職業安全管理與安全技術為提昇勞工作業安全水準的兩個基本要素，事業單位利用建置職安衛管理系統達到系統化管理的目的，並利用相關技術配合管理系統的執行。然而，事業單位即使有良好的管理系統，但倘若未有合適的風險評估機制，也無法發揮職安衛管理系統應有的功能，甚至耗費許多不必要的資源。事件樹分析具有可提供分析多重防護設施功能的有效性、分析事件的演變過程並建立不同的事故情境、獲得較客觀的量化風險等特性，較能符合職安衛管理系統對於風險評估的要求，但缺點為在建構事故情境時，防護層或影響因子的陳述方式沒有一致性的排序原則，如此會造成樹的分枝過度複雜及評估上的不一致性。本研究利用事件樹結合保護層分析做為職業安全風險評估機制，並以矽甲烷供應系統為例，展現事件樹分析應用的特色。此外，本研究亦嘗詴將職安衛管理系統行政控制措施與防護層概念結合，並藉由職業災害案例呈現可能的應用方式，提供事業單位進行風險管理程序時，一項較可行的風險評估方法。

Hazard identification, risk assessment and risk control constitute the critical tasks of occupation health and safety management system standard OHSAS 18001. Risk assessment provides organizations with an in-depth understanding of occupational health and safety risks and are the basis for the design and implementation of risk control mechanisms. Among the available risk assessment techniques, risk matrix is most commonly used because of its simplicity. However, the usefulness applicability of risk matrix depends on appropriate estimation of frequency and consequence of an identified risk. It can become inconclusive or even misleading when inappropriate estimations of frequency and consequence are made. In this study, event tree analysis, ETA, is applied for accident type identification and frequency estimate.
Event tree analysis is widely used for safety analysis and risk assessment as it is capable of specifying how a sequence of events, or scenario, develops under various conditions. The formalized scenarios can then be used to identify the accident characteristics and the likelihood of a specific scenario being fully developed. However, there is a lack of consideration for the functions protection layers and their effectiveness in event tree analysis. The so-called onion model is applied in this study to analyze sequence of the protection layers.
Two different categories of risks are addressed in this study. The first case study focuses on the likelihood and consequence analyses for a bulk silane storage and supply system. Protection layers and safety integrity level of each layer are used to construct the branches of ETA. The bulk silane system is selected due to its wide applications in Taiwan’s high-tech industry and its extremely hazardous nature. Protection layers such as gas detector, UV/IR flame detector, sprinkler system, and
various automatic shut-off mechanisms used in a solar cell module manufacturing facility are considered as the protection layers. The second case study attempts to use event tree to analyze possible scenarios of hazardous manual tasks. The administrative control in occupational safety and health management system is incorporated with the concept of protective layers. Results of the case studies confirm the applicability of the proposed risk assessment technique proposed in this study.

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