Armoured cable is electrical cable with a layer of metal wire or tape wrapped around its core, designed to protect the internal conductors from physical damage. It’s the standard choice for outdoor, underground, and industrial installations where ordinary cable would be too vulnerable to crushing, impact, or interference from rodents and tools. The most common type, steel wire armoured (SWA) cable, is rated at 600/1000V and used for everything from mains power supply to underground transport networks.
How Armoured Cable Is Built
Armoured cable has the same basic components as any power cable, with one critical addition: a protective metal layer sandwiched between the inner and outer sheaths. Working from the inside out, the layers are:
- Conductor: Copper or aluminum wires that carry the electrical current.
- Insulation: Cross-linked polyethylene (XLPE) surrounds each conductor, chosen for its strong electrical properties and water resistance.
- Bedding: A PVC inner sheath that separates the insulated conductors from the armour layer, preventing the metal from cutting into the insulation.
- Armour: The defining layer. Steel wires or steel tape are wrapped around the bedding to resist crushing, impact, and penetration.
- Outer sheath: A final PVC jacket that holds everything together, seals out moisture, and adds one more barrier against the environment.
The armour layer is what separates this cable from standard types. Without it, the cable is just insulated conductors in a plastic jacket. With it, the cable can handle direct burial in soil, exposure to construction activity, and installation in areas where animals might chew through unprotected wiring.
Types of Armour
The three main categories of armour are steel wire armour (SWA), galvanized steel wire braid (GSWB), and aluminum wire armour (AWA). Each suits different installation conditions.
Steel Wire Armour (SWA)
SWA is the most widely used type. Galvanized steel wires are wrapped (not braided) around the cable core, providing a minimum of 90% coverage. This makes SWA extremely tough but relatively stiff, so it works best in fixed installations where the cable won’t need to flex after it’s laid. Underground power networks, sewage systems, and outdoor supply runs are typical applications. SWA can withstand the weight of buried soil, resist damage from drilling and digging, and hold up in waterlogged conditions.
Galvanized Steel Wire Braid (GSWB)
GSWB cable uses the same galvanized steel, but the wires are braided rather than wrapped. This changes two things. First, coverage drops to around 80% minimum, meaning slightly less raw mechanical protection. Second, the braided construction stays flexible when the cable bends, maintaining its protective coverage even around curves. That flexibility makes GSWB the preferred choice for marine and offshore environments, machinery connections, and instrumentation where the cable may need to be routed through tight spaces or moved during maintenance. The galvanization also gives it strong corrosion resistance in salt air and wet conditions.
Aluminum Wire Armour (AWA)
AWA substitutes aluminum for steel, which reduces the cable’s overall weight. This matters in long overhead runs or applications where minimizing load on cable trays is a priority. Aluminum is also naturally resistant to certain types of corrosion, though it provides less mechanical strength than steel for the same wire diameter.
Voltage Ratings and Cable Standards
In the UK and countries following British Standards, armoured cables fall under two key specifications. BS 5467 covers cables with PVC bedding and outer sheathing, which is the standard construction for most outdoor and underground work. BS 6724 covers cables with low smoke zero halogen (LSZH) sheathing, designed for indoor and enclosed environments where fire safety is a concern.
Both standards define two voltage ratings. The most commonly specified is 600/1000V, meaning 600V from any conductor to earth and 1,000V between phases. A higher rating of 1,900/3,300V is available for heavier industrial applications.
PVC vs. Low Smoke Zero Halogen Sheathing
The choice between PVC and LSZH sheathing comes down to where the cable will be installed. PVC is durable, flexible, and cost-effective for outdoor and underground use. But when PVC burns, it releases dense black smoke and toxic halogen gases, including chlorine. In an open trench or outdoor environment, that’s manageable. Inside a building, it’s dangerous.
LSZH sheathing produces significantly less smoke and no halogen gases when exposed to fire. Under standardized testing (the “3m cube test”), LSZH materials allow at least 60% light transmittance through the smoke they generate, meaning occupants can still see exits and escape routes. The absence of corrosive gases also protects nearby electronics and infrastructure. A small fire that fills a server room or hospital corridor with acidic PVC smoke can cause more damage to equipment and people than the flames themselves.
LSZH armoured cable is particularly worth specifying in data centers, healthcare facilities, schools, escape routes, and any enclosed space where evacuation could be slow or equipment density is high.
How Armoured Cable Is Terminated
You can’t simply strip armoured cable and connect it like ordinary wire. The steel armour needs to be properly clamped and earthed at each end using a cable gland. The most common type for indoor SWA installations is the BW gland, which provides mechanical retention, clamps the armour wires in place, and maintains earth continuity through the armour to the enclosure. BW glands carry an IP 2X ingress protection rating, suitable for dry indoor environments. For outdoor or weatherproof installations, CW and other sealed gland types offer higher protection against moisture and dust.
Getting the gland right matters for two reasons. Mechanically, an unsecured armour layer can spring loose and damage insulation or create a hazard. Electrically, the gland is what connects the armour to the earthing system of whatever enclosure it enters.
Using the Armour as an Earth Conductor
One practical advantage of SWA cable is that the steel armour itself can serve as the circuit protective conductor (the earth path). In many cable sizes, particularly up to 95mm², the cross-sectional area of the armour is at least as large as the current-carrying conductors inside the cable, giving it more than enough capacity to safely carry fault current back to the supply. This can eliminate the need for a separate earth wire, simplifying installation and reducing cost. The armour must be properly terminated with the correct gland at both ends for this to work, since the gland is what creates the electrical connection between the armour and the earthing system.
Where Armoured Cable Is Used
The most common application is supplying mains electricity to buildings, outbuildings, and external equipment. If you need to run power from a consumer unit to a detached garage, garden office, or external lighting installation, SWA cable buried in a trench is the standard approach. It’s also used for:
- Underground power distribution: Street lighting circuits, housing developments, commercial site supply.
- Industrial installations: Factory floor wiring, conveyor systems, heavy machinery connections.
- Outdoor surface runs: Cable clipped along external walls or run through conduit where it’s exposed to weather, impact, or UV light.
- Marine and offshore: GSWB-type armoured cable for platforms, vessels, and dockside infrastructure where flexibility and corrosion resistance are essential.
Armoured cable is not typically used for internal domestic wiring within walls and ceilings, where standard twin-and-earth cable is sufficient. Its strength is overkill for protected indoor routes, and its rigidity makes it harder to work with in tight spaces. The point of armouring is to survive conditions that would destroy ordinary cable, so it belongs in the installations where that protection is genuinely needed.