65% of home electrical fires start with an arc.

An arc fault is a sustained electrical discharge across a gap or through degraded insulation, hot enough to ignite the materials around it. Arc temperatures can exceed 6,000 °C, well above the autoignition point of cable sheathing, dust, and framing timber.

• Series arc.A break or loose connection in line with a load. Current still flows but jumps the gap, often inside a wall, behind an outlet, or at an appliance terminal.
• Parallel arc.Current jumping between two conductors, or to earth, through damaged insulation. Often caused by a nail, screw, rodent damage, or simple ageing of the cable.

Arcs can persist for weeks or months without tripping conventional protection. They typically announce themselves first as a faint smell, a discoloured outlet, or unexplained flickering. Many do not announce themselves at all.

The NFPA Home Electrical Fires research report^[1] classifies US home electrical fires by ignition mechanism. Two figures combine to produce the 65% claim.

• 88%.Share of US home electrical fires that fall in the NFPA category 'heat from powered equipment', the umbrella for fires caused by energised wiring or devices rather than, for example, a candle near a curtain.
• 74%.Share of fires inside that category attributed to electrical arcing (the rest being thermal overload, friction, and other minor mechanisms).

The figure is not a Basis statistic. It is a direct derivation from primary NFPA data, presented so any reader can reproduce it from the source.

Why other sources cite a lower number

Schneider Electric and the UK IET ^[7]commonly cite “around a third” for arc-fault share of electrical fires. The lower figure counts only fires forensically attributed to a specific arc-fault subcategory (typically excluding short circuits and equipment arcing). The 65% derivation here aggregates all arcing inside NFPA's parent “heat from powered equipment” class, which is the population AFDD protection addresses. Both figures describe the same underlying ignition mechanism.

A default residential switchboard ships with two protective devices on each circuit. Neither is designed to detect an arc.

• Miniature circuit breaker (MCB).Trips on sustained overcurrent, typically at multiples of the rated current. A series arc draws less than the rated load. The MCB sees a normal circuit.
• Residual current device (RCD).Trips on imbalance between line and neutral, i.e. current leaking to earth, at 30 mA in residential. A high resistance parallel arc may leak less than 30 mA, or nothing to earth at all. The RCD sees a balanced circuit.

What distinguishes an arc from a normal load is the shape of the current waveform, not its magnitude. Detecting it requires a different class of protection that watches waveform signatures rather than amplitude or earth-leakage imbalance. That class is the AFDD (section 04).

The arc fault detection device, or AFDD, is the international product class designed for this job. IEC 62606^[2] specifies how an AFDD must analyse the current waveform, what arc signatures it must recognise, and the maximum trip times at each current level. Any AFDD product claim should reference back to IEC 62606 testing.

AFDDs use signal processing on the live circuit waveform to detect arc signatures, then open the circuit when those signatures persist beyond a defined threshold. The decision is made on waveform shape, not on current magnitude, which is what closes the gap left by the MCB and the RCD.

AS/NZS 3000 ^[3] (the Wiring Rules every NZ electrician works to) mandates RCD coverage on residential lighting and socket circuits. It does not require AFDD protection. A switchboard installed last week to the current standard contains nothing that catches an arc.

FENZ data on the resulting fire load:

• 2,378.House fires in New Zealand in 2025. 15 of them fatal [5].
• 830 of 2,432.Structure fires attributed to electrical causes in 2023, roughly 1 in 3 [4].

Internationally, the US National Electrical Code (NEC 210.12)^[6] has required AFCI (the US equivalent of AFDD) on most residential branch circuits since 2014. The European and UK regimes encourage AFDD use without yet mandating it universally. New Zealand currently sits behind both regimes on this specific protection class.

Every Basis circuit module samples its current waveform at more than 10,000 samples per second. The on-device arc-fault model evaluates each circuit against the IEC 62606 arc signatures continuously, cycle by cycle, with no homeowner action required.

• Per circuit, not per board.Detection runs independently on every circuit. A failure on the oven branch does not blind the lighting branch.
• Continuous self-test.The board verifies its own protection chain on a schedule, so silent failure of a protective device surfaces in the app rather than at the moment it was supposed to act.
• Updateable.The arc detection model improves over the field life of the board. Updates ship over the air to every installed unit.