Protection

AFDD (Arc Fault Detection Device)

Listens for the crackle of a bad connection — and trips before the arc becomes a fire.

Photorealistic arc fault detection device on a DIN rail
Real part — tap a marker

AFDD (Arc Fault Detection Device)

Listens for the crackle of a bad connection — and trips before the arc becomes a fire.

Terminals
  • L in / N in (top) — supply from the MCB or RCBO upstream
  • L out / N out (bottom) — the protected final circuit
  • Functional earth / neutral fly-lead on some designs
  • Microprocessor — samples current and looks for the arc signature
  • Series and parallel arc detection — broken conductor vs L-to-N tracking
  • Tripping relay — opens both poles on a detected arc
  • Toggle lever and test button
  • DIN-rail clip and cable shield
Cutaway of an AFDD arc fault detection device showing the microprocessor board, arc-sensing transformer, filtering and the tripping relay
Cutaway — tap a marker
⚡ Meet it in a circuit

Terms used here

Defined in the glossary — each one opens at its own entry.

⚠ Safety note

An AFDD does not replace an MCB or an RCD — it is normally an AFDD-RCBO that combines all three in one module. Never fit one to defeat nuisance tripping: repeated arc trips are telling you a connection, a flex or an accessory is breaking down, and that fault needs finding.

More components
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Now wire it for real.

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