Arc Fault Detection Devices (AFDDs) Explained

How AFDDs Work and Why Circuit Design Matters

Arc Fault Detection Devices, commonly known as AFDDs, are one of the most discussed topics in modern electrical installations.

Electricians often ask the same questions:

• What do AFDDs actually detect?

• Why were AFDDs introduced into BS 7671?

• Do ring circuits and radial circuits affect AFDD behaviour?

• Why do AFDDs sometimes nuisance trip?

Understanding how arc fault detection works requires understanding how electrical waveforms behave on different types of circuits.

This article explains the fundamentals behind AFDD operation, including waveform disturbance, circuit topology and modern electrical loads.

What Is an AFDD?

An Arc Fault Detection Device (AFDD) is a protective device designed to detect electrical arcs that may lead to fire.

Traditional protective devices respond to different electrical conditions:

MCBs respond to overcurrent.RCDs respond to earth leakage or imbalance.

AFDDs operate differently.

Instead of measuring current magnitude alone, AFDDs analyse waveform disturbance on the electrical circuit.

When an arc fault develops, the current waveform becomes irregular due to rapid interruptions and restrikes within the conductor.

These disturbances are the patterns that AFDD technology is designed to detect.

Why Arc Faults Can Be Difficult to Detect

One of the reasons AFDDs were introduced is because series arc faults may not create enough current to trip traditional protective devices.

A loose terminal, damaged insulation or poor connection can produce an arc without generating an overcurrent condition.

This means an MCB may not operate even though a fault is developing.

AFDDs help address this type of situation by analysing the electrical waveform behaviour rather than simply measuring current magnitude.

Ring Circuits vs Radial Circuits

Circuit design also influences how disturbances travel through an electrical installation.

In a radial circuit, there is a single path between the load and the consumer unit.

Any waveform disturbance created on the circuit travels back along that single conductor path.

In a ring final circuit, current can travel around the circuit in two directions depending on the position of the load.

Because of this, disturbances created on a ring circuit may travel back toward the protective device along two parallel paths.

The fault itself is the same, but the signal environment seen by the AFDD can be different.

Understanding this behaviour is important when considering how AFDDs operate in real installations.

Modern Electrical Loads and Waveform Noise

Modern electrical installations include many electronic devices that introduce switching noise onto electrical circuits.

Examples include:

LED lighting drivers

Switch mode power supplies

EV chargers

Heat pumps

These devices produce periodic switching noise that can appear on the electrical waveform.

Arc disturbances behave differently.

Switching noise tends to be predictable and repetitive, while arc faults are typically irregular and intermittent.

AFDD design must distinguish between these two types of disturbance.

Installation Quality Still Matters

Most arc faults originate from installation issues such as:

Loose terminals

Damaged insulation

Poor connections

AFDDs are designed to reduce fire risk caused by developing arc faults, but they do not replace good installation practice.

Correct inspection, testing and termination remain the most important factors in preventing electrical faults.

AFDDs and BS 7671

Arc fault detection devices are referenced within BS 7671 as a fire risk reduction measure in certain types of premises.

Their purpose is to help reduce the risk of electrical fires caused by arc faults that may not be detected by traditional protective devices.

Understanding how AFDDs interpret waveform disturbance helps electricians make more informed decisions when designing or installing electrical circuits.

Watch the Video or Download the Full White Paper

This article summarises the key concepts behind Arc Fault Detection Devices (AFDDs) and waveform disturbance.

For a more detailed explanation, including diagrams and practical examples, you can:

Watch the full video explanation or Download the Navitas AFDD technical white paper.

Both resources explore the subject in greater detail and explain how arc fault detection works in real electrical installations.