A Bead on the Neutral: Arc Mapping Without an Established Origin
A 1998 wood-frame house; fire damage is confined to one second-floor bedroom. The bedroom branch circuit is 14/2 NM-B copper on a 15 A single-pole breaker. Evidence: (1) a 10 mm copper bead with a sharp, irregular, slightly porous surface on the white neutral conductor where the cable was stapled to a stud 0.3 m above the floor plate; (2) the black conductor at that staple shows 40 mm of missing insulation and bright, unoxidized copper; (3) the breaker contacts are pitted but the handle is ON, and the homeowner says no breaker was ever reset; (4) no tracking or arcing is visible in the panel; (5) the deepest charring in the room is on the wall containing that stud, with a 0.6 m wide area of charred subfloor at its base. You have found no other arcing anywhere on the circuit.
1. What physical characteristics distinguish an arc bead from copper melted by fire, and what must you do before calling this bead an ignition source?
Read the worked answer
Arc origin: an arc concentrates energy at a small root, melting a tiny volume of copper at its 1083 degrees C melting point and resolidifying it as a small, sharp-edged, sometimes porous or cup-shaped bead, typically with a localized carbonized track or pinhole in the adjacent insulation and a sharply defined transition from melted to unaffected metal.
Fire origin: a conductor in a compartment fire is heated along a length, oxidizes heavily (black cupric and red cuprous oxide), necks and sags, and its insulation is consumed over a broad span rather than at a point. Its surface is dull and oxidized; an arc bead is bright or has a thin, distinctly structured oxide.
Judgment point: a compartment fire can exceed 1000 degrees C in the plume, so the presence of a bead proves only that copper was melted, not that an arc caused it. The bright unoxidized copper on the black conductor cuts both ways: it may indicate a protected, recently exposed surface, or it may simply be where the fire never reached after the metal was exposed.
Required before opinion: have the sample examined metallurgically (SEM/EDS, cross-section) with 300 mm of conductor on each side, cut with a saw rather than side-cutters to preserve the bead; confirm the circuit was energized and its overcurrent protection intact; reconstruct the circuit path and panel schedule; and if you lack the metallurgical background, retain a qualified electrical engineer rather than extrapolating from appearance.
2. Apply arc-mapping logic. Can you classify the cause as electrical on this evidence? What classification is defensible, and why?
Read the worked answer
Arc mapping principle: arcs on a circuit that lie downstream of, or within, the area of fire origin are as likely to be consequences of fire attack on energized conductors as they are to be causes. Arcing outside the fire-damaged area, or a pattern of multiple arcs converging toward the origin, has probative value. A single bead inside the area of heaviest damage has very little.
Applying it here: the bead is on the wall with the deepest charring and sits at the base of that wall where a fire would burn longest and hottest. That is exactly where a fire-caused arc is expected. There is no second arc, no upstream pattern, and no independent origin determination pinpointing that stud.
Breaker evidence: pitting at the contacts is consistent with arcing at the panel, but the panel shows no tracking and the handle position proves nothing about the pre-fire state, because a breaker can be reset by an occupant. The homeowner's statement is a statement, not physical evidence.
Defensible classification: area of origin in that corner is well supported; the cause is undetermined, with electrical as one unranked hypothesis to be tested. Declaring an electrical cause on one bead is the conclusion the evidence cannot carry, and it is the kind of opinion that fails a reliability challenge.
3. The cable is 14 AWG on a 15 A breaker. Does that change your analysis, and what additional documentation will you collect to close the case?
Read the worked answer
Ampacity: 14 AWG copper on a 15 A breaker is code-compliant, so the installation itself is not a defect and cannot be used to argue an overload cause. That removes one hypothesis and sharpens the inquiry toward a mechanical or connection fault.
The staple hypothesis: a staple driven tight against the cable is a recognized site for insulation damage and a high-resistance series connection that can glow and carbonize without drawing enough current to trip a thermal-magnetic breaker promptly. It is a plausible mechanism, but plausibility is not proof, and the same staple sits in the hottest part of the room.
Collect and document: the full circuit path and every outlet on it; the panel schedule and the breaker's rating and trip curve; the load connected at the time of the fire; the removed cable in labeled, sealed packaging with photographs in place before cutting; the receptacle and any cord ahead of it; and a comparison set of undamaged cable from the same run. Interview the occupant about prior symptoms (flickering, warm plates, a breaker that tripped and was reset).
Bottom line: the correct product of this work is a documented, tested hypothesis set, not a cause.