Why safety integrity level matters

In factory automation, safety systems are no longer the peripheral add-ons of the past. They are integral to the design, operation and long-term viability of production lines. As automation increases and machinery becomes more interconnected, engineers are routinely tasked with specifying safety controllers, networks, sensors and safety I/O devices that carry a defined safety integrity level (SIL).

SIL is intended to quantify how reliably a safety function reduces risk. Defined in standards such as IEC 61508 and IEC 62061, it measures the probability that a safety system will fail dangerously. In simple terms, the higher the SIL number, the lower the allowable probability of failure.

The challenge for automation engineers and engineering managers is that SIL is not a “more is better” metric. Specifying a SIL that is either too low or too high for a given application can introduce serious problems. The goal is not maximum SIL, but the correct SIL based on a sound risk assessment.

The consequences of under-specifying SIL

Choosing a SIL that is too low for an application carries the most obvious and severe consequences. At its core, an under-specified safety system does not reduce risk to a tolerable level. That increases the likelihood of injury, loss of life or significant equipment damage when failures occur.

From a compliance standpoint, this creates immediate exposure. Functional safety standards require that achieved risk reduction match the risk identified during assessment. If an incident occurs and investigations show the safety function did not meet the required SIL, the decision becomes difficult to defend. Regulators, insurers and courts will focus on whether the hazard was foreseeable and whether reasonable measures were taken to mitigate it.

Operationally, a low SIL can create a false sense of security. Operators and managers may believe a process is protected when it is not. Lower diagnostic coverage and higher probabilities of dangerous failure increase the chance that faults remain undetected until a hazardous event occurs.

There is also a long-term cost implication. If audits or insurance requirements later mandate a higher SIL, upgrading an operating line can be far more expensive than designing it correctly from the start. Retrofitting sensors, logic solvers and final elements often means downtime, revalidation and retraining.

In short, specifying too low a SIL leaves unacceptable residual risk in the system.

The hidden costs of over-specifying SIL

Over-specifying SIL is more common than many engineers admit. It is often driven by caution, lack of experience or a desire to future-proof a system. While this approach may feel safer, it brings its own set of drawbacks.

The most immediate impact is cost. SIL-rated controllers, safety I/O, certified networks and redundant architectures come at a premium. As SIL increases, the number of acceptable components decreases, limiting supplier options and driving up prices. Engineering hours also rise due to stricter documentation, verification and validation requirements.

Higher SIL systems are more complex to design and maintain. Redundancy, diagnostics and fault-handling must be carefully engineered and tested. Maintenance teams face increased proof-testing requirements and more rigorous recordkeeping. Any modification, even a small one, may trigger reanalysis and revalidation.

Availability is another concern. Poorly designed diagnostics can lead to nuisance trips and unnecessary shutdowns. Over time, this frustrates operators and maintenance staff and increases the temptation to bypass safety functions—an outcome that ultimately reduces safety.

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Overly conservative systems can also limit flexibility. As production needs evolve, highly constrained safety architectures may make it harder to adapt or expand a line without significant rework.

The result is a system that costs more, takes longer to commission and delivers little additional safety benefit beyond what a correctly specified SIL would provide.

Matching SIL to risk, not preference

The correct SIL emerges from a structured risk assessment. This includes identifying hazards, estimating severity, evaluating frequency and duration of exposure and considering the possibility of avoidance. Methods such as risk graphs, layers of protection analysis or mappings between performance level and safety integrity level help translate these factors into a required risk reduction.

Once that requirement is known, the safety function can be designed so the combined performance of sensors, logic solvers and final elements meets, but does not significantly exceed the target SIL.

A balanced outcome

When SIL is specified correctly, the benefits are clear. The system achieves compliance and credible risk reduction. Capital and engineering costs remain proportional to the hazard. Maintenance requirements are manageable, and the production line operates reliably without unnecessary interruptions.

For automation engineers and managers, the key lesson is that safety is not maximized by choosing the highest number on the scale. It is optimized by understanding the risk, applying standards rigorously, and designing a system that does exactly what is required—no more and no less.

In functional safety, precision matters. Getting SIL right is not just good engineering practice, it is essential to building manufacturing systems that are safe, defensible and sustainable over the long term.