Safety and Security: Two Sides of the Same Coin

A Weakness in Security Creates Increased Risk, Which in Turn Creates a Decrease in Safety, so Safety and Security Are Directly Proportional, but Are Both Inversely Proportional to Risk


By Eric Byres, PE, and John Cusimano, CFSE

In many languages, there is only one word for safety and security. In German, for example, the word is 'Sicherheit,' in Spanish it is 'seguridad', in French it is 'sécurité' and in Italian it is 'sicurezza.'

According to Merriam-Webster, the primary definition of safety is "the condition of being free from harm or risk," which is essentially the same as the primary definition of security, which is "the quality or state of being free from danger." However, there is another definition for security; that is, "measures taken to guard against espionage or sabotage, crime, attack or escape," and this is generally the definition we are using when we refer to industrial security.

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Using these definitions, we can better understand the relationship between safety and security. The relationship is such that a weakness in security creates increased risk, which in turn creates a decrease in safety. So safety and security are directly proportional, but are both inversely proportional to risk. While this may all seem elementary, understanding the relationship between safety and security is very important to understanding how to integrate the two. Those that own and operate industrial facilities, especially those that many governments have defined as critical infrastructures, certainly understand the meaning and importance of safety and security relative to their operations.

In the context of industrial automation and control systems, safety systems are special control systems whose function is to detect a hazardous condition and take action (typically shut down the process) to prevent a hazard. They are typically one of many layers of defense in an overall protection scheme for the facility. Whereas, control system security refers to the capability of a control system to provide adequate confidence that unauthorized persons and systems can neither modify the software and its data nor gain access to the system functions, and yet to ensure that this is not denied to authorized persons and systems.

Until recently, the engineering disciplines of safety system design and control system security were effectively on separate, but parallel paths. Safety standards and associated engineering work practices are mature and well-established, based on decades of learning. On the other hand, control system security is a much newer field and has its roots in information system or IT security. Some say control system security is where safety system engineering was about 10 years ago.

So why is there sudden interest in integrating safety system and control system security disciplines? One reason is that safety integrated systems (SIS), once totally isolated, are increasingly becoming connected to or integrated with process control systems that connect to the outside world. This is a significant because a security breach of a SIS could directly prevent the SIS from performing its intended protection function, which could lead directly to a catastrophic event. On the other hand, a security incident in a control system, while still having the potent to be very damaging, should be limited to causing a process shutdown because the SIS is there to prevent a dangerous situation, provided it was designed properly and was not also compromised. The integration of control and safety systems raises significant concerns about the possibility of a common security vulnerability affecting both systems.

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Another reason for the sudden interest is a growing recognition of the many similarities between the safety and security life cycles, and that there are improvements and efficiencies to be gained by combining the two approaches. By addressing both safety and security fundamentally from the beginning, asset owners will be able to head off the need to perform a second costly process later to find and address security vulnerabilities.

This interaction between the safety of a critical system and security became painfully obvious to the owners of the Hatch Nuclear facility in March of 2008. According to data supplied by the Repository of Industrial Security Incidents (, the Hatch Nuclear Power Plant near Baxley, Ga., was forced to shut down for 48 hours after a contractor updated software on a computer that was on the plant's business network. The computer was used to monitor chemical and diagnostic data from one of the facility's primary control systems. The software was designed to synchronize data on both systems. When the updated computer rebooted, it reset the data on the control system, causing the safety system to interpret the lack of data as a drop in water reservoirs that cool the plant's radioactive nuclear rods. The safety system behaved as designed and triggered a shutdown. The engineer was not aware that the control system would be synchronized as well or that a reboot would reset the control system.

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