3.3 Industrial Wiring Systems: Trunking, Tray, Ladder, and Armoured Cables

3.3 Industrial Wiring Systems: Trunking, Tray, Ladder, and Armoured Cables

Industrial installations demand wiring containment systems capable of withstanding mechanical impact, chemical exposure, vibration, and heavy thermal loads. Under SS 638, cable selection and containment design must balance mechanical protection, current-carrying capacity, heat dissipation, and structural support.

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1. Metallic Cable Containment Systems

Galvanized Screwed Steel Conduit (BS EN 61386)

Heavy-gauge screwed steel conduit provides maximum mechanical protection in severe industrial environments (e.g., machine shops, hazardous chemical areas, exposed factory columns).

• Earthing Continuity: All joints must be screwed tightly into threaded conduit couplers or cast iron conduit boxes to maintain low earth fault loop impedance. Locknuts and serrated washers must be installed at switchboard entries.
• Maximum Space Factor (40% Rule): Under SS 638, the total cross-sectional area of all cables enclosed within a conduit must not exceed 40% of the internal cross-sectional area of the conduit. This prevents conductor overheating due to restricted airflow and prevents insulation damage caused by mechanical jamming during cable pulling.

Galvanized Iron (GI) Sheet Steel Trunking (BS EN 50085)

Surface-mounted GI trunking is used to group multi-circuit distribution feeds across factory halls.

• Maximum Space Factor (45% Rule): The sum of overall cable areas must not exceed 45% of the internal cross-sectional area of the trunking.
• Fire Barriers (SCDF Fire Code / SS 638): Wherever trunking passes through fire-rated walls or floor slabs, internal intumescent fire-stop barriers (fire-rated pillows or expanding intumescent mastic) must be installed inside the trunking to prevent smoke and fire propagation between fire compartments.

Cable Trays and Cable Ladders (BS EN 61537)

• Perforated Cable Trays: Manufactured from hot-dip galvanized steel, used primarily for light power circuits, control cabling, and instrumentation.
• Heavy-Duty Cable Ladders: Feature side rails and welded rungs, designed to support heavy multi-core armoured power cables.
• Thermal Dissipation Advantage: Open tray and ladder containment allows free air circulation around cables. This yields superior current-carrying capacity rating factors ($C_i \approx 1.0$) compared to enclosed trunking ($C_i \approx 0.7 - 0.8$).

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2. XLPE / SWA / PVC Armoured Cables

Cross-linked Polyethylene (XLPE) insulated, Steel Wire Armoured (SWA), PVC sheathed copper power cables are the industry standard for main feeders, underground trenches, and outdoor exposed industrial runs.

Construction Layers and Engineering Functions

1. Copper Conductors: High-conductivity plain annealed stranded copper (BS EN 60228 Class 2).
2. XLPE Primary Insulation: Cross-linked polyethylene allows a maximum continuous conductor operating temperature of 90°C (compared to 70°C for standard PVC). This permits higher current-carrying capacity for a given conductor cross-section.
3. PVC Bedding: Extruded PVC layer forming a smooth circular cushion over insulated cores to protect them from sharp wire armour edges.
4. Steel Wire Armour (SWA): Single layer of galvanised steel wires wrapped helically around the bedding. Serves two functions:
   • Provides extreme mechanical protection against crushed loads, impacts, and rodent damage.
   • Functions as a Circuit Protective Conductor (CPC) for earth fault currents, provided its earth loop impedance complies with SS 638.
5. PVC Outer Sheath: Outer jacket providing weather, moisture, and chemical resistance. Maximum outer sheath temperature is 70°C.

Minimum Bending Radius (SS 638)

To prevent damaging internal XLPE core insulation or displacing steel armour wires:

• Cables up to 25 mm overall diameter: Minimum bending radius = $6 \times$ overall cable diameter ($D$).
• Cables exceeding 25 mm overall diameter: Minimum bending radius = $8 \times D$.

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3. Industrial Cable Cleating and Support Spacing

Armoured cables mounted on ladder racks or wall surfaces must be securely clamped using heavy-duty two-bolt aluminum or synthetic cable cleats.

Purpose of Cleating

1. Static Load Support: Preventing cable sag and mechanical strain at gland entry points.
2. Dynamic Short-Circuit Withstand: During a heavy 3-phase short-circuit fault, electromagnetic repulsion forces between adjacent conductors generate severe mechanical stress. Cleats must be rated to withstand peak fault forces without breaking.

SS 638 Support Spacing Guidelines for SWA Cable

• Horizontal Runs: Cable cleats/supports spaced at maximum intervals of 1.2 m to 1.5 m.
• Vertical Runs: Cable cleats spaced at maximum intervals of 300 mm to 400 mm. Tighter vertical spacing prevents the self-weight of heavy vertical cable risers from imposing tension on top gland terminations.

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4. Cable Glands, Armour Termination, and Earth Continuity

Armoured cables entering switchboards or motor terminal boxes must be terminated using brass cable glands to ensure mechanical retention, environmental sealing, and electrical earth continuity.

Cable Gland Selection: BW vs CW Glands

Armour Termination Procedure & Earth Tags

1. Strip outer PVC sheath to expose steel armour wires.
2. Pass armour wires into gland cone and tighten gland nut to sandwich armour firmly between cone and body, creating low-resistance electrical contact.
3. Fit a Brass Earth Tag (Banana Tag) or Earthing Nut over the entry thread inside the switchboard enclosure.
4. Connect a green-and-yellow protective bonding conductor from the earth tag directly to the SDB Earth Busbar using a crimped copper lug and bolt assembly. This guarantees an uninterrupted earth fault loop return path back to the MET.