SWA construction: layers and materials
Cable Sizing · Lesson 16 · Intermediate
Purpose
Identify SWA cable layers and assign electrical, mechanical and environmental functions without calling the armour insulation.
Learning objectives
- Name SWA layers
- Distinguish bedding and sheath
- Interpret cable markings
- Match construction to environment
A typical multicore SWA cable combines conductors, insulation, fillers/bedding, steel wire armour and an oversheath. Each layer has a distinct declared function.
Core theory
The exact product standard matters: common UK thermosetting, low-smoke and fire-resistant families are not interchangeable. Read conductor material, voltage rating, insulation, sheath, armour and reaction/fire data from the manufacturer declaration.
Steel wire armour provides mechanical protection and may form all or part of a protective-conductor path only after design verification. Bedding beneath armour is not automatically equivalent to a certified insulating sheath.
Glands, shrouds, banjos/earth tags, enclosures and corrosion control must preserve mechanical retention, ingress protection and reliable electrical continuity.
| Term | Meaning | Symbol | Unit |
|---|---|---|---|
| Bedding | Layer forming a bed beneath armour | Not applicable | Not applicable |
| Armour | Metal wire layer providing declared mechanical protection | SWA | Not applicable |
| Oversheath | Outer environmental/protective covering | Not applicable | Not applicable |
Assumptions: No manufacturer schedule exists.
- Gap: SWA describes construction family, not complete environmental performance.
- Specify: Define standard, conductor/insulation/sheath, voltage, fire and chemical/UV requirements.
- Coordinate: Select compatible glands, enclosure materials and corrosion measures.
The description is incomplete and cannot establish product suitability.
Reasonableness check: The outer environment acts on sheath and terminations, not the generic acronym.
- Calling armour an insulating layer
- Selecting by core size alone
- Ignoring gland/sheath compatibility
Where this appears in practice
SWA is widely used for outdoor, buried and industrial distribution where engineered mechanical protection is required.
Knowledge check
Does the word SWA define the exact insulation and fire performance?
No. The product standard and manufacturer declaration do.
Answer: No. The product standard and manufacturer declaration do.
Practical exercise
Annotate a supplied cable datasheet and termination drawing with every layer and interface function.
Summary
- Layers have distinct functions
- Product standard matters
- Terminations complete the system
Sources and review
- BS 7671:2018+A4:2026 Requirements for Electrical Installations: IET/BSI; Current edition and corrigenda must be confirmed; United Kingdom
- Earthing and Bonding FAQs: SWA armour as CPC: IET; Current online guidance; United Kingdom
- Avoiding danger from underground services (HSG47): Health and Safety Executive; Third edition; Great Britain
Editorial review date: 2026-08-22. Professional electrical review is pending.