Safety vs Functional Safety

I've often seen misconceptions about the concepts of safety and functional safety. While both are critical to maintaining the integrity of railway operations, they address different facets of risk management. As railway systems increasingly rely on complex automation and electronic controls, understanding these distinctions becomes more crucial than ever. This article will break down the differences between safety and functional safety,

Why is this distinction important? As our railways become more automated, functional safety plays an increasingly critical role.

The importance of functional safety grows exponentially with the increasing complexity and automation of railway systems. For instance, the introduction of computer-based interlocking systems in the 1980s and 1990s necessitated a much more rigorous approach to ensuring system safety and fail-safe operation.

One of the most significant developments in this field was the introduction of IEC 61508, the international standard for functional safety of E/E/PE safety-related systems, first published in 1998 and later revised. This standard, and its sector-specific derivatives like EN 50126 for railways, provided a structured approach to managing functional safety throughout a system's lifecycle

What is Safety in Railways? : The Broad Umbrella

In its broadest sense, safety?refers to the condition where the risk of harm is minimized to an acceptable level across all aspects of the railway system. This includes physical infrastructure, operational procedures, human factors, and emergency responses. Safety is the umbrella term that encompasses all measures taken to protect passengers, railway staff, and the public from accidents or injuries. It is defined as:

• Freedom from unacceptable risk
• Applicable to all systems, whether general or functional
• Addressing risks from any source

Safety is defined as "freedom from unacceptable risk." It refers to the overall condition of a system being safe, considering all possible hazards and risks. The focus is on the comprehensive protection of people, the environment, and assets from harm.

What is Functional Safety? : A Specialised Focus

Functional safety, on the other hand, has a more specific definition: Functional safety specifically deals with the aspect of safety that arises from the malfunction of electrical, electronic, or programmable electronic (E/E/PE) systems. According to the International Electrotechnical Commission (IEC 61508), functional safety involves the correct operation of safety-related systems under all conditions, particularly in response to faults or failures

Imagine a system that automatically applies the brakes if a train is going too fast. Functional safety ensures that this system works reliably, even if there’s a malfunction. It is defined as:

• Absence of unacceptable risk due to hazards caused by malfunction of Electrical/Electronic/Programmable electronic systems (E/E/PE)
• Only concerned with unacceptable levels of risk due to E/E/PE systems
• Involves processing information and making automated decisions

Functional Safety is a part of overall safety that depends on a system or equipment operating correctly in response to its inputs.

The functional safety requirements indicate how to prevent/mitigate the identified hazards through a functional behavior (state).

Key Hazards in Functional Safety Unique to Automation

In my experience, there are two primary types of hazards we focus on in functional safety:

1. Failure to operate when required Example: Emergency braking not applied when needed, or a failure of the Brake Control Unit (BCU) to apply Emergency Brake (EB).
2. Unintended operation when not required. Example: Train doors opening during movement or a failure of the Door Control Unit (DCU) system to keep train doors locked and closed while the train is in motion.

Functional safety is particularly relevant in modern railway systems due to the increasing reliance on electronic circuits and automated systems. As an example, these systems include automatic train protection (ATP), signalling systems, and train control units. Etc., where the failure of a component could lead to catastrophic outcomes. For instance, an ATP system must ensure that emergency braking is applied when required and that no unintended actions, such as doors opening during train movement, occur.

Different Sources of Risk

The main distinction between general safety and functional safety lies in the sources of risk they address. General safety covers risks that could arise from any part of the railway system, including human error, environmental factors, and infrastructure failures. Functional safety, however, is concerned specifically with risks that stem from electrical, electronic, or programmable electronic (E/E/PE) system malfunctions.

Conclusion

Safety and functional safety work together, but they focus on different risks. Safety looks at the whole system, while functional safety zooms in on the electronic parts that control critical functions. As railways become more automated, making sure these systems are both safe and functional becomes more important.

In my upcoming article, I'll explore the differences between Passive Safety Vs Active Safety. Be sure to check it out—happy reading!