Safety Engineering: Practices
Safety Engineering: PRACTICES
Safety is one of the primary goals of engineering. In most ethical codes for engineers safety is mentioned as an essential area of professional competence and responsibility.
In everyday language, the term safety is often used to denote absolute safety, that is, certainty that accidents or other harms will not occur. In engineering practice, safety is an ideal that can be approached, but never fully attained. What can be achieved is relative safety, meaning that it is unlikely but not impossible that harm will occur. The safety requirements in regulations and standards represent different (and mostly high) levels of relative safety. Industries with high safety ambitions, such as airway traffic, are characterized by continuous endeavors to improve the level of safety.
The ambiguity between absolute and relative safety is a common cause of misunderstandings between experts and the public. Both concepts are useful, but it is essential to distinguish between them.
In decision theory, lack of knowledge is divided into the two major categories: "risk" and "uncertainty." In decision-making under risk, the probabilities of possible outcomes are known, whereas in decision-making under uncertainty, probabilities are either unknown or known with insufficient precision. In engineering practice, both risk and uncertainty have to be taken into account. Even when engineers have a good estimate of the probability (risk) of failure, some uncertainty remains about the correctness of this estimate.
Safety has often been defined as the antonym of risk, but that is only part of the truth. In order to achieve safety in practical applications, the dangers that originate in uncertainty are equally important to eliminate or reduce as those that can be expressed in terms of risk. Many safety measures in engineering are taken to diminish the damages that would follow from possible unknown sources of failures. Such measures protect against uncertainty rather than risk.
Several methods are used by engineers to achieve safety in the design and operation of potentially dangerous technology.
Inherently safe design. The first step in safety engineering should always be to minimize the inherent dangers in the process as far as possible. Dangerous substances or reactions can be replaced by less dangerous ones. Fireproof materials can be used instead of flammable ones. In some cases, temperature or pressure can be reduced.
Safety reserves. Constructions should be strong enough to resist loads and disturbances exceeding those that are intended. In most cases, the best way to obtain sufficient safety reserves is to employ explicitly chosen safety factors.
Negative feedback. Dangerous operations should have negative feedback mechanisms that lead to a self-shutdown in critical accident situations or when the operator loses control. Two classical examples are the safety valve that lets out steam when the pressure becomes too high in a steam boiler and the "dead man's handle" that stops the train when the driver falls asleep. One of the most important safety measures in the nuclear energy industry is to ensure that a nuclear reactor closes down automatically when a meltdown approaches.
Multiple independent safety barriers. In order to avert serious dangers, a chain of barriers is needed, each of which is independent of its predecessors so that if the first fails, then the second is still intact, and so on. Typically the first barriers are measures to prevent an accident, after which follow barriers that limit the consequences of an accident, and finally rescue services as the last resort. One of the major lessons from the Titanic disaster (1912) is that an improvement of the early barriers is no excuse for reducing the later barriers (such as access to lifeboats).
Maintenance and inspections. Many severe accidents have resulted from insufficient maintenance of installations or pieces of equipment that were originally in excellent shape. Regular inspections by persons with sufficient competence and mandate are an efficient means to prevent this from happening.
Educated and responsible operators. Human mistakes are an important source of accidents. An efficient countermeasure is to educate workers, authorize them to temporarily stop processes they consider to be acutely dangerous, and encourage them to take initiatives to improve safety.
Incidence reporting. Experience from air traffic and nuclear energy shows that systems for reporting and analyzing safety incidents are an efficient means to prevent accidents. Systems for anonymous reporting facilitate the reporting of human mistakes.
Safety management. Safety can be achieved only in an organization whose top management gives priority to safety and aims at continuous improvement.
SVEN OVE HANSSON
Marshall, Gilbert. (2000). Safety Engineering, 3rd edition. Des Plaines, IL: American Society of Safety Engineers. General safety principles and their application on industrial workplaces.