Arc mapping
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Arc mapping is a technique used in
fire investigation Fire investigation, sometimes referred to as origin and cause investigation, is the analysis of fire-related incidents. After firefighters extinguish a fire, an investigation is launched to determine the origin and cause of the fire or explosion ...
that relies on finding the locations of electrical arcs and other
electrical faults In an electric power system, a fault or fault current is any abnormal electric current. For example, a short circuit is a fault in which a live wire touches a neutral or ground wire. An open-circuit fault occurs if a circuit is interrupted by a fa ...
that occurred during a fire; the locations of the electrical faults can then, under some circumstances, indicate the progression of the fire over time. It is usually performed by a
forensic Forensic science, also known as criminalistics, is the application of science to Criminal law, criminal and Civil law (legal system), civil laws, mainly—on the criminal side—during criminal investigation, as governed by the legal standard ...
electrical engineer Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems which use electricity, electronics, and electromagnetism. It emerged as an identifiable occupation in the l ...
. It The technique relies on the assumption that, when heat from fire (or from hot gases caused by the fire) impinges on an electrical line (whether or not protected by a conduit), it will melt the wire insulation and cause an electrical fault at the first point that it reaches on the electrical line. For this to occur, the electrical line must be energized at the time that fire hits it.


Origin of fire

Arc mapping often aims to determine the point of origin of the fire. A common assumption used in reaching this goal is that the fire expands uniformly in all directions as it burns, and maintains a circular shape centered on the point of origin; therefore, when an electrical fault occurs on an electrical line, the point of origin is on a perpendicular to that point. Contrary to this assumption, however, in many cases fire does not extend uniformly; in particular, large local
fuel loads A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy or to be used for work. The concept was originally applied solely to those materials capable of releasing chemical energy but ...
, venting, and air currents have a strong effect on fire progression. Several other factors affect the locations where electrical faults occur. Electrical faults do not always occur where fire first reaches a conduit, but preferentially occur at bends in a conduit or at locations where wires are pressed together. The elevation of an electrical line has a strong effect on its exposure to heat, since temperatures in a fire are generally highest near ceiling level, except in the immediate vicinity of the point of origin. Protection from the fire is an important consideration: being located within a wall or being covered in fiberglass insulation will offer some protection to an electrical line, and will delay electrical faults.


Electrical faults

An electrical fault will only occur if an electrical line is energized - more specifically, if two conductors are at different potentials, ''and'' have the capacity to source/sink significant current. An electrical arcing event will often (though not always) cause a loss of power downstream on the electrical line, such as by severing the conductors or tripping a
circuit breaker A circuit breaker is an electrical safety device designed to protect an electrical circuit from damage caused by an overcurrent or short circuit. Its basic function is to interrupt current flow to protect equipment and to prevent the risk ...
; as a result, a significant part of arc-map analysis is determining the order in which sections of electrical lines lost power. In most cases, the electrical faults that occurred furthest downstream occurred first. Electrical faults can also energize conductors or components that are not normally energized, such as conduits (normally grounded). ("Energized" usually refers to a conductor being connected to hot power, as opposed to
neutral Neutral or neutrality may refer to: Mathematics and natural science Biology * Neutral organisms, in ecology, those that obey the unified neutral theory of biodiversity Chemistry and physics * Neutralization (chemistry), a chemical reaction in ...
. More generally, electrical faults occur between points at different potentials, such as between hot and neutral, between hot and ground, or between hots of two different phases.)


Electrical arcing

Investigators distinguish between electrical arcing and copper that melted due to the high temperatures of the fire. The term "electrical arcing", in fire investigation, often refers to "melted copper that indicates that electrical arcing occurred". "Melted copper" generally refers not to copper that is currently melted, but to "previously melted, resolidified copper".
NFPA 921 NFPA 921, "Guide for Fire and Explosion Investigations", is a peer reviewed document that is published by the National Fire Protection Association (NFPA). Its purpose is "to establish guidelines and recommendations for the safe and systematic inv ...
and ''
Kirk's Fire Investigation ''Kirk's Fire Investigation'' by David J. Icove and Gerald A. Haynes has long been regarded as the primary textbook in the field of fire investigation.J.F. Decker and B.L. Ottley, Arson Law and Prosecution, Carolina Academic Press, , 2009, p. 22 ...
'' give some guidelines and illustrations on distinguishing between electrical arcing and other melted copper; however, these strict guidelines occasion much debate.


References

{{reflist Fire Forensic techniques