Navigating SIF logic solver selection: Why the Moore Industries SLA stands out

In industries where hazardous processes are part of daily operations, the importance of safety systems cannot be overstated. Safety Instrumented Functions (SIF) are at the heart of these systems, designed to bring a process to a safe state when specific hazardous conditions arise. Each SIF is typically designed with its primary purpose to reduce risk, with its effectiveness quantified by its Safety Integrity Level (SIL) capability. SIL ranges span from SIL 1, offering basic risk reduction, up to SIL 4, which delivers the highest safety integrity and reliability.

Most SIFs are comprised of three key components: sensors to detect a hazardous condition, a logic solver to interpret the signal and make decisions, and final control elements to act on that decision. Among these, the logic solver is particularly critical — it is, quite literally, the brain of the system. Choosing the right logic solver is essential not only for compliance but for ensuring that when the moment comes, the SIF performs as designed and takes the hazardous condition to a safe state.

To meet the SIL requirements of any application, each device in the SIF must be properly evaluated. This evaluation focuses on whether the device has a proven track record or provides sufficient safety data — metrics like Safety Failure Fraction (SFF), Probability of Failure on Demand (PFDavg), and Systematic Capability (SC). These parameters are typically detailed in the manufacturer’s Failure Modes Effects and Diagnostic Analysis (FMEDA) report or a formal safety certificate.

Logic solver selection generally falls into one of three categories. First is the proven in use approach, where the device's historical performance data demonstrates its suitability. Second is selecting a device supported by a manufacturer’s FMEDA report, which provides calculated failure data. Third — and increasingly viewed as the gold standard — is choosing a device that has been fully designed and manufactured in compliance with IEC 61508, complete with third-party certification and supporting safety documentation.

While proven-in-use data is an acceptable method to verify a device’s effectiveness, it often proves elusive in practice. Logic solvers, with their inherent complexity, rarely come with sufficient, verifiable historical performance data that satisfy stringent safety requirements. This leaves many safety professionals turning to manufacturer-provided failure data. While certainly easier to obtain, this data has its limitations. Most modern logic solvers fall into the ‘Type B’ category under IEC 61508, meaning they incorporate complex electronics and software. These systems can experience systematic failures — problems that arise not from predictable wear and tear, but from underlying design or firmware flaws. Unfortunately, failure data alone cannot fully capture these risks.

This is why the most reliable and future-proof path is to select a logic solver that is not just well-documented but fully compliant with IEC 61508 and certified by an independent third party. Devices that meet this rigorous standard have undergone extensive scrutiny of both hardware and software development processes, addressing systematic faults through documented fault avoidance and control measures.

Enter the Moore Industries SLA logic solver. The SLA stands out because it ticks all the critical boxes for SIF logic solver selection. Designed from the ground up in full compliance with IEC 61508, the SLA is certified for SIL 3 applications by independent assessors. Its FMEDA report provides transparent, detailed safety data, enabling risk reduction calculations with confidence. But beyond meeting the standard, the SLA excels in usability. It avoids the unnecessary complexity often seen in safety PLCs, offering streamlined configuration menus for alarming, Excel-like maths and logic functions, and simple 2oo3 or even 5oo8 voting architecture safety schemes. In a world where safety integrity is paramount and compliance can be a labyrinth, the Moore Industries SLA provides a reassuringly straightforward answer. It combines rigorous safety design with practical simplicity, reducing the burden on safety practitioners while enhancing overall system reliability. For those tasked with ensuring that their safety systems are bulletproof when it matters most, the SLA is more than a logic solver: it is peace of mind.