earthed to provide a path for fault currents and ensure personnel safety. Failure to earth the armour can result in the armour becoming live under fault conditions, creating a shock hazard. Use appropriate armour glands (BW, CW, or CX type) with earth tags to ensure reliable earthing.
5. Total Cost of Ownership: Beyond the Per-Metre Price
The upfront material cost tells only part of the story. A 25-year lifecycle analysis reveals where each armour type delivers value.
| Cost Factor | SWA | AWA | STA |
|---|---|---|---|
| Material cost (per m) | 1.0x (baseline) | ~1.2–1.4x | ~0.85–0.95x |
| Installation labour | Standard | ~10–15% less (lighter pulling) | Standard |
| Support structure cost | Standard | Lower (lighter trays/cleats) | Standard |
| Replacement risk (wrong spec) | High if misapplied to single-core AC | Low | High if misapplied to single-core AC |
| Scrap value (end of life) | Moderate (steel) | Higher (aluminium) | Moderate (steel) |
The arithmetic: For a 500m single-core AC feeder at 630A, specifying SWA instead of AWA would risk thermal runaway from induced eddy currents—a failure requiring full replacement at 2-3x the original installation cost. The 20-40% premium on AWA material is negligible against the replacement risk.
Example Cost Comparison (indicative, 2026 market)
| Cable Type | 3×185mm² SWA (per km) | 3×185mm² AWA (per km) |
|---|---|---|
| Material cost | £12,000–15,000 | £14,500–20,000 |
| Installation cost | £3,000–4,000 | £2,500–3,500 (lighter) |
| Total installed | £15,000–19,000 | £17,000–23,500 |
Same 3-core 185mm² construction, differing only in armour material. AWA costs more due to higher aluminium material price, partially offset by lighter weight reducing installation labour. For single-core AC circuits requiring AWA (see §4), the cost premium is justified against the risk of eddy-current heating in steel armour.
6. Quality Identification: How to Verify Armoured Cable
When inspecting armoured cable deliveries, verify these points:
1.
Armour wire/tape condition: Galvanisation should be uniform, no rust spots. Aluminium wires should have consistent diameter (±0.1mm per BS 5467/6724).
2.
Armour coverage: Helical layers should be tightly applied with no gaps exceeding one wire diameter. For STA, tape overlap should be minimum 25% of tape width.
3.
Bedding integrity: The extruded bedding between inner sheath and armour must be uniform—irregularities indicate poor extrusion that may allow moisture ingress.
4.
Bend test: The cable should withstand 6× cable diameter bend radius (armoured) without armour wire fracture or tape lifting.
5.
Marking: Look for batch numbers and metre markings that enable full traceability to the production lot and third-party test certificates.
6.
Certification check: Verify BASEC, KEMA, or LUL certification marks. Request test certificates for the specific batch.
7. Q&A
Can SWA cable be used for single-core AC applications?
No. Steel armour is magnetic and will induce significant eddy current heating in single-core AC circuits, particularly at higher currents (typically above 100A). Use AWA (aluminium wire armour) for single-core AC installations.
What is the difference between BS 5467 and BS 6724?
BS 5467 covers PVC-sheathed armoured cables (PVC bedding + PVC outer sheath). BS 6724 covers LSZH (Low Smoke Zero Halogen) armoured cables (LSZH bedding + LSZH outer sheath). BS 6724 is required in public buildings, tunnels, and enclosed spaces where halogen gas release in a fire poses a health risk. Both standards cover the same voltage ratings: 0.6/1kV and 1.9/3.3kV.
Is STA cable as strong as SWA?
STA provides comparable crush resistance but is significantly less flexible due to the taped construction. STA also offers moderate impact resistance compared to SWA's high impact resistance. STA is suitable for fixed installations where the cable will not be re-routed or subject to frequent movement. SWA is preferred for direct burial and installations requiring periodic re-termination.
Why is AWA cable more expensive than SWA?
Aluminium wire costs more than galvanised steel wire per unit of armour coverage, and the raw material price of aluminium is higher. However, AWA's lighter weight reduces installation labour and structural support costs, partially offsetting the material premium. For single-core AC applications, AWA is essential regardless of cost—using SWA would result in thermal issues.
What is the typical lifespan of properly installed SWA cable?
BS 5467/BS 6724 cables installed in accordance with BS 7671 have a design life of 40+ years. The galvanised steel armour should remain corrosion-free within the bedding and outer sheath system. Premature failure is almost always caused by mechanical damage during installation or water ingress at poorly sealed terminations.
Do I need to earth the armour layer?
Yes, absolutely. The armour layer must be earthed to provide a path for fault currents and ensure safety. Use appropriate armour glands with earth tags, and ensure the earthing connection is mechanically secure and corrosion-resistant.
Can I use SWA for solar PV DC cables?
Yes. DC circuits do not induce eddy currents in magnetic armour, so both SWA and AWA are suitable for solar PV DC applications. SWA is more commonly used due to lower cost and higher mechanical protection for buried or exposed installations.
SORIVO Armoured Cable Offering
| Parameter | Market Standard | SORIVO Premium |
|---|---|---|
| Conductor | IEC 60228 Class 2 (stranded) | IEC 60228 Class 2, oxygen-free copper ≥99.9% purity |
| Armour material | Standard galvanised steel/aluminium | Double-galvanised steel (SWA/STA) or 5000-series aluminium alloy (AWA) — superior corrosion resistance |
| Bedding | Thin-wall extruded | Thick-wall extruded bedding, +15% over minimum BS requirement |
| Sheath | Standard PVC/LSZH | UV-stabilised, enhanced abrasion resistance |
| Certification | Self-declared CE | BASEC / KEMA / LUL third-party certified |
| Traceability | No batch marking | Full metre-mark traceable to production lot and test certificate |
| Warranty | 1-5 years | 25-year design life warranty |
Need certified armoured cable for your next infrastructure project?
Contact SORIVO for technical selection support and batch test certificates.
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References
- BS 5467: Armoured cables with thermosetting insulation and PVC sheathing
- BS 6724: Armoured cables with thermosetting insulation and low smoke zero halogen sheathing
- BS 6622: Cables with extruded XLPE or EPR insulation, 3.8/6.6 kV up to 19/33 kV — PVC
- BS 7835: Cables with extruded XLPE or EPR insulation, 3.8/6.6 kV up to 19/33 kV — LSZH
- IEC 60228: Conductors of insulated cables
- BS 7671: Requirements for electrical installations (IET Wiring Regulations)