Arc mapping, a technique used in fire investigation is usually performed by a forensic electrical engineer. It relies on finding the locations of electrical arcs and other electrical faults that occurred during a fire; the locations of the electrical faults can then, under some circumstances, indicate the progression of the fire over time. 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.