Globules and Beads: What Do They Indicate About Small-Diameter Copper Conductors that Have Been Through a Fire?

Abstract

Many fire investigators believe the presence of a rounded mass of re-solidified copper at the end of a small-diameter copper conductor means the conductor was energized at the time of the fire. While testing performed in the course of fire investigations by others and in this work has demonstrated that beads of re-solidified copper may be present on conductors known to have voltage present at the time of the fire, there may also be globules of re-solidified copper on conductors known not to have been energized. Even if the copper conductor orientation during a fire is considered, the external appearance of these beads and globules on small, 34 AWG conductors has not been found to be a reliable indicator of the cause of formation, often requiring further analysis. In an effort to identify and determine potential differences between beads created by electrical faulting alone and globules created by fire attack, testing was performed in which 34 AWG conductors were exposed to laboratory flames and burning bedding with and without being energized. The resulting beads and globules were then analyzed considering both their external appearances and internal microstructures. The analysis of the internal microstructures considering grain boundaries, voids, damage, and signs of re-melting provided insight concerning the cause of such features on tinned, 34 AWG conductors that can be found at fire scenes. Specifically, in unpowered 34 AWG conductors the copper microstructure variations between globules and the immediately adjacent wires has shown that at least a 432°C temperature differential can be created without electrical faulting in this work. Beads created by electrical faulting alone have been found to have characteristics that distinguish them from globules caused by fire attack in this work as well. The microstructures in beads resemble the heat-affected and fusion zones of a fusion weld, while the proximal unmelted wires do not reach the annealing temperature of copper. This condition creates a sharp line of demarcation between the melted and unmelted conductor surfaces as a result of the localized heat from electrical faulting seen within the beads. This sharp line of demarcation can be used to identify beads as opposed to globules.

A Diferenciação da Pérola de Fusão Primária e Secundário por Meio da Metalografia

“…Apesar de não haver uma característica definitiva para a diferenciação, verifica-se que há uma tendência na literatura em reconhecer características distintivas entre fios energizados e não energizados, representadas por uma linha de demarcação devido às diferenças entre os cristais de cobre fundidos e aqueles mantidos a temperatura ambiente 2,5,6,9,13,14 . Para se chegar a uma conclusão pericial, essas características devem ser corroboradas com os demais elementos materiais do local dos fatos, exigindo o conhecimento da cena do incêndio e a aplicação do método científico.…”

“…Alguns dos artigos 14 confirmaram a existência de um gradiente acentuado de temperatura nas pérolas criadas por falhas elétricas após o corte da seção transversal, indicando que o exame das microestruturas pode fornecer um método para identificar a causa, confirmando a aplicabilidade da metalografia.…”

“…As abordagens investigadas pela literatura 14 geralmente incluem: (1) comparação da aparência externa dos condutores após serem expostos a chamas e curtos-circuitos, (2) comparação do cobre para sinais de recristalização em grãos equiaxiais e (3) comparação do teor de carbono entre as esferas e glóbulos. Nessa referência 14 , os autores descobriram que, em casos específicos, as duas primeiras técnicas fornecerão evidências suficientes para apoiar uma conclusão.…”

“…Sendo pesquisa científica relacionada ao incêndio de grande valia na realização de perícias, sobretudo para a determinação da causa e da origem do incêndio, o presente trabalho busca estudar características que possibilitem a referida distinção nos fios de cobre, por meio do exame metalográfico, em condutores obtidos em cenas reais de incêndio. Para tanto, os resultados encontrados serão discutidos e comparados com os achados da literatura mais recente sobre o tema [13][14][15] .…”

Microstructural Characteristics of Arc Beads with Overcurrent Fault in the Fire Scene

“…If continued for long periods of time, the conduction will be molten, and due to the released energy arc beads will be formed where the wires are melted. Analyzing the characteristics of arc beads at the fire scene will provide a reference for determining the cause of the fire [ 7 , 8 , 9 , 10 ]. Using optical microscopy (OM), Takak et al [ 11 ] produced metallographic photographs of short-circuit arc beads for the first time and found that the short-circuit microstructure of an arc bead was composed of fine cellular crystals.…”