10.1 Thermoplastic, Armoured & Flexible Cables

10.1 Thermoplastic, Armoured & Flexible Cables

Cables form the physical conduits through which electrical energy is conveyed from supply intakes and distribution boards to current-using equipment. Selecting the correct cable type, cross-sectional area, and containment method is a fundamental competency for every electrical installer. In the United Kingdom, cable design, conductor colouring, and mechanical protection must comply rigorously with BS 7671 (The IET Wiring Regulations) and relevant British Standards (such as BS 6004, BS 5467, and BS 6724).

An installation operative must understand not only how to route and secure cables, but also the physical properties of their metallic conductors, dielectric insulation, protective bedding, mechanical armouring, and environmental sheathing.

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Flat Thermoplastic Sheathed Cable (BS 6004 Table 9 / 6242Y)

Commonly referred to across the electrotechnical trade as twin and earth (T&E), flat thermoplastic PVC-sheathed cable manufactured to BS 6004 (code designation 6242Y) is the predominant wiring system for domestic and light commercial surface or concealed installations.

  ┌──────────────────────────────────────────────────────────┐
  │                    PVC Outer Sheath                      │
  │  ┌──────────────┐      ┌────────────┐     ┌───────────┐  │
  │  │  Line Core   │      │  Bare CPC  │     │  Neutral  │  │
  │  │ (Brown PVC)  │      │ (Uncovered)│     │(Blue PVC) │  │
  │  │  ┌────────┐  │      │  ┌──────┐  │     │ ┌───────┐ │  │
  │  │  │ Copper │  │      │  │Copper│  │     │ │ Copper│ │  │
  │  │  └────────┘  │      │  └──────┘  │     │ └───────┘ │  │
  │  └──────────────┘      └────────────┘     └───────────┘  │
  └──────────────────────────────────────────────────────────┘

Multi-Layer Construction

1. Conductors: Plain annealed high-conductivity copper. Conductors up to and including 2.5 mm² are typically solid circular conductors (Class 1). Larger cross-sectional areas (4.0 mm², 6.0 mm², 10.0 mm², and 16.0 mm²) utilize compact stranded conductors (Class 2) to maintain sufficient flexibility for pulling through building voids and joists.
2. Insulation: Thermoplastic Polyvinyl Chloride (PVC) rated for a maximum continuous conductor operating temperature of 70°C under normal load, with a limiting short-circuit temperature of 160°C.
3. Circuit Protective Conductor (CPC): Positioned centrally between the insulated live conductors. The CPC is uninsulated (bare plain annealed copper) and has a reduced cross-sectional area compared to the live conductors on cable sizes above 1.0 mm².
4. Outer Sheath: Tough, extruded thermoplastic PVC compound (coloured grey or white), providing moisture resistance and light mechanical protection against abrasion during pulling.

Standard Dimensions and Reduced CPC Sizing

Installers must memorize the relationship between live conductor size and the corresponding factory-fitted CPC cross-sectional area, as this dictates circuit earth fault loop impedance (R1 + R2) and adiabatic thermal withstand (S) calculations:

Mandatory CPC Sleeving

Because the CPC inside 6242Y cable is bare, BS 7671 requires it to be covered by insulating sleeving wherever the sheath is removed at joints and terminations, and protective conductors must be identified by the green-and-yellow colour combination, which is reserved exclusively for protective conductors.

Installers must slide green-and-yellow PVC sleeving over the entire exposed bare copper length, right up to the conductor terminal inside back boxes, luminaires, switches, and consumer units. Leaving the CPC bare inside an accessory enclosure exposes adjacent live phase terminals to flashover if the conductor flexes, representing a major inspection defect.

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Three-Core and Earth Thermoplastic Cable (6243Y)

Manufactured to identical dielectric and sheathing standards as 6242Y, three-core and earth cable (6243Y) incorporates three fully insulated conductors alongside the uninsulated reduced CPC. Conductor insulation complies with the harmonized colour standards of BS 7671 Table 51:

• Brown: Harmonized Line 1 identifier
• Black: Harmonized Line 2 identifier
• Grey: Harmonized Line 3 identifier

  ┌──────────────────────────────────────────────────────────────────┐
  │                       PVC Outer Sheath                           │
  │  ┌────────────┐   ┌────────────┐   ┌────────────┐   ┌──────────┐ │
  │  │ Brown Core │   │ Black Core │   │  Bare CPC  │   │Grey Core │ │
  │  │(Line/Perm) │   │ (Strapper) │   │ (Uncovered)│   │(Strapper)│ │
  │  └────────────┘   └────────────┘   └────────────┘   └──────────┘ │
  └──────────────────────────────────────────────────────────────────┘

Trade Applications and Conductor Function Identification

1. Two-Way and Intermediate Lighting Circuits: Running between master and slave switches. The Brown core typically conveys the permanent line feed to the common terminal (COM), while the Black and Grey cores serve as strappers between terminals 1 and 2 (or L1 and L2).
2. Mains-Interlinked Domestic Smoke and Heat Alarms: Interconnecting ceiling-mounted detector bases to BS 5839-6. Brown provides continuous 230 V unswitched power, Grey is over-sleeved with blue tape to serve as Neutral, and Black serves as the interconnect (interlink) signal conductor between the alarms.
3. Central Heating Controls: Connecting multi-channel central heating programmers, motorized 2-port or 3-port mid-position zone valves, room thermostats, and cylinder thermostats.

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Steel Wire Armoured (SWA) Cable (BS 5467 / BS 6724)

Where electrical sub-mains, industrial distribution circuits, or outdoor cabling run underground, externally on building facades, or through plant rooms, cables are exposed to severe mechanical damage, impact, crushing, rodents, and moisture. In these environments, Steel Wire Armoured (SWA) cable is the standard UK wiring system.

Multi-Layer Construction

                          ┌──────────────────────────────────────┐
                          │ 1. Stranded Copper Conductors (Cu)   │
                          └──────────────────┬───────────────────┘
                                             ▼
                          ┌──────────────────────────────────────┐
                          │ 2. XLPE Insulation (90°C Dielectric) │
                          └──────────────────┬───────────────────┘
                                             ▼
                          ┌──────────────────────────────────────┐
                          │ 3. Extruded Bedding (PVC or LSOH)    │
                          └──────────────────┬───────────────────┘
                                             ▼
                          ┌──────────────────────────────────────┐
                          │ 4. Galvanised Steel Wire Armour (SWA)│
                          └──────────────────┬───────────────────┘
                                             ▼
                          ┌──────────────────────────────────────┐
                          │ 5. Tough Outer Sheath (PVC or LSOH)  │
                          └──────────────────────────────────────┘

1. Conductors: Stranded plain annealed copper conductors (Class 2), available in single-core, two-core, three-core, four-core, and five-core variants.
2. Insulation: Cross-linked Polyethylene (XLPE). XLPE is a thermosetting polymer with superior thermal and electrical properties compared to PVC:
   • Maximum continuous operating temperature: 90°C (compared to 70°C for PVC).
   • Short-circuit temperature limit: 250°C (compared to 160°C for PVC).
   • Higher current-carrying capacity for an equivalent conductor cross-sectional area.
3. Bedding Layer: An extruded layer of PVC (BS 5467) or Low Smoke Zero Halogen compound (BS 6724) applied over the insulated cores. The bedding forms a smooth, cylindrical cushion that protects the delicate conductor insulation from being crushed or abraded by the steel armour wires.
4. Armour: A single layer of circular galvanised steel wires wound helically around the bedding. This armouring provides immense tensile strength, resists penetration by spades and digging tools, protects against rodent attack, and serves as an integral earth return path.
5. Outer Sheath: A tough, extruded oversheath of black PVC or black Low Smoke Zero Halogen (LSOH) polymer, resisting ultraviolet (UV) degradation, moisture, and chemical attack.

Indoor vs. Outdoor SWA Glands (BS 6121)

Armoured cables must be terminated using matched brass compression glands. Installing the wrong gland compromises ingress protection (IP) and earth fault integrity:

  Weatherproof CW Gland Assembly:
  [Cable Outer Sheath] ──► [Outer Seal Nut] ──► [Outer Rubber Seal]
                       ──► [Armour Clamping Ring] ──► [Brass Cone]
                       ──► [Entry Component / Gland Body]
                       ──► [Banjo Tag] ──► [Enclosure Wall] ──► [Brass Locknut]

Step-by-Step SWA Glanding Procedure

1. Outer Sheath Stripping: Measure back from the cable end according to the manufacturer's gland sizing gauge. Cut around the outer sheath using a junior hacksaw or utility knife, taking care not to nick the underlying steel armour wires. Strip off the outer sheath.
2. Armour Wire Trimming: Slide the outer compression nut and clamping ring onto the cable. Align the steel armour wires flat against the cable bedding. Using a junior hacksaw, make a light circular cut around the armour wires at the specified gauge distance. Score the wires evenly, then flex each wire outward until it snaps cleanly along the score line.
3. Bedding Exposure: Strip the exposed bedding back to within approximately 5 mm of the clamped armour wire shoulder, revealing the XLPE insulated conductors.
4. Cone Assembly: Flare the trimmed armour wires outward evenly over the gland's internal brass cone. Push the gland body forward so the cone slips underneath the armour wires. Tighten the compression cone nut onto the gland body with appropriate open-ended spanners, locking the steel wires rigidly between the cone and gland shoulder.
5. Earthing the Banjo Tag: Slide a brass earth tag (banjo) over the threaded entry shank of the gland body. Pass the threaded shank through the enclosure knockout. Place a serrated shakeproof star washer over the threads inside the enclosure and tighten a brass locknut securely.
6. Earth Fly-Lead Bonding: Drill and bolt a copper lug to the banjo tag using a brass or zinc-plated steel bolt, shakeproof washer, and nut. Connect an insulated green-and-yellow copper fly-lead from the banjo directly to the main earthing terminal (MET) or distribution board earth bar.

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Flexible Cords and Flexible Cables

Fixed wiring cables (such as 6242Y and SWA) feature rigid Class 1 or Class 2 conductors designed for stationary containment. In contrast, electrical equipment that is portable, adjustable, or subjected to continuous vibration requires flexible wiring.

Flexible Cord vs. Flexible Cable

• Flexible Cord: A flexible cable in which the cross-sectional area of each conductor does not exceed 4.0 mm².
• Flexible Cable: A flexible cable in which the conductor cross-sectional area exceeds 4.0 mm².

Conductor Metallurgy

Flexible cords utilize Class 5 finely stranded copper conductors, comprising dozens or hundreds of micro-fine annealed copper strands bunched or twisted together. This fine stranding prevents work hardening, metal fatigue, and conductor breakage caused by repeated flexing, coiling, and vibration.

Common Types and Sheath Classifications