Metallurgical analysis of the ‘cause’ arc beads pattern characteristics under different short-circuit currents

Highlights

• Bead pattern characteristics were divided into six types.

• The relationship between the number of arc beads and the diameter was obtained.

• Oxygen content and arc temperature determined the oxide form.

Abstract

Electric arc beads commonly cause large fire and explosion accidents, especially during the powder process industries. During such fires, the original equipment is often severely burned, rendering electrical fire investigation impossible. Therefore, identifying the beads caused by electrical faults through metallurgical investigation is crucial. This study focused on the ‘cause’ of arc beads and determined their pattern characteristics through metallurgical analysis of copper wires with short-circuit currents of 100, 160, 200, and 240 A. According to microstructure characteristics and metallographic knowledge, bead pattern characteristics were divided into six types: hypereutectic (A), dendritic eutectic (B), chilled layer (C), cellular crystal (D), dendritic (E), and coarse columnar crystal (F). Oxide levels of each category were A (4.5%–7.6%) > C (2.0%–3.8%) > B (1.1%–2.4%) > D (1.0%–2.1%) > E (0.6%–1.9%) > F (0.1%–0.7%). When the short-circuit currents were the same, there were a greater number of small particle beads than large ones. As the current increased, the number of distributed beads with a particle size d < 1 mm decreased, with 1 mm ≤ d < 2 mm increased, and with d ≥ 2 mm were stable. Bead size and distribution had a negative linear correlation at 200 A. When the current increased, the proportion of beads with A and C structures gradually decreased, and those with D structures increased to more than 50%. Oxygen content and arc temperature determined the oxide form. These results are vital for electrical fire investigations and provide theoretical support for fire material evidence extraction and cause identification.

Introduction

Data from US fire departments indicate an estimated 45,210 home structure fires annually between 2010 and 2014, in which several types of electrical failure or malfunction contributed to ignition. These fires caused 420 deaths, 1370 injuries, and USD1.4 billion in direct property damages annually (Babrauskas, 2008, 2017a). Electric arc beads pose an ignition hazard in flammable gas mixtures exist, which commonly cause large fire and explosion accidents, especially during the powder process industries (Klippel et al., 2015, Li et al., 2020). Moreover, electrical equipment failure arc ignition of flammable materials is the main cause of industries fires. Arc faults are more difficult than other electrical faults to detect and quickly fix and can readily cause serious consequences. The overload is severe enough to melt the wire, such as in short-circuit faults, which typically eject hot particles and have centre temperatures approximately 6500–12,000 K (Rallis and Mangaya, 2002). Melted metal particles can ignite flammables or other combustibles, expand the combustion range, and increase fire risk (Babrauskas, 2001; He et al., 2016; Kobayashi et al., 2017; Yuan et al., 2018; Dubaniewicz and DuCarme, 2016). Therefore, for electrical fire investigation, the arc beads patterns as a vital evidence, which is particularly essential to accurately identify patterns characteristics.

Research on metal melting patterns formed by arcs concentrates primarily on copper and aluminium wires in electrical systems. In 1971, Takaki used the metallographic method to distinguish between short-circuited beads and globules (Takaki, 1971). Subsequently, numerous scholars have used stereo microscopes, metallographic microscopes, scanning electron microscopes, Raman spectroscopes, and other instruments to systematically analyse the short-circuit and fire-melting beads of copper and aluminium wires (Gray et al., 1983; Babrauskas, 2003a; Ao et al., 2014; Wright et al., 2015). In 2012, China issued the national standard ‘GB/T16840 electrical fire pattern evidence technical identification method’ to distinguish typical characteristics and methods of copper and aluminium wire arc high-temperature melting metal patterns (GB/T 16840, 2012). In 2015, Wang studied the beads of copper wire with the density and metallographic methods (Wang et al., 2015a). Singh proposed that arcing results in obvious CuO or Cu₂O grain structures near the end, gradually decreasing from the end (Sai et al., 2008). Most studies have examined the residual patterns of copper–aluminium wire ends, but few have investigated the particle patterns of hot beads. At present, the University of California, Berkeley is a research leader in investigating fires caused by hot particles and has cooperated in in-depth research with the University of Science and Technology of China (Wang et al., 2015b, 2015c; Yang et al., 2016). In 2011, Rory heated steel balls of various sizes to a certain temperature to study the effects of particle size on ignition and determined the critical conditions of the temperature and size of metal particles required for open flame combustion and smouldering (Hadden et al., 2011). In 2015, the James team at the University of California, Berkeley, heated variously sized metal particles of stainless steel, copper, and aluminium to different temperatures and analysed the ignition capacity of hot particulate matter for underlying fibre flammables (Zak, 2015; Urban et al., 2015, 2017, 2018, 2019). Relevant studies have generally focused on heating metal particles to certain temperatures for experimental research. These simplified theoretical models neglect that metal splashing out of the arc molten pool is still in a high-temperature liquid state and must undergo a series of processes, such as metal liquid cooling, solidification, and metal oxidation. These processes have a considerable effect on ignition capacity, thereby affecting the evolution of pattern characteristics of arc beads.

Because they are located at the bottom of the collapsed accumulation layer, arc beads are minimally damaged by fire, and their pattern characteristics are easy to retain. Therefore, arc beads are crucial to proving the occurrence of electrical faults. This study investigated the causal factors of arc bead pattern characteristics by applying metallographic analysis of copper wire exposed to various short-circuit currents and analysed the differences among metallographic characteristics, corresponding oxide content, and beads sizes.