Ring & Radial Final Circuits and Spurs (Lecture Extract)

Circuit Types and Configurations

• Two principal arrangements repeatedly referenced:
  • Ring Final Circuit
  • Conductors loop from the consumer unit, run around the area, and re-enter the same protective device, forming a closed “ring”.
  • Typically wired in 2.5\,\text{mm}^2 copper twin-&-earth cable.
  • Intended mainly for socket-outlet groups in domestic properties.
  • Radial Circuit
  • Conductors start at the consumer unit and terminate at the last outlet with no return path.
  • Cable size chosen to carry the full design current to the farthest point; common sizes: 2.5\,\text{mm}^2 for short radials, 4\,\text{mm}^2 for longer or higher-load runs.
• Each circuit—ring or radial—must be fed from its own protective device (e.g.
  miniature circuit-breaker, MCB) and labelled accordingly so users can identify it easily.

Regulatory Requirements (Regulation 411.3.3)

• Regulation 411.3.3 of BS 7671 states that socket-outlets rated up to 32\,\text{A}, and other points that may reasonably be expected to supply portable equipment for use outdoors, MUST be provided with additional protection by an RCD not exceeding 30\,\text{mA}.
• The phrase “accessible to ordinary persons” includes virtually all sockets in domestic premises.
• Therefore every ring, radial, or spur feeding such outlets must either:
  • Be on an RCD/RCBO at the consumer unit, or
  • Incorporate an RCD-protected spur device.

Common Domestic Practice

• Modern installations usually employ two ring finals in an average dwelling:
  • One ring serving the front/ground-floor sockets.
  • One ring serving the rear/upper-floor sockets.
• Smaller properties may still have a single ring for all outlets; larger or rewired houses often add extra rings or radials for kitchens, utility rooms, or dedicated loads.
• Whatever the distribution method, electricians must affix clear identification at the consumer unit (and on any inspection schedule) indicating which protective device and cable route supply each area.

Spurs on a Ring Final Circuit

• A spur is an extension taken from a ring final circuit after initial commissioning, commonly to feed an extra socket or a lighting point.
• Two categories exist:
  1. Non-Fused Spur
  • Connected directly to a socket, junction box, or the back of an accessory on the ring.
  • Must use cable of the same cross-section (normally 2.5\,\text{mm}^2).
  • May feed ONLY:
    • One single socket-outlet, or
    • One double socket-outlet, or
    • One fixed item of equipment whose load does not exceed the plug rating (e.g.
      boiler, extractor fan).
  1. Fused Spur (via a Fused Connection Unit – FCU)
  • Spur cable again generally 2.5\,\text{mm}^2 but downstream of the FCU may be reduced (e.g.
    1.5\,\text{mm}^2) because the FCU limits current.
  • FCU incorporates a BS 1362 fuse, selectable up to 13\,\text{A}.
  • Can supply multiple sockets or mixed loads provided the aggregated current is within the fuse rating.
• Wiring detail:
  • The live and neutral of the spur must terminate in the same protection device as the rest of the ring, not alongside its outgoing leg alone, preventing overheating due to imbalance.
  • Spurs should be shown on updated circuit charts to aid future testing.
• Practical example (implied in transcript):
  • Desire to add two lighting outlets from a downstairs ring.
  • Installer fits an FCU with a 13\,\text{A} fuse, then runs 1.5\,\text{mm}^2 twin-&-earth to the lighting junction box.

Cable Sizes, Protective Devices & Current Ratings

• Standard U.K. domestic socket circuit values:
  • Ring final: 2.5\,\text{mm}^2 cable protected by 32 A Type B MCB.
  • Radial (≤ 50 m): 2.5\,\text{mm}^2 cable, 20 A MCB.
  • Radial (> 50 m or high load): 4\,\text{mm}^2 cable, 32 A MCB.
• Lighting circuits typically use 1.0 – 1.5\,\text{mm}^2 cable with a 6 A MCB, but when fed from an FCU spur the fuse rating governs the downstream cable size.

Identification & Labelling

• Regulations require that each protective device is clearly marked with:
  • Circuit description (e.g.
    “Kitchen Ring Final”).
  • Cable size.
  • Protective device rating.
  • RCD reference if applicable.
• During alterations (e.g. adding spurs) the electrician must update:
  • Consumer unit schedule.
  • On-site test results & inspection records.

Good Practice & Safety Notes

• Avoid over-populating a single ring; where heavy loads (washing machines, tumble-dryers) cluster in a small area, provide a dedicated radial.
• Verify that all ring ends are present in their protective device after work; use the end-to-end resistance test:
  R_{1, \text{end-to-end}} \approx 0.26\,\Omega for a typical 50\,\text{m} loop of 2.5\,\text{mm}^2 copper.
• “Figure-of-eight” (r1-rn, r1-r2) cross-coupled readings confirm intact continuity before energising.
• Spurs must never be looped from an existing spur unless protected by an FCU; chaining non-fused spurs risks overloading the ring conductor.
• All new socket outlets must be RCD-protected regardless of whether the original circuit was; if necessary retrofit an RCBO or an RCD-protected spur unit.