**2. Critical infrastructure system description**

The issue of critical infrastructure protection began to be addressed in the United States in response to a terrorist bombing on a federal building in Oklahoma City in 1995 [2]. Over the following years, other countries also started tackling these problems, e.g., from 1998 in Canada and from 1999 in the United Kingdom, Germany, Sweden, and Switzerland. Following the September 11, 2001 attacks, the majority of European countries proceeded to define "Critical Infrastructure" and began to take actions aimed at its protection [3].

commodities (e.g., energy and water supply) or sectors providing technical services (e.g., transport or ICT systems). The socioeconomic infrastructure is composed of sectors that provide social or economic services (e.g., health care, financial and currency markets, emergency services, and public administration). There are significant dependencies between two types of critical infrastructure [5]. For instance, all of the socioeconomic sectors require the unrestricted availability of commodities produced by the technical infrastructure sectors, whereas the technical infrastructure, by contrast, fully depends on the socioeconomic sectors, espe-

Failures in a Critical Infrastructure System http://dx.doi.org/10.5772/intechopen.70446 77

The sector level is composed of the individual sectors and subsectors of a critical infrastructure. This level represents the classification of specific sectors and their mutual linkages. The transportation sector, for example, is made up of five subsectors, namely road transport, rail transport, air transport, inland waterways transport, and ocean and short-sea shipping and ports [1]. The individual elements that form the element level are the basic building blocks of the critical infrastructure system. These elements reach different degrees of relevance within the system, depending on the extent of the impact that their disruption or failure can poten-

It is imperative that a critical infrastructure system be viewed in a comprehensive manner, taking into account its networked arrangement where individual subsystems are interlinked via various types of linkages. The basic structure of these linkages arises from their character and includes one-way linkages, which represent an influence or dependency, and two-way linkages involving interdependency. Rinaldi et al. [6] have classified interdependencies in more detail as physical, cybernetic, geographic, and logical in nature and noted that interdependencies increase the risk of failures or disruptions in multiple infrastructures. Pederson

et al. [5] have subsequently further classified these linkages for lower levels of detail.

**3. Impacts of critical infrastructure system failures on dependent** 

Like any other complex system, a critical infrastructure system includes a multitude of elements with different levels of importance, categorized into several levels and interconnected by linkages of various types and intensity. Such a structural arrangement leads to a broad correlation between individual subsystems, which determines the manner and intensity of propagation of impacts from critical infrastructure system failures on dependent subsystems

The functioning of a critical infrastructure system is constantly being threatened by a wide range of security threats. These threats can be generally categorized into five basic groups [7]: • climatological threats (including natural disasters such as floods, tornadoes, heavy snow-

cially in crisis situations.

tially produce.

**subsystems and society**

fall, or extensive fires);

**3.1. Critical infrastructure system failures**

and society.

The US Department of Homeland Security (DHS) currently defines a critical infrastructure as "*systems and assets, whether physical or virtual, so vital to the United States that the incapacity or destruction of such systems and assets would have a debilitating impact on security, national economic security, national public health or safety, or any combination of those matters*" [4]. A critical infrastructure at the European Union level is specified in a Council Directive [1], defining a critical infrastructure as "*an asset, system or part thereof located in Member States which is essential for the maintenance of vital societal functions, health, safety, security, economic or social well-being of people, and the disruption or destruction of which would have a significant impact in a Member State as a result of the failure to maintain those functions*."

The hierarchic arrangement of a critical infrastructure system has three levels that constitute a vertical classification [3]: system level, sector level, and element level (see **Figure 1**). The system level is the basic classification of a critical infrastructure according to its functions. This level comprises two areas, namely the technical infrastructure and the socioeconomic infrastructure. The technical infrastructure includes sectors producing and providing specific

**Figure 1.** Hierarchic arrangement in a critical infrastructure system.

commodities (e.g., energy and water supply) or sectors providing technical services (e.g., transport or ICT systems). The socioeconomic infrastructure is composed of sectors that provide social or economic services (e.g., health care, financial and currency markets, emergency services, and public administration). There are significant dependencies between two types of critical infrastructure [5]. For instance, all of the socioeconomic sectors require the unrestricted availability of commodities produced by the technical infrastructure sectors, whereas the technical infrastructure, by contrast, fully depends on the socioeconomic sectors, especially in crisis situations.

The sector level is composed of the individual sectors and subsectors of a critical infrastructure. This level represents the classification of specific sectors and their mutual linkages. The transportation sector, for example, is made up of five subsectors, namely road transport, rail transport, air transport, inland waterways transport, and ocean and short-sea shipping and ports [1]. The individual elements that form the element level are the basic building blocks of the critical infrastructure system. These elements reach different degrees of relevance within the system, depending on the extent of the impact that their disruption or failure can potentially produce.

It is imperative that a critical infrastructure system be viewed in a comprehensive manner, taking into account its networked arrangement where individual subsystems are interlinked via various types of linkages. The basic structure of these linkages arises from their character and includes one-way linkages, which represent an influence or dependency, and two-way linkages involving interdependency. Rinaldi et al. [6] have classified interdependencies in more detail as physical, cybernetic, geographic, and logical in nature and noted that interdependencies increase the risk of failures or disruptions in multiple infrastructures. Pederson et al. [5] have subsequently further classified these linkages for lower levels of detail.
