Purpose

Distinguish fused and unfused spurs and explain why accessory count alone is not a substitute for Appendix 15 assessment.

Learning objectives

• Define a spur
• Compare fused and unfused arrangements
• Trace downstream protection
• Identify loading and topology risks

A spur branches from a ring final rather than forming part of its return path. Whether it is acceptable depends on origin, conductor, outlets, load, and protective arrangement.

Core theory

An unfused spur relies on the ring-final protective arrangement and is restricted by the relevant design guidance; it must not become an uncontrolled branch network.

A fused connection unit limits downstream current to its fuse rating and can supply suitable fixed equipment or a permitted downstream arrangement.

Spur assessment must consider existing spurs, junction accessibility, conductor size, connection capacity, likely load, RCD protection, voltage drop, and manufacturer instructions.

Assumptions: No special engineered justification.

1. Trace: The proposal creates a multi-outlet branch outside the ring path.
2. Protection: No local fuse limits the branch independently.
3. Decision: Do not approve from a sketch; redesign using a compliant topology and verify against current Appendix 15 guidance.

The proposed unfused multi-outlet branch should not be accepted as an ordinary spur arrangement.

Reasonableness check: A branch conductor should not carry uncontrolled aggregate load based on a mistaken ring-current-sharing assumption.

Knowledge check

Answer: A local overcurrent limit for the downstream circuit. The fuse bounds downstream current when correctly selected and installed.

Practical exercise

Classify four paper circuits as ring point, fused spur, unfused spur, or unclear; explain what evidence is missing for each unclear case.

Summary

• A spur is outside the ring return path
• An FCU limits downstream current
• Trace topology before judging compliance