Inside an AFDD: Microsecond Arc Plasma & Waveform Signature Physics

1. The Physics of the Series Arc: Why 500V DC Insulation Testers Miss It

A series arc fault occurs when a current-carrying conductor suffers a physical discontinuity—such as a loose terminal screw, a damaged stranded wire in a flex, or a cracked junction box connection. Because the break is in series with the load, the circuit current is limited by the load impedance (typically 2 A to 20 A in domestic and commercial circuits).

When disconnected and tested with a standard insulation resistance tester at 500 V DC (as required by BS 7671 Chapter 64), the test instrument reads >500 MΩ to earth and neutral because the dielectric between live conductors remains intact. Traditional protective devices are completely blind to this hazard:

• MCBs & Fuses: Fail to trip because the load current is well below thermal and magnetic overcurrent thresholds (e.g. 10 A on a 32 A Type B MCB).
• RCDs: Fail to trip because all current flowing through the series arc returns through the neutral conductor, yielding zero residual differential flux in the toroidal core.

2. Dynamic Arc Characteristics: The Zero-Crossing "Flat Spot"

Unlike a linear resistor or a steady inductor, an electrical arc in air is a non-linear plasma discharge governed by the dynamic energy balance of the ionization channel:

In a 50 Hz alternating current system, as the supply voltage approaches the zero crossing, the electrical power input \(u \cdot i\) falls below the cooling dissipation rate \(P_0\). The plasma de-ionizes, extinguishing the arc. Current stops completely, producing the signature current flat spot (typically 0.5 ms to 2.5 ms per half-cycle). Only when the rising instantaneous voltage exceeds the re-ignition breakdown voltage \(V_\text{re-ign} \approx 40\text{--}70\text{V}\) does Townsend avalanche ionization restrike the plasma channel, causing a step-change in \(\frac{di}{dt}\) and emitting broadband RF noise from 10 MHz to 50 MHz.

3. DSP Algorithm Discrimination: Benign Commutation vs Hazardous Arcs

An AFDD uses high-speed digital signal processors sampling at multi-megahertz rates to evaluate three simultaneous conditions:

1. Broadband High-Frequency Energy: Detects elevated RF power in the 10–50 MHz band via an internal inductive pickup coil.
2. Synchronous Waveform Flat-Spots: Verifies that current cessation aligns precisely with the 50 Hz supply voltage zero crossings.
3. Cycle-to-Cycle Irregularity: Normal motor commutators (e.g. power drills) produce regular, periodic spark harmonics synchronized to rotor RPM. In contrast, thermal air turbulence and molten copper sputtering create highly chaotic, non-repeating cycle envelopes.

4. Statutory & Regulatory Context (BS 7671 & BS EN 62606)

Under BS 7671:2018+A3:2024 Regulation 421.1.7, AFDDs complying with BS EN 62606 are mandated for final AC circuits supplying socket-outlets up to 32 A in Higher Risk Residential Buildings (HRRBs), Houses in Multiple Occupation (HMOs), and purpose-built student accommodation.