Arc Mapping in Fire Investigation

Arc mapping is a survey method. You locate electrical arcing damage on circuit conductors, then plot each site on a scaled diagram of the structure. The distribution of those sites can help bound the area of origin. The limits deserve the same attention as the data.

What arc mapping is

Arcing requires an energized conductor and a fault path. Fire supplies the fault path. Heat degrades insulation, insulation fails, and conductors short to one another or to a grounded surface. The arc leaves localized damage at the point of failure.

Arc mapping treats each of those points as a data point. An arc site is consistent with fire reaching that conductor while the circuit was energized. Other sources produce arcing on the same wiring. Mechanical damage during suppression, overhaul, or collapse can fault an energized conductor long after the fire started. Pre-existing damage from a staple, a nail, or a pinched conductor can predate the fire entirely. Work through those alternatives before you treat a site as fire related.

Survey the wiring across the involved area and you have a set of mapped sites, fixed in space on a diagram.

What the survey supports

Arc mapping is one input among several. It works alongside fire pattern analysis, heat and flame vector analysis, depth of char and calcination surveys, witness statements, and fire dynamics analysis. Agreement across independent methods strengthens an origin hypothesis. Disagreement calls for more work before you write.

• Bounding the area where fire reached energized wiring
• Testing an origin hypothesis developed from fire patterns and witness accounts
• Corroborating or contradicting a reported point of first observed flame
• Identifying areas that warrant closer excavation
• Establishing that a given circuit was energized when fire reached it

Arc severing and melting

An arc is a short, high energy event confined to a small area. It removes metal at the point of contact and leaves the adjacent conductor at or near its original diameter. Severed ends are often blunt or notched. Resolidified copper forms beads or craters with a sharp line of demarcation against undamaged metal.

Fire melting acts over length and over time. Copper held at sustained high temperature thins gradually, tapers, sags, and can form drips and pooled masses. The transition from damaged metal to sound metal is gradual. Melting commonly appears on many conductors in the same area at similar elevation, without regard to which circuit they serve.

The descriptions above apply to copper. Aluminum melts at a lower temperature and behaves differently under fire exposure. Aluminum branch and feeder conductors can be consumed over long runs, leaving globules, oxide residue, or nothing recoverable. Severing criteria developed for copper transfer poorly to aluminum, and the housing stock still holds aluminum branch wiring. Identify the conductor metal before you apply any severing criteria. Record the metal for every site.

Arcing through carbonized insulation can produce a series of small sites along a length of cable. Treat that series as one related event. Record its full extent, both ends and every site between.

Field indicators

The indicators below describe copper conductors. Confirm the conductor metal first.

Indicators consistent with arcing:

• Localized damage with a sharp boundary against unaffected conductor
• Original conductor diameter preserved immediately adjacent to the damage
• Beads, craters, notches, or blunt severed ends
• Damage on one or two conductors while neighbors in the same cable remain intact
• Corresponding damage on a nearby grounded surface, box, raceway, or cable strap
• A matching mark on the opposing conductor or the fault surface

Fire and mechanical damage

Indicators consistent with fire melting and mechanical damage:

• Gradual thinning and tapering over a length of conductor
• Rough, porous, or irregular surfaces
• Drips, pooled metal, and resolidified masses below the conductor run
• Similar damage on multiple adjacent conductors regardless of circuit
• Damage distributed along the heat gradient and independent of circuit routing
• Clean shear or stretch failures consistent with suppression, overhaul, or collapse

Circuit tracing and the panel

An arc site has limited value until you know which circuit it belongs to and where the overcurrent protection sat. Panel work comes first.

• Photograph and document the panel before anything is moved. Record every breaker position and rating.
• Treat found breaker and switch positions as evidence to be verified. Heat, collapse, suppression, and handling by fire or utility personnel can change a handle position.
• Record who accessed the panel, when, and what they reported doing.
• Identify the service, the main, all subpanels, and the state of each disconnect.
• Identify any alternate supply: generator, transfer switch, photovoltaic array, battery storage, uninterruptible power supply.
• Determine when utility power was removed, by whom, and at what point in the incident.
• Trace each damaged conductor back toward its overcurrent device.
• Record cable type, conductor metal, conductor count, and gauge at each site.
• Note fixed appliances, receptacles, and devices served by the circuit.

Sequence and energization

Once an overcurrent protective device opens, conductors downstream lose power from that source. Arcing found downstream of an interruption point can indicate that the arc preceded operation of the device.

A circuit with no arcing damage supports little on its own. The circuit may have been switched off, de-energized early, protected before fire reached it, or never involved.

The sequencing inference is conditional. Verify and document each of the following before you rely on it.

• Establish that the mapped conductor was the energized party. A de-energized conductor can take arc damage from an adjacent energized one.
• Establish that the device was not reset, restored, or replaced during or after the incident.
• Exclude alternate supplies feeding the circuit downstream of the interruption point.
• Verify the found position of the device against witness accounts and physical evidence.
• Document the circuit tracing that connects the mapped site to that device.
• State in the report which conditions you verified and which you could not.

Documentation

Suppression, overhaul, and debris removal move wiring. Document position before anything is lifted. A site recorded after the conductor has been pulled loses most of its mapping value.

• Establish a scaled floor plan before excavation. Grid the structure and work the grid.
• Photograph each conductor in place with an overall, a mid range, and a close up frame.
• Fix each site by measurement to two permanent references, and record elevation.
• Tag conductors before cutting. Mark the direction toward the panel.
• Record the circuit number and the corresponding breaker for every site.
• Cut well away from the damage. Package to prevent contact damage to severed ends.
• Maintain chain of custody for every item removed.
• Plot sites on the diagram as you go. Use a legend that separates confirmed arcing from suspected arcing.

Limits

Arc mapping supports origin analysis. Cause is a separate determination resting on separate evidence.

Laboratory examination of arc bead microstructure has not been shown to reliably separate an arc that caused a fire from an arc produced by a fire. Treat any claim to the contrary with care, and be prepared to address it on cross examination.

Conditions that reduce or eliminate the value of the survey:

• Structures with little recoverable wiring in the involved area
• Aluminum conductors consumed to the point that severing criteria cannot be applied
• Heavy collapse or deep debris loading
• Circuits de-energized before or early in the fire
• Extensive post fire disturbance before your arrival
• Older or unclear wiring methods with routing you cannot confirm
• Areas served by very few circuits, which yield too few points to bound anything

Keeping the determination defensible

NFPA 1033 defines the professional qualifications and job performance requirements for the fire investigator. Origin, cause, and classification are determinations the certified investigator makes. A survey method produces data. A diagram organizes it.

State plainly what the arc data shows and where it stops. Record the alternate hypotheses you considered and the basis for setting each aside. Note the data you could not obtain and the effect on your confidence. Undetermined remains a valid finding. A well documented undetermined holds up under examination.

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