電氣溶融痕による火災原因の鑑定法に關する硏究 : Study on identification method of fire cause by molten marks on elenctric wires

이의평 橫浜國立大學 2001 해외박사

In a building fire, electric molten marks (EMMs) are made to cords of electric appliances, fixed electric wires and the portable cords etc. in many cases. Moreover, also in a vehicles fire, EMMs are made to electric wiring. As described above, EMMs are often left in a spot of fire outbreak. They may give a big clue of fire cause when the fire is investigated after fire fighting. A molten mark formed by short-circuit which causes fire is called as the primary molten mark (PMM). A molten mark formed by short-circuit which is caused by fire is called as the secondary molten mark (SMM). However, the technique to distinguish between the PMM and the SMM has not been confirmed yet. Therefore, the identification methods of EMMs were studied systematically as follows. 1. The following two cases were experimentally studied as a fundamental research which is necessary to establish the method of distinguishing the Primary Molten Marks (PMMs) from the Secondary Molten Marks (SMMs); ① In order to research whether thermal histories affects the grain size of non-melted wire which is adjacent to EMMs or not, the grain size of copper wire exposed to various heating and cooling conditions is examined. ② In addition, it was also researched that there were some differences of properties in external features, cavities and metallographic structures of the PMMs and SMMs made under the several different conditions. As a result, the following were obtained. 1) The grain size of the copper wire has no relationship with the cooling conditions and spontaneous heat of wires prior to the heating, but depends on the highest heating temperature only. Therefore, it is ineffective to use the grain size to distinguish PMM from SMM. 2) It is unreasonable to discriminate between the PMMs and the SMMs by the external features and cavity distribution of the EMMs. 3) When the surface of the copper wire was oxidized after the exposition to the air, the oxidation structures, which reflect the effect of the oxidation before the production of EMMs, are observed from the EMMs. And oxidation structure is formed not only by oxidation of surface before short-circuit, but also by oxidation during coagulation after short-circuit through absorption of ambient oxygen. 4) It considered that the oxygen of the air is absorbed at the melting and coagulating process and then the oxidation structures could be found even when the oxygen concentration is low as in case of fire, since the oxidation structure was admitted by the ambient of oxygen concentration 2.5%. 2. The oxidation structure in an EMM has been often observed in electric wire of fire scene. In this study, I examined to estimate the surrounding temperature at the formation of an EMM by DAS (Dendrite Arm Spacing) and oxygen concentration in oxidation structure. The results were as follows; The DAS was dependent on functions of oxygen concentration and cooling rate. The oxidation structure in an EMM, even if it was exposed in fire again, did not change as long as it was not re-molten state. The surrounding temperature at the formation of an EMM was estimated by measuring DAS and oxygen concentration. 3. I studied the crystal structure of the carbon which was caught in PMMs and SMMs prepared artificially, and examined the difference in crystal structure between the PMMs and SMMs. I found that both graphitized carbon and amorphous carbon were found in the PMMs. However, only amorphous carbon could be found in the SMMs. This result shows that graphitized carbon can be used as an indicator of the PMMs. 4. In a fire site, there sometimes molten marks exist in blades of the attachment plug and the receptacle. When the fire cause is investigated, these molten marks have possibilities to become a big clue. However the technique to discriminate between the primary molten marks (PMMs) which are cause of fire and the secondary ones (SMMs) which are produced in the fire, has not been established yet. Therefore I artificially made PMMs and SMMs, and investigated how PMMs differs from SMMs. As a result, the followings were obtained. 1) Though cut fusion of two blades was made at the PMMs and the SMMs, cut fusion of single blade and melted mass of two blade were made at the SMMs. 2) DAS (Dendrite Arm Spacing) apparently differed between PMMs and SMMs. DAS of PMMs was 3.5∼14.0㎛, but DAS of SMMs(ambient temperature when molten marks were made : 450℃~ 900℃) was 24.5∼56.8㎛. 3) If DAS in molten marks can be measured, the ambient temperature when molten marks were made can be estimated from by the DAS, and the discrimination between PMM and SMM becomes possible. 5. One of main causes of electrical fire is short-circuit. The short-circuit due to thermal degradation is one main cause though there are many other causes. The circuit-short mechanism of PVC cords at defective heat loss was studied as following two cases: ① Temperature of the covering rises rapidly by joule heating etc., or ②Temperature of the covering rises slowly by thermal degradation at comparatively low temperature for a long time, then PVC cords are short-circuited. As a result, the followings were obtained: 1) Though PVC cord is not short-circuited in the rating current (20A) of divergence breaker when it is extended on the straight line and there is no defective heat loss at the room temperature, wound cord is short-circuited at the rating current of PVC cord regardless of the protection equipment such as the divergence breaker. 2) When temperature of the covering of PVC cord rises rapidly to nearly melting point by joule heat, PVC cord is short-circuited in a few minutes or less by the direct contacting with electrical wires of two poles due to softening and melting of the covering. On the other hand, when PVC cord is short-circuited due to the thermal degradation of a long term, it is short-circuited by absorbed moisture on CaCl_(2) or by insulation failure due to the decrease in the insulation resistance of the covering. 3)HCl generated by thermal degradation process reacts with CaCO_(3) which is added as filling of PVC covering, and hygroscopic CaCl_(2) is produced. The short-circuited mechanism by absorbed moisture on CaCl_(2) is certain, and the generation of CaCl_(2) can be quantitatively measured. As mentioned above, the identification methods of electric molten marks on electrical wires and in blades of the attachment plug and the receptacle were studied systematically. Moreover, the short circuit mechanism by thermal degradation of covering of an electric wire was also clarified. 消防では、「火災」から得られる原因と損害の調査によって、火災による人命の救濟と財産の保全を圖ることを目的として、火災調査を行っている。火災調査は、「火災の原因調査」と「火災及び消火のために受けた損害の調査」と分けられる。火災原因調査の中心とするところは、出火原因を究明することであるが、延燒擴大の原因や死傷者發生の要因を調査することも重要な範圍である。火災の原因調査は火災現象の究明はもとより、火災予防の施策ないし措置の成果を檢討し、その是正改善を圖り、もって火災予防の徹底に資することを目的としている。出火原因の究明には、①出火個所の判定、②出火した發火源の判定、③出火した經過などの判定のステップがある。 出火個所を明らかにした後、出火個所で考えられる發火源について消去法などによりその檢討がなされ、"出火した發火源" を明らかにする。出火した個所はある一定の範圍(空間)であり、その範圍內の個個の發火源についてその可能性を檢討する。 一般に建物火災や車兩火災などの場合は出火すると、大體天井の固定電線や室內のコ-ドや電氣製品などの電源コ-ドや製品の中の配線などに電氣溶融痕ができる。このように、出火した場所には電氣溶融痕が存在することが多いので、火災原因硏究明時には、この電氣溶融痕が過負荷など電氣的要因にできたもの(1 次溶融痕)かそれとも火災により被覆が燃やされて短絡したもの(2 次溶融痕)かを究明する必要がある。 しかしながら、1次ㆍ2次溶融痕の判別方法については、外觀、溶融痕中のボイド、炭化物卷き入み、表面分析などの方法が檢討されてきたが、まだ精度の高い判別方法は確立されていないのが現狀である。 そこで、1次ㆍ2次溶融痕の判別に關して樣樣な角度から1次ㆍ2次溶融痕にはどのような差があるかを檢討して、有效な判別方法を明らかにする硏究が必要であり、本論文では、1次ㆍ2次溶融痕の判別方法に關する、以下の4つの項目について檢討をした。 ① 溶融痕の外觀(形態、大きさなど)及び氣孔分布、溶融痕近傍の非溶融部の結晶粒度、そして酸化組織の出現條件 ② 酸化組織中の酸素濃度と樹枝狀晶の枝の間隔(DAS:Dendrite Arm Sparcing)によって生成時の雰圍氣溫度推定による方法 ③ 卷き入み炭化殘渣中の炭素結晶構造による判別方法 ④ プラグの刃とコンセントの受け刃の溶融痕跡中の金屬組織による判別方法 硏究結果は、以下のとおりである。 ① 從來、火災現場では電氣溶融痕の外觀や氣孔の分布をみて判斷する傾向が强かったが、これらの判別方法の有效性を樣樣な實驗により檢討したところ、電氣溶融痕の外觀や氣孔の分布によって判斷することは有效な判別方法ではない。 ② 溶融痕近傍の非溶融(銅線)部分の結晶粒度は、被熱履歷は殘さず最高暴露溫度のみに依存するので、1次溶融痕か2次溶融痕かの判別に利用できない。 ③ 酸化組織は短絡前の表面酸化により生じるばかりではなく、短絡して凝固する段階で雰圍氣中の酸素を吸收してもできる。 ④ 現場から採集した電氣溶融痕を硏磨して金屬顯微鏡で觀察するとしばしば酸化組織(銅と亞酸化銅の合金組織)が觀察される。この酸化組織中の樹枝狀の枝の間隔(DAS)と硏磨面の酸素濃度を測定すると、溶融痕生成時の雰圍氣溫度が推定できる。 ⑤ 電氣溶融痕の中には、多くの炭化殘渣が卷き入まれている。この炭化殘渣の成分は電線の被覆の成分であるPVCと充塡材に起因するC, Cl, Ca, Clなどである。1次溶融痕中にはグラファイト化炭素とアモルファス炭素が卷き入まれているが、2次溶融痕中にはアモルファス炭素しか卷き入まれていない。 ⑥ コ-ドにできる溶融痕ばかりでなく、プラグとコンセントにできる溶融痕についても、樹枝狀の枝の間隔(DAS)を測定すると、生成時の雰圍氣溫度が定量的に推定できる。 以上のように電氣溶融痕の判別方法を多角度から檢討し、體系的にまとめた。