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WORKPLACE SAFETY


DESIGNING A FUNCTIONALLY SAFE SIL 3 ANALOGUE OUTPUT MODULE


WITH SIL 2 COMPONENTS Manufacturers requiring a safety integrity level (SIL) 3 solution face several challenges when using SIL 2 components. As Revision 3 of the industrial functional safety standard IEC 61508 is released, new methods must be employed. This article from Brian Condell, product applications engineer at Analog Devices, outlines a solution to overcome the challenges of successful SIL 3 implementation and reduce time to market.


T


here has been a marked uptake in industrial functional safety systems over the last number of years, which has been driven by several factors such as:


Manufacturers’ desire to use new complex technology to lower costs (for example, use of safe torque off instead of adding a second contactor)


The use of robots, specifically collaborative robots, which has been shown to bring productivity improvements across many factory floors


Recognising that using safety certified equipment improves overall reliability


Acknowledging that the use of diagnostics improves throughput in many factories and plants


The introduction of new safety requirements


An additional driver has been the introduction of stringent requirements for the energy, oil, and gas sectors combined with regulatory obligations.


Before getting into too much detail, let us take a look at some basic definitions to help readers of all levels better appreciate this article.


WHAT IS SAFETY?


Safety is considered to be freedom from unacceptable risk. For example, an unprotected rotating machine on a factory floor would be considered unsafe.


WHAT IS A SAFETY FUNCTION?


This defines an operation that must be carried out to achieve or maintain safety. The purpose of a safety function is to reduce risk in the system. For example, if that same rotating machine had a light curtain installed in front of it, the safety function would be to detect the broken light beam when a hand passes through it and stop the rotating machine before the hand has time to touch it.


Figure 1. A typical safety function.


Typically, a safety function has three subsystems. Figure 1 shows a safety system that is used to detect the level of a hazardous liquid and turn off the flow when it is full.


An input subsystem used to detect a value or state (sensor, like a level sensor)


A logic subsystem that decides if the state is hazardous (programmable logic controller (PLC))


An output subsystem that can take an action to maintain safety (actuator)


WHAT IS FUNCTIONAL SAFETY?


This deals with confidence that a system will carry out its intended safety function when required to do so. It is effectively a measure of how confident a functional safety engineer is that the light curtain and stopping safety function of the motor will operate when the light beam is broken.


A system is considered functionally safe if the hardware metrics (random errors), systematic capability (SC), and common cause failures (CCF) do not lead to malfunctioning of the safety system, injury or death in humans, damage to the environment, nor loss of production. Now with some of the basic safety definitions explained, let’s consider some functional safety standards that must be adhered to when designing a functional safety system and what benefits they bring. When following a functional safety development process like IEC 61508 or


14 SUMMER 2024 | INDUSTRIAL COMPLIANCE Figure 2. Safety standards.


This article will focus on IEC 61508 for industrial applications and specifically how to design a SIL 3 solution with SIL 2 components using identical redundancy.


REDUNDANCY, HIGH AVAILABILITY, AND HARDWARE FAULT TOLERANCE


No matter how reliable a system is, systems will eventually fail! Two common failure types are systematic and random. See Figure 3.


ISO 26262 for example, there are many benefits to manufacturers like:


Improved up front requirements clarity Fewer bugs during testing


Greater consistency within the software written


Fewer defects found during integration More thorough testing Fewer defects in the field


Improved differentiation compared to the competition


There are many safety standards (see


Figure 2), most of which have been derived from the industrial IEC 61508 standard. It is worth noting that 90 per cent to 95 per cent of the IEC 61508 requirements are similar across all standards.


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