Cable Construction

Layers

• Conductor — copper is standard; aluminium is used for large power cables where weight and cost dominate, with terminations that suit it. Conductors are solid for small sizes and stranded for flexibility and larger sizes.
• Insulation — determines the voltage rating and the maximum conductor operating temperature.
• Screen — for medium voltage cables, a semiconducting layer and metallic screen control the electric field and carry fault and capacitive current. For instrument cables, a foil or braid screen keeps interference out.
• Bedding — a layer separating the cores from the armour.
• Armour — steel wire armour for multicore cables, aluminium wire for single core cables to avoid heating from circulating magnetic effects. Armour gives mechanical protection and in many installations serves as the protective conductor.
• Sheath — the outer protective layer, chosen for the environment: sunlight, chemicals, hydrocarbons, abrasion and rodents.

Insulation materials

• XLPE — cross-linked polyethylene. High operating temperature, good electrical properties, the general standard for power cables.
• EPR — flexible, good for medium voltage and for cables that must move.
• PVC — cheap, lower temperature rating, and it emits dense toxic smoke and corrosive gas when burned.
• LSZH (low smoke zero halogen) — specified for enclosed and manned spaces, offshore and control rooms, where smoke and corrosive gas from a fire would endanger people and equipment.
• Mineral insulated and fire resistant cables — maintain circuit integrity during a fire, used for emergency lighting, fire alarms, shutdown systems and firewater pumps. The fire survival time of these circuits is a design requirement and must not be compromised by using an ordinary cable for part of the run.

Special types

Instrument cables with individual and overall screens and defined capacitance for intrinsically safe circuits; downhole and submersible cables for electric submersible pumps; trailing and reeling cables for mobile equipment; and heat trace cables for freeze protection and viscosity control.