Hazardous event: definition

Emphasis should be placed on the last phrase of the SIF definition, "specific hazardous event." This phrase helps one clearly identify what equipment is included in the safety instrumented function versus auxiliary equipment not actually needed to provide protection against the hazard. 

Each hazardous event, once categorized, can then be represented on a risk matrix shown in Fig. 10.6, and prioritized with respect to the urgency of risk control measures that should be implemented to reduce the risk from that particular type of event. A commonly used set of definitions for each risk category on this matrix is given in Table 10.3. 

The high-level list of hazardous events derived from the above process was subdivided into events where the frequency or the consequences of each event are significantly different. This process resulted in the definition of 122 hazardous events that form the basis of version 3 of the SRM. There is a separate cause/consequence model for each hazardous event. Examples from the list of hazardous events are presented in Table 1 at the end of the text. 

The rationale behind the definitions of iow demand mode and high demand or continuous mode in lEC 61508 is based on the failure behaviour of a safety-related system due to random hardware faults. Underlying much of the reasoning is the distinction between safety-functions that only operate on demand and those that operate continuously . A safety function that operates on demand has no influence until a demand arises, at which time the safety function acts to transfer the associated equipment into a safe state. A simple example of such a safety function is a high level trip on a liquid storage tank. The level of liquid in the tank is controlled in normal operation by a separate control system, but is monitored by the safety-related system. If a fault develops in the level control system that causes the level to exceed a pre-determined value, then the safety-related system closes the feed valve. With such a safety function, a hazardous event (in this case, overspill) will only occur if the safety function is in a failed state at the time a demand (resulting from a failure of the associated equipment or equipment control system) occurs. A failure of the safety function will not, of itself, lead to a hazardous event. This model is illustrated in Figure 4. 

A system is a part of the universe within a certain domain in space and time. What is an environment Outside the frontier of the system is the environment [1], Here, system shall have an identity, that is, deterministic. There shall be an external boundary to the system. An external boundary is determined by what aspect of system performance is of concern. This is stated here because for quantitative hazard analysis, boundary definition is extremely important. Also, the interface part needs to be considered (See Fig. V/3.0-l). The process definition for qualitative risk analysis is Qualitative Risk Analysis assesses the priority of identified risks using their probability of occurring, the corresponding impact [...] as well as other factors such as the time frame and risk tolerance [..On the contrary, quantitative risk analysis (QRA) as per DNV is Typically, a QRA can be defined as the formal and systematic approach of identifying potentially hazardous events, estimating the likelihood and consequences of those events, and expressing the results as risk to people, the environment or the husiness. ... 

This case study is based on a typical electric vehicle architecture (technology-specific details have been abstracted for reasons of commercial sensitivity), in which a basic Item Definition and hazardous event are considered. The purpose of the case study is to examine the product-based safety rationale arguments, discussed in Section 2, for the corresponding Safety Goal and Functional Safety Concept. 

For the purposes of this investigation-rather than adopting any single definition of a reactive chemicaT-CSB focuses on the broadest range of practices to identify reactive hazards and to manage the risk of reactive incidents. A reactive chemical may include any pure substance or mixture that has the capability to create a reactive incident. CSB defines a reactive incident as a sudden event involving an uncontrolled chemical reaction-with significant increases in temperature, pressure, or gas evolution-that has caused, or has the potential to cause, serious harm to people, property, or the environment. 

Fault tree analysis is based on a graphical, logical description of the failure mechanisms of a system. Before construction of a fault tree can begin, a specific definition of the top event is required for example the release of propylene from a refrigeration system. A detailed understanding of the operation of the system, its component parts, and the role of operators and possible human errors is required. Refer to Guidelines for Hazard Evaluation (CCPS, 1992) and Guidelines for Chemical Process Quantitative Risk Assessment (CCPS, 2000). 

Of course it is impossible, as noted above, to certify that a drug is absolutely safe, ie, free of all risk. It is possible, however, to identify most of the hazards likely to be associated with use of a new drug and to place some statistical limits on frequency of occurrence of such events in the population under study. As a result, an operational and pragmatic definition of "safety" can usually be reached that is based on the nature and incidence of drug-associated hazards compared with the hazard of nontherapy of the target disease. 

In the event that no useable information on aeute and/or chronic aquatic hazard is available for one or more relevant components, it is concluded that the mixture cannot be attributed (a) definitive hazard category(ies). In this situation the mixture should be classified based on the known components only, with the additional statement that x percent of the mixture consists of components(s) of unknown hazards to the aquatic environment . 

The Institution also developed definitions for a chemical hazard and a major hazard. It called a chemical hazard a hazard involving chemicals or processes which may realize its potential through fire, explosion, toxic or corrosive effects and, a major hazard as an imprecise term for a larger scale chemical hazard, especially one which may be realized through an acute event . 

The methodology of PSA is in principle a combination of event tree and fault tree analyses. The target of the analysis decisively determines the selection and the definition of modes in which the technical system is at the end of an event sequence. These modes comprise both the safe ones resulting from successful safety measures and also those unwanted ones characterized by a certain release of hazardous materials. A PSA is extended to a PRA, if the consequences of the different system modes are determined and linked with the event frequencies of those modes. 

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