Accepted Manuscript Title: Analysis on Micro-regional Characters of the Resistance Wires Surface in Electrical Immersion Heater by SEM/EDX Author: Haixin Gu Lin Xue Yongfeng Zhang Liying Cao Minju Ding Hao Huang Renlie Bao PII: DOI: Reference:
S0379-0738(14)00094-2 http://dx.doi.org/doi:10.1016/j.forsciint.2014.03.001 FSI 7532
To appear in:
FSI
Received date: Revised date: Accepted date:
19-3-2013 2-3-2014 3-3-2014
Please cite this article as: Haixin GuLin XueYongfeng ZhangLiying CaoMinju DingHao HuangRenlie Bao Analysis on Micro-regional Characters of the Resistance Wires Surface in Electrical Immersion Heater by SEM/EDX (2014), http://dx.doi.org/10.1016/j.forsciint.2014.03.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Analysis on Micro-regional Characters of the Resistance Wires
2
Surface in Electrical Immersion Heater by SEM/EDX
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Haixin GU1*, Lin XUE1, Yongfeng ZHANG1, Liying CAO1, Minju DING2, Hao HUANG1, Renlie BAO1 1, Shanghai Fire Research Institution of the Ministry of Public Security, 918 Minjing Road, Shanghai, China 2, Shanghai Institute of Forensic Sciences, 803 Zhongshan North 1 Road, Shanghai, China
*Corresponding author information: Haixin GU, Email：[email protected], Tel: +86-021-65240172, Fax:+86-021-65240052
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Analysis on Micro-regional Characters of the Resistance Wires
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Surface in Electrical Immersion Heater by SEM/EDX Abstract: We present a novel method for identifying the conditions of electrical resistance wires before fire basing on micro-regional characters. The morphology and elemental compositions of resistance wires surface under normally used, misused and external heating conditions were respectively studied by scanning electron microscopy (SEM) and energy dispersive X-Ray spectroscopy (EDX). The results show that the normally used wire has a smooth surface and stable metal composition. While the resistance wire under misused condition for the certain time could be characterized as a coarse surface covered by compacted and silica-embedded oxide layer, and the proportion of elemental compositions of different metals is also changed with higher content of Al and lower content of Fe and Cr. The external heating condition has little influence on inside wires. The mechanism of changes formed under misused condition was explained. We demonstrate that this approach is practical and functional to aid fire investigators in determining the cause of related fires.
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1. Introduction
32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55
The incidences and cost related with fires caused by electrical immersion heater have been increasing recent years. For the fire investigation involving laboratory analysis of this kind of fire materials, one of major tasks is to determine whether the heaters are ever under electrical accident or misused before fire. Short circuit as the electrical damage may be a potential ignition source in many cases, and much attention has been paid on electrical wiring and its identification methods [1~4]. However, the heaters themselves do not generally cause fire [5]. One of the most probable causes of fire is that the heating element contacted with and thus ignited the surrounding combustible materials under misused condition [6]. The major difference of heating element between normally used and misused condition is the external surroundings, namely, using water as heating medium for the former and directly exposed to air for the latter. For the selection of sample to be examined, the heating tube is directly influenced by high temperature of fire scene, which may complicate the discriminations of self-forming and fire-burning. On the other hand, it suffers from the drawbacks of contaminants from fire scene and rust by oxidation. Hence, the selection of inside resistance wire as analytical object seems appropriate. However, up to date, few studies have generally been performed on fire identification methods on this subject. Since the heaters under misused condition differ from the normally used ones, we have reasons to make an attempt to find out what happen to the wires. Scanning electron microscopy coupling with an energy dispersive X-ray spectroscopy (SEM-EDX) is a non-destructive detection method which can provide combined information of both morphological structure and elemental compositions. Consequently, SEM-EDX would also be supposed to serve as a powerful tool for forensic purpose of providing fire investigators with technical support by determining the conditions resistance wire once experienced before fire. In the present study, the micro-regional characters of morphology and metal elemental
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Keywords: Fire investigation, immersion heater, resistance wire, surface morphology and elemental compositions, identification
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compositions of resistance wires under normally used and misused condition were respectively investigated by SEM/EDX. We specially characterized the differences and then analyzed the mechanism of transformation phenomenon. The external heating experiment was also conducted to help in understanding whether fire would affect internal wires. To the best of our knowledge, we find out for the first time that the SEM/EDX detection on electrical resistance wires was used for fire cause identification.
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2. Material and methods
63 64 65 66 67 68 69 70 71 72 73 74 75 76 77
2.1 Material and instrumentation The electrical heating coil (Hongjian Electric Appliance Plant, HJ-555-1) was used as a typical immersion heater. This product consists of heating tube, silica sand and internal resistance wire. The silica sand is filled in the tube not only to fix wire but also serve as heating medium. The obtained samples were divided by cutting machine (Struers Inc. Labotom-3), and 2~3 cm length of resistance wire was taken from the central part of heating tube. Attachment on the surface of wires was removed by ultrasonic cleaner (Shanghai Kudos Co., LTD, SK200H) for 2 mins. SEM/EDX instrument (JEOL Inc., JSM-6390LA/EX-54175LMU) was used for detection at magnifications from 30× to 2000×. Due to silicon sand may be distributed at random in scanning area, relative percentage method was used to calculate each metal composition in order to avoid the interference of non-metal compositions. The temperature of internal wire could not be measured directly in this work. However thermal infrared imager (FLIA Systems Inc., SC620) was used to observe change and distribution of the temperature on tube surface and indirectly investigate the temperature of wire. Muffle Furnaces (Themo Fisher Scientific Inc. F48000) was used to simulate fire burning condition.
78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95
2.2 Preparation of samples under different conditions Following experiments were respectively performed to simulate three different conditions. Fig. 1 shows the design of experiment process. i) To simulate the normally used condition (NC), the electrical heating coils were immersed and electrically heated in water. The water would maintain boiling for a certain duration time, and then the electrical power would be turned off. After the electrical heating coils naturally cooled down to room temperature, this process would be repeated. Sample NC1, NC2 and NC3 were prepared with different single duration time and cumulative time, 10minutes and 60hours(NC1), 30minutes and 60hours(NC2), 60hours and 180hours(NC3) respectively. The water boiled at about 7 minutes and was supplemented timely. ii) In the experiments of misused conditions(MC), the electrical heating coils were exposed to air. In order to investigate the thermal-induced change processes of wire, the coils were heated for a certain time of 30s(MC1), 45s(MC2), 1 mins(MC3), 2 mins(MC4), 3 mins(MC5), 5mins (MC6), 10mins (MC7)and 20mins (MC8) respectively. iii) The influence of external fire on the inside electrical wires cannot be ignored. Since the reconstruction of an fire scene is always unavailable, muffle furnace was used to conduct experiment of burning condition (BC). The normally used(NB) wire and misused heaters(MB) wire were heated for 30mins at temperature of 600 (1), 800 (2), and 1000 (3) respectively.
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3.1 SEM/EDX analysis of electrical wires under normally used condition The new wire has a smooth, even and clear striated surface without silica sands attached (Fig.2 a, b), and composes of three metal elements, namely Fe, Cr and Al of 70.62%, 24.80%, and 4.58% respectively(Table 1). Samples of NC1 and NC2 were used for the comparison of different usage, while sample of NC3 for accelerating the aging process. Tab.1 shows that metal compositions of normally used wires are slightly changed by compared with the new ones. No significant variation tendency could be found when the usage duration of electrical wire advanced. The relative contents of Fe, Cr and Al reach a stable proportion at about 73%, 21% and 6% respectively. It is also demonstrated that both different usage method and even long time aging make little influence on morphological features of wire surface under normally used condition (NC), as showed in Fig.2.
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3. Results
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Fig.1 Schematic representation of the experimental design
Fig. 2 SEM images of new wire (a and b) and normally used wire NC3(c and d) with different magnifications Table.1 Temperature of heating tube and relative content of metal composition of wire Sample
Number
Temperature of tube /
relative content of metals(%）
Sample
Temperature of tube /
relative content of metals(%)
Fe
Cr
Al
Number
Fe
Cr
Al
New wire
/
70.62
24.80
4.58
MC5（3min）
955
54.56
17.02
28.42
NC1
/
73.42
20.63
5.95
MC6（5min）
937
52.07
15.71
32.23
NC2
/
72.76
21.14
6.10
MC7（10min）
951
30.60
15.01
54.39
NC3
/
73.11
20.88
6.01
MC8（20min）
956
30.12
14.87
55.71
MC1（30s）
80
74.67
20.26
5.06
NB3
1000
74.04
20.02
5.94
Page 4 of 9
MC2（45s）
186
73.24
20.58
6.18
MB3
1000
32.31
14.22
MC3（1min）
492
66.46
21.30
12.24
Area a
/
72.93
20.43
6.64
MC4（2min）
847
59.57
19.33
21.10
Area b
/
33.68
15.52
50.80
53.47
113
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3.2 SEM/EDX analysis of electrical wires under misused condition Fig.3 indicates the progression of morphological changes of wire surface under misused condition. The misused wire within 45 secs is hard to discriminate by comparison with the normally used ones (Fig.3a and Fig2d). The silica sand begins to gather and attach on some part of wire surface at the time of 1 min (Fig.3 b), making wire surface coarse. In the next 1 minute, an oxide layer (red arrow) embedded with silica sands (white arrow) could be clearly observed on the surface of electrical wire (Fig.3c). As the degree of transformation proceeds, silica sands would collect and grow up (white arrow in fig.3 d, e). Furthermore, the oxide layer becomes compact and thick (white arrow in Fig.3e). Fig.3 (f) demonstrates that this phenomenon happens on the whole wire surface. Meanwhile, as shown in table 1, metal elemental compositions of wire are also changed. Briefly, the relative content of aluminum greatly increases, nearly 9 times that of NC from 6.01% to 55.71% within 20mins. Conversely, the relative contents of ferric and chromium decrease gradually from 73.11% to 30.12% and 20.88% to 14.87%, respectively.
127 128 129 130 131 132 133 134 135 136 137
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Fig. 3 SEM images of misused wires: a) MC2; b) MC3; c) MC4; d) MC6; e and f) MC8
3.3 Influence of external temperature on internal wires In fire scene, the high temperature is a definite factor possibly influencing the internal wires. However, few changes could be found in normally used wires as well as misused ones before and after the experiments of burning condition (Fig.4 and Tab.1) at the given temperature. The high temperature from outside surroundings could not make the surface of normally used wire coarse and neither make its metal compositions changed (Tab.1). Furthermore, the characters of wire caused under misused condition still remain. The electrical wire could be well protected by tube, and especially by fire-resistant silica sand. We demonstrate that this method would practically avoid the influence of fire and make the identification results more dependable.
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Fig. 4 SEM images of wires under external burning condition: a & b) normally used wire (NB3), c and d) misused wire of (MB3)
4. Discussion
142 143 144 145 146 147 148 149 150 151 152 153 154 155
A reasonable explanation for this dissimilarity in both microscopic features and metal composition is the difference in the course wires experienced. In the process of normally used condition, the heat generated from electrical wires is transferred from intermediate fillings (silica sand) to tube, and then to the external water. When temperature of water reaches the boiling point at 100 , the entire heat transmission process maintains balance. Due to effective release of heat, electrical wire takes the heat and temperature within its capacity and makes its micro-regional character unchanged. However, for the misused condition, due to the poor thermal conductivity of air, the heat generating from tube is incapable to release out efficiently, contributing to increasing temperature of the wire. Thermal infrared images (Fig.5) show that the temperature of tube electrically heated in the air increases remarkably from room temperature to maximum temperature at 955 within 3 mins, and then maintains a new balance at about 950 (Fig.6). When tube temperature reaches about 490 at the time of 1 minute, chemical and physical changes on the wire surface originate. Meanwhile the temperature of internal wire should be much higher than the tube.
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Fig.5 Thermal infrared images of tubes in the air with different heating time: a) 1 min, b) 2 min, c) 3 min
158 159 160 161 162
Along with the wire surface getting coarser under misused condition, the change of metal content is also subjected to duration time and cumulated heating. As shown in Fig.6, during the next 7mins after tube temperature reaching its peak of 955 , the relative content of aluminum still increases from 28.42% to 54.39%, and then tended to a stability. On the contrary, the Ferric and chromium continue to decrease by 23.96%and 2.01% respectively.
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Fig.6 Relative content of metals and temperature of tube with variable electric-heating time
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Fig.7 shows the SEM image and EDX spectrum of area “a” and “b” on misused wire. Their relative contents are respectively showed in table 1. According to the characters of morphology and composition, the uncovered area “a” could be confirmed as a part of original surface of wire, meanwhile the area “b” is the oxide layer visually traced as a coating substance growing from area “a”. This regional discovery would forcefully explain the explanations of phenomenon. When temperature exceeds the capacity of wire, aluminum with lowest melt point is relatively separated outward and precipitated on the surface gradually, forming a molten layer mixed with surrounding silica sands. Once the electrical power is cut off, the wire surface solidifies with cooling and form a compacted and silica-sand-embedded oxide layer. And thus, the coarse wire surface with higher content of aluminum could be witnessed.
175 176 177
Fig. 7 SEM image of wire surface and its EDX spectrums (The picture of “a” and “b” on the right side respectively
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5. Conclusions
179 180 181 182 183 184 185 186 187 188 189 190
In this paper, we have demonstrated a novel method well suited for identifying the conditions of electrical immersion heaters before fire. Internal wire is chosen for detection to obtain original information of the electrical heater and avoid the influence of external fire effectively. The normally used and the misused wire could be identified by the significant differences of morphology and elemental compositions. Namely, the normally used wire has the smooth surface of no attachments and stable metal composition, while the misused one is characterized by the oxide-layer-covered coarse surface where silica sand is embedded and the elemental compositions are redistributed. Analysis with fewer sampling procedure was performed by SEM/EDX visually and metrically. Non-destructive method is employed to detect the target part of sample, making the preservation of physical evidence available. Consequently, the method we presented above could be a well-established technique to aid fire investigators in determining the conditions of electrical immersion heater before fire and its possibility of causing the fire.
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6. References
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[1] V. Babrauskas, How do electrical wiring faults lead to structure ignitions, Fire and Materials Conference. (2005)39-51. [2] Eui Pyeong Lee, Hideo Ohtani, Yoshiyuki Matsubara, Tsutomu Seki, Hideo Hasegawa, Shuji Imada, Isao Yashiro, Study on discrimination between primary and secondary molten marks using carbonized residue, Fire Safety J. 37(2002)353-368. [3] Yasuaki Hagimoto, Hiroki Yamamoto, Analysis of a Soldered Wire Burnt in a Fire, J. Forensic
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showed the EDX spectrum of “a” and “b” area marked by white rectangle in SEM image on the left side.)
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Sci. 51(2006)187-89. [4]H.B. Land, Determination of the cause of arcing faults in low voltage switchboards, 2005 IEEE Electric Ship Technologies Symposium. (2005)125-132. [5]Goodson M, Electric Water Heater Fires, Fire & Arson Investigator 2000. 51(2000)17-20. [6]Chris Lennard, Fire Cause & Fire Debris Analysis (A Review: 1998 to 2001), 13th INTERPOL Forensic Science Symposium. (2001)70-96.
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Acknowledgements
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This research was supported by Plan Project for Application of Innovation of M.P.S (2009YYCXSHXF149) and the Scientific Research Project of Shanghai Municipal Science and Technology Commission (11231201303).
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S0379-0738(14)00094-2 http://dx.doi.org/doi:10.1016/j.forsciint.2014.03.001 FSI 7532
To appear in:
FSI
Received date: Revised date: Accepted date:
19-3-2013 2-3-2014 3-3-2014
Please cite this article as: Haixin GuLin XueYongfeng ZhangLiying CaoMinju DingHao HuangRenlie Bao Analysis on Micro-regional Characters of the Resistance Wires Surface in Electrical Immersion Heater by SEM/EDX (2014), http://dx.doi.org/10.1016/j.forsciint.2014.03.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Analysis on Micro-regional Characters of the Resistance Wires
2
Surface in Electrical Immersion Heater by SEM/EDX
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cr us
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Haixin GU1*, Lin XUE1, Yongfeng ZHANG1, Liying CAO1, Minju DING2, Hao HUANG1, Renlie BAO1 1, Shanghai Fire Research Institution of the Ministry of Public Security, 918 Minjing Road, Shanghai, China 2, Shanghai Institute of Forensic Sciences, 803 Zhongshan North 1 Road, Shanghai, China
*Corresponding author information: Haixin GU, Email：[email protected], Tel: +86-021-65240172, Fax:+86-021-65240052
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Page 1 of 9
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Analysis on Micro-regional Characters of the Resistance Wires
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Surface in Electrical Immersion Heater by SEM/EDX Abstract: We present a novel method for identifying the conditions of electrical resistance wires before fire basing on micro-regional characters. The morphology and elemental compositions of resistance wires surface under normally used, misused and external heating conditions were respectively studied by scanning electron microscopy (SEM) and energy dispersive X-Ray spectroscopy (EDX). The results show that the normally used wire has a smooth surface and stable metal composition. While the resistance wire under misused condition for the certain time could be characterized as a coarse surface covered by compacted and silica-embedded oxide layer, and the proportion of elemental compositions of different metals is also changed with higher content of Al and lower content of Fe and Cr. The external heating condition has little influence on inside wires. The mechanism of changes formed under misused condition was explained. We demonstrate that this approach is practical and functional to aid fire investigators in determining the cause of related fires.
31
1. Introduction
32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55
The incidences and cost related with fires caused by electrical immersion heater have been increasing recent years. For the fire investigation involving laboratory analysis of this kind of fire materials, one of major tasks is to determine whether the heaters are ever under electrical accident or misused before fire. Short circuit as the electrical damage may be a potential ignition source in many cases, and much attention has been paid on electrical wiring and its identification methods [1~4]. However, the heaters themselves do not generally cause fire [5]. One of the most probable causes of fire is that the heating element contacted with and thus ignited the surrounding combustible materials under misused condition [6]. The major difference of heating element between normally used and misused condition is the external surroundings, namely, using water as heating medium for the former and directly exposed to air for the latter. For the selection of sample to be examined, the heating tube is directly influenced by high temperature of fire scene, which may complicate the discriminations of self-forming and fire-burning. On the other hand, it suffers from the drawbacks of contaminants from fire scene and rust by oxidation. Hence, the selection of inside resistance wire as analytical object seems appropriate. However, up to date, few studies have generally been performed on fire identification methods on this subject. Since the heaters under misused condition differ from the normally used ones, we have reasons to make an attempt to find out what happen to the wires. Scanning electron microscopy coupling with an energy dispersive X-ray spectroscopy (SEM-EDX) is a non-destructive detection method which can provide combined information of both morphological structure and elemental compositions. Consequently, SEM-EDX would also be supposed to serve as a powerful tool for forensic purpose of providing fire investigators with technical support by determining the conditions resistance wire once experienced before fire. In the present study, the micro-regional characters of morphology and metal elemental
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Keywords: Fire investigation, immersion heater, resistance wire, surface morphology and elemental compositions, identification
Page 2 of 9
compositions of resistance wires under normally used and misused condition were respectively investigated by SEM/EDX. We specially characterized the differences and then analyzed the mechanism of transformation phenomenon. The external heating experiment was also conducted to help in understanding whether fire would affect internal wires. To the best of our knowledge, we find out for the first time that the SEM/EDX detection on electrical resistance wires was used for fire cause identification.
62
2. Material and methods
63 64 65 66 67 68 69 70 71 72 73 74 75 76 77
2.1 Material and instrumentation The electrical heating coil (Hongjian Electric Appliance Plant, HJ-555-1) was used as a typical immersion heater. This product consists of heating tube, silica sand and internal resistance wire. The silica sand is filled in the tube not only to fix wire but also serve as heating medium. The obtained samples were divided by cutting machine (Struers Inc. Labotom-3), and 2~3 cm length of resistance wire was taken from the central part of heating tube. Attachment on the surface of wires was removed by ultrasonic cleaner (Shanghai Kudos Co., LTD, SK200H) for 2 mins. SEM/EDX instrument (JEOL Inc., JSM-6390LA/EX-54175LMU) was used for detection at magnifications from 30× to 2000×. Due to silicon sand may be distributed at random in scanning area, relative percentage method was used to calculate each metal composition in order to avoid the interference of non-metal compositions. The temperature of internal wire could not be measured directly in this work. However thermal infrared imager (FLIA Systems Inc., SC620) was used to observe change and distribution of the temperature on tube surface and indirectly investigate the temperature of wire. Muffle Furnaces (Themo Fisher Scientific Inc. F48000) was used to simulate fire burning condition.
78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95
2.2 Preparation of samples under different conditions Following experiments were respectively performed to simulate three different conditions. Fig. 1 shows the design of experiment process. i) To simulate the normally used condition (NC), the electrical heating coils were immersed and electrically heated in water. The water would maintain boiling for a certain duration time, and then the electrical power would be turned off. After the electrical heating coils naturally cooled down to room temperature, this process would be repeated. Sample NC1, NC2 and NC3 were prepared with different single duration time and cumulative time, 10minutes and 60hours(NC1), 30minutes and 60hours(NC2), 60hours and 180hours(NC3) respectively. The water boiled at about 7 minutes and was supplemented timely. ii) In the experiments of misused conditions(MC), the electrical heating coils were exposed to air. In order to investigate the thermal-induced change processes of wire, the coils were heated for a certain time of 30s(MC1), 45s(MC2), 1 mins(MC3), 2 mins(MC4), 3 mins(MC5), 5mins (MC6), 10mins (MC7)and 20mins (MC8) respectively. iii) The influence of external fire on the inside electrical wires cannot be ignored. Since the reconstruction of an fire scene is always unavailable, muffle furnace was used to conduct experiment of burning condition (BC). The normally used(NB) wire and misused heaters(MB) wire were heated for 30mins at temperature of 600 (1), 800 (2), and 1000 (3) respectively.
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3.1 SEM/EDX analysis of electrical wires under normally used condition The new wire has a smooth, even and clear striated surface without silica sands attached (Fig.2 a, b), and composes of three metal elements, namely Fe, Cr and Al of 70.62%, 24.80%, and 4.58% respectively(Table 1). Samples of NC1 and NC2 were used for the comparison of different usage, while sample of NC3 for accelerating the aging process. Tab.1 shows that metal compositions of normally used wires are slightly changed by compared with the new ones. No significant variation tendency could be found when the usage duration of electrical wire advanced. The relative contents of Fe, Cr and Al reach a stable proportion at about 73%, 21% and 6% respectively. It is also demonstrated that both different usage method and even long time aging make little influence on morphological features of wire surface under normally used condition (NC), as showed in Fig.2.
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3. Results
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Fig.1 Schematic representation of the experimental design
Fig. 2 SEM images of new wire (a and b) and normally used wire NC3(c and d) with different magnifications Table.1 Temperature of heating tube and relative content of metal composition of wire Sample
Number
Temperature of tube /
relative content of metals(%）
Sample
Temperature of tube /
relative content of metals(%)
Fe
Cr
Al
Number
Fe
Cr
Al
New wire
/
70.62
24.80
4.58
MC5（3min）
955
54.56
17.02
28.42
NC1
/
73.42
20.63
5.95
MC6（5min）
937
52.07
15.71
32.23
NC2
/
72.76
21.14
6.10
MC7（10min）
951
30.60
15.01
54.39
NC3
/
73.11
20.88
6.01
MC8（20min）
956
30.12
14.87
55.71
MC1（30s）
80
74.67
20.26
5.06
NB3
1000
74.04
20.02
5.94
Page 4 of 9
MC2（45s）
186
73.24
20.58
6.18
MB3
1000
32.31
14.22
MC3（1min）
492
66.46
21.30
12.24
Area a
/
72.93
20.43
6.64
MC4（2min）
847
59.57
19.33
21.10
Area b
/
33.68
15.52
50.80
53.47
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3.2 SEM/EDX analysis of electrical wires under misused condition Fig.3 indicates the progression of morphological changes of wire surface under misused condition. The misused wire within 45 secs is hard to discriminate by comparison with the normally used ones (Fig.3a and Fig2d). The silica sand begins to gather and attach on some part of wire surface at the time of 1 min (Fig.3 b), making wire surface coarse. In the next 1 minute, an oxide layer (red arrow) embedded with silica sands (white arrow) could be clearly observed on the surface of electrical wire (Fig.3c). As the degree of transformation proceeds, silica sands would collect and grow up (white arrow in fig.3 d, e). Furthermore, the oxide layer becomes compact and thick (white arrow in Fig.3e). Fig.3 (f) demonstrates that this phenomenon happens on the whole wire surface. Meanwhile, as shown in table 1, metal elemental compositions of wire are also changed. Briefly, the relative content of aluminum greatly increases, nearly 9 times that of NC from 6.01% to 55.71% within 20mins. Conversely, the relative contents of ferric and chromium decrease gradually from 73.11% to 30.12% and 20.88% to 14.87%, respectively.
127 128 129 130 131 132 133 134 135 136 137
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Fig. 3 SEM images of misused wires: a) MC2; b) MC3; c) MC4; d) MC6; e and f) MC8
3.3 Influence of external temperature on internal wires In fire scene, the high temperature is a definite factor possibly influencing the internal wires. However, few changes could be found in normally used wires as well as misused ones before and after the experiments of burning condition (Fig.4 and Tab.1) at the given temperature. The high temperature from outside surroundings could not make the surface of normally used wire coarse and neither make its metal compositions changed (Tab.1). Furthermore, the characters of wire caused under misused condition still remain. The electrical wire could be well protected by tube, and especially by fire-resistant silica sand. We demonstrate that this method would practically avoid the influence of fire and make the identification results more dependable.
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Fig. 4 SEM images of wires under external burning condition: a & b) normally used wire (NB3), c and d) misused wire of (MB3)
4. Discussion
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A reasonable explanation for this dissimilarity in both microscopic features and metal composition is the difference in the course wires experienced. In the process of normally used condition, the heat generated from electrical wires is transferred from intermediate fillings (silica sand) to tube, and then to the external water. When temperature of water reaches the boiling point at 100 , the entire heat transmission process maintains balance. Due to effective release of heat, electrical wire takes the heat and temperature within its capacity and makes its micro-regional character unchanged. However, for the misused condition, due to the poor thermal conductivity of air, the heat generating from tube is incapable to release out efficiently, contributing to increasing temperature of the wire. Thermal infrared images (Fig.5) show that the temperature of tube electrically heated in the air increases remarkably from room temperature to maximum temperature at 955 within 3 mins, and then maintains a new balance at about 950 (Fig.6). When tube temperature reaches about 490 at the time of 1 minute, chemical and physical changes on the wire surface originate. Meanwhile the temperature of internal wire should be much higher than the tube.
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Fig.5 Thermal infrared images of tubes in the air with different heating time: a) 1 min, b) 2 min, c) 3 min
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Along with the wire surface getting coarser under misused condition, the change of metal content is also subjected to duration time and cumulated heating. As shown in Fig.6, during the next 7mins after tube temperature reaching its peak of 955 , the relative content of aluminum still increases from 28.42% to 54.39%, and then tended to a stability. On the contrary, the Ferric and chromium continue to decrease by 23.96%and 2.01% respectively.
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Fig.6 Relative content of metals and temperature of tube with variable electric-heating time
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Fig.7 shows the SEM image and EDX spectrum of area “a” and “b” on misused wire. Their relative contents are respectively showed in table 1. According to the characters of morphology and composition, the uncovered area “a” could be confirmed as a part of original surface of wire, meanwhile the area “b” is the oxide layer visually traced as a coating substance growing from area “a”. This regional discovery would forcefully explain the explanations of phenomenon. When temperature exceeds the capacity of wire, aluminum with lowest melt point is relatively separated outward and precipitated on the surface gradually, forming a molten layer mixed with surrounding silica sands. Once the electrical power is cut off, the wire surface solidifies with cooling and form a compacted and silica-sand-embedded oxide layer. And thus, the coarse wire surface with higher content of aluminum could be witnessed.
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Fig. 7 SEM image of wire surface and its EDX spectrums (The picture of “a” and “b” on the right side respectively
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5. Conclusions
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In this paper, we have demonstrated a novel method well suited for identifying the conditions of electrical immersion heaters before fire. Internal wire is chosen for detection to obtain original information of the electrical heater and avoid the influence of external fire effectively. The normally used and the misused wire could be identified by the significant differences of morphology and elemental compositions. Namely, the normally used wire has the smooth surface of no attachments and stable metal composition, while the misused one is characterized by the oxide-layer-covered coarse surface where silica sand is embedded and the elemental compositions are redistributed. Analysis with fewer sampling procedure was performed by SEM/EDX visually and metrically. Non-destructive method is employed to detect the target part of sample, making the preservation of physical evidence available. Consequently, the method we presented above could be a well-established technique to aid fire investigators in determining the conditions of electrical immersion heater before fire and its possibility of causing the fire.
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6. References
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[1] V. Babrauskas, How do electrical wiring faults lead to structure ignitions, Fire and Materials Conference. (2005)39-51. [2] Eui Pyeong Lee, Hideo Ohtani, Yoshiyuki Matsubara, Tsutomu Seki, Hideo Hasegawa, Shuji Imada, Isao Yashiro, Study on discrimination between primary and secondary molten marks using carbonized residue, Fire Safety J. 37(2002)353-368. [3] Yasuaki Hagimoto, Hiroki Yamamoto, Analysis of a Soldered Wire Burnt in a Fire, J. Forensic
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showed the EDX spectrum of “a” and “b” area marked by white rectangle in SEM image on the left side.)
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Sci. 51(2006)187-89. [4]H.B. Land, Determination of the cause of arcing faults in low voltage switchboards, 2005 IEEE Electric Ship Technologies Symposium. (2005)125-132. [5]Goodson M, Electric Water Heater Fires, Fire & Arson Investigator 2000. 51(2000)17-20. [6]Chris Lennard, Fire Cause & Fire Debris Analysis (A Review: 1998 to 2001), 13th INTERPOL Forensic Science Symposium. (2001)70-96.
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Acknowledgements
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This research was supported by Plan Project for Application of Innovation of M.P.S (2009YYCXSHXF149) and the Scientific Research Project of Shanghai Municipal Science and Technology Commission (11231201303).
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