SWA as protective conductor?
Cable Sizing · Lesson 18 · Intermediate
Purpose
Prove whether SWA armour is adequate as all or part of the CPC using continuity, fault-loop and thermal constraints.
Learning objectives
- Calculate armour suitability
- Use steel material data
- Include reactance where needed
- Verify terminations
Armour can be a protective conductor only when the entire fault path is demonstrably adequate. The IET directs designers to armour calculations, steel k values and applicable guidance, not an assumption based on the letters SWA.
Core theory
Check minimum protective-conductor sizing and the adiabatic relation using the armour's verified effective cross-sectional area and applicable steel k value. Copper-equivalent shortcuts must preserve the correct thermal/material basis.
Verify earth-fault loop impedance and disconnection time using actual armour resistance and, where significant, reactance. IET guidance highlights that SWA armour reactance can matter even where line conductors are relatively small.
Every gland, enclosure, earth tag, parallel CPC and bonding connection belongs to the fault path. Check corrosion, circulating currents, parallel-path current sharing, enclosure capability and continuity at both ends.
| Term | Meaning | Symbol | Unit |
|---|---|---|---|
| Circuit protective conductor | Protective conductor connecting exposed-conductive-parts to the earthing arrangement | CPC | Not applicable |
| Adiabatic check | Short-duration thermal withstand verification | S ≥ √(I²t)/k | Not applicable |
| Armour reactance | Inductive component of armour fault-path impedance | Not applicable | Ω |
S ≥ √(I²t) / kUse compatible fault current, clearing time, armour area and the current standard's material/temperature k value.
mm²Assumptions: Values share the same fault case; k is explicitly supplied for the declared material/limits.
- Energy: √(I²t) = 900×√0.20 = 402.49 A√s.
- Area: Smin = 402.49/46 = 8.75 mm².
- Limits: Compare with verified armour data, then separately prove Zs, reactance, terminations and mechanical/environmental integrity.
Illustrative adiabatic minimum is 8.75 mm² on the supplied basis; this alone does not approve the armour as CPC.
Reasonableness check: Longer clearing time or higher fault current increases required area.
- Assuming armour is always enough
- Using copper k for steel
- Ignoring gland continuity or reactance
Where this appears in practice
SWA feeder schedules should record armour data, CPC arrangement, Zs model, adiabatic result and termination details.
Knowledge check
Does passing the adiabatic equation alone prove armour is a satisfactory CPC?
No. Loop impedance, disconnection, continuity, reactance and terminations must also pass.
Answer: No. Loop impedance, disconnection, continuity, reactance and terminations must also pass.
Practical exercise
Audit a supplied SWA CPC calculation and termination schedule for missing fault-path evidence.
Summary
- Armour capability must be calculated
- Steel and reactance matter
- Terminations are part of the CPC
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.