Arc faults are one of the leading causes of electrical fires in low-voltage installations. Unlike overloads or short circuits, arc faults may not generate sufficient current to trip traditional protective devices such as MCBs or RCDs. This is where Arc Fault Detection Devices (AFDDs) become critical.

AFDD requirements are defined under BS 7671 issued by BSI, particularly for higher-risk premises.

What Is an Arc Fault?

An arc fault occurs when electricity jumps across a gap due to damaged insulation, loose connections, or cable degradation. There are two main types:

• Series arc faults – occur in series with the load, often caused by loose terminals.
• Parallel arc faults – occur between conductors, such as line-to-neutral or line-to-earth.

These arcs generate high temperatures that can ignite surrounding materials without necessarily exceeding the trip threshold of an MCB.

How AFDDs Work

AFDDs monitor electrical waveforms and detect the unique high-frequency signatures produced by arc faults. Using microprocessor-based technology, they distinguish between harmless operational arcs, such as those from switching devices, and dangerous fault arcs.

Once a hazardous arc pattern is detected, the AFDD disconnects the circuit rapidly.

Application in UK Installations

AFDDs are recommended or required in certain installations including:

• HMOs
• Care homes
• Student accommodation
• Buildings with sleeping accommodation
• Timber-framed structures

They are typically installed at the consumer unit, often combined as AFDD/RCBO devices for full circuit-level protection.

Technical Considerations

Electricians must consider:

• Circuit type and load characteristics
• Compatibility with existing boards
• Nuisance tripping risk
• Cost-benefit analysis

AFDDs are particularly effective on final circuits supplying socket outlets up to 32A.

Limitations

AFDDs do not replace overcurrent or earth leakage protection. They operate in addition to MCB and RCD functionality. Proper circuit design and termination practices remain essential.

Conclusion

AFDDs represent an advanced layer of fire protection in UK electrical installations. As regulations evolve and fire safety standards tighten, their use is becoming increasingly relevant in both residential and commercial environments.