Safety Integrity Level (SIL) is not just a math exercise or a line in a spec; it is the bridge between your risk assessment and how safe, reliable, and usable your machines actually are. For manufacturing professionals, the real question is less “What is SIL?” and more “What happens to my robot cell, press, or conveyor when we get SIL wrong—or right?”

Quick recap: what SIL is doing for you

In our previous article, we covered the basics: SIL is a measure of how reliably a safety function reduces risk over time, expressed through probabilities of dangerous failure and risk-reduction factors. It ties directly to how severe the hazard is, how often people are exposed, and how easily they can avoid it.

In practice, SIL is how you:

• Translate a risk assessment (severity, frequency, avoidance) into a target performance level for each safety function.

• Choose architectures, components, diagnostics, and proof-test intervals that match that target—no more, no less.

• Demonstrate to yourself (and auditors) that the safety-instrumented functions on your machines meet the risk reduction you claim.

Done well, SIL keeps you out of two equally dangerous ditches: underspecifying safety (“cheap but risky”) and overspecifying safety (“gold-plated but painful to live with”).

The danger of “too low” and “too high” SIL

When SIL is too low

Undershooting the required SIL effectively means accepting more risk than your own analysis said was tolerable. The practical consequences are:

• People and equipment are more exposed than you think. A single undetected fault in a sensor, relay, or output can turn into a dangerous failure when demanded.

• Compliance is hard to defend. After an incident, it quickly becomes obvious if the implemented loop never had a shot at achieving the required risk reduction.

• Fixes are disruptive and expensive. Upgrading a live machine from a lower to a higher SIL can mean new controllers, re-wiring, revised architectures, and re-validation—with real downtime.

When SIL is too high

Overshooting is more subtle but can be just as harmful:

• You pay more than you need to, in both hardware and engineering.

• You increase system complexity—more channels, diagnostics, and test intervals for maintenance to manage.

• You risk nuisance trips and frustrated operators, which is when bypasses and “temporary” fixes start to appear.

The sweet spot is “just right”: a SIL level that honestly reflects the risk, uses appropriate technology, and can be maintained by your team without heroics.

From here, the discussion gets real when you see how these three cases—too low, too high, and just right—play out in actual machines.

Example 1: Robot tending cell

Imagine a small robot cell tending a CNC or press brake—exactly the type of application many shops are now automating with collaborative and traditional robots.

Too low: “It stops when the door opens… usually”

A “too low” implementation might look like this:

• A single non-monitored gate switch on the access door.

• An e-stop loop wired through standard relays, no diagnostics, no rated safety logic.

• The robot’s safe torque off (STO) is not being used, or is wired in a non-redundant way.

On paper, someone might claim the risk is “low” because “we don’t go in there often.” In reality, changeovers and troubleshooting happen daily, and a single stuck contact could leave power available with the gate open. One random failure can defeat the entire safety chain.

Too high: “We trip constantly and nobody knows why”

On the other extreme, imagine specifying a very high SIL (or equivalent performance level) for a modest risk cell:

• Overly complex, multi-channel safety logic and devices, chosen simply because they have the highest rating.

• Aggressive diagnostics that trip the cell on minor or poorly filtered faults.

• Operators entering frequently for short tasks (like clearing chips or checking parts) and being hit with long restart sequences and unexplained trips.

The outcome:

• Operations pressure to “just make it run,” leading to taped-over sensors, bypass plugs, or permanent overrides.

• Maintenance spending too much time chasing nuisance faults instead of real issues.

Here, a “higher” SIL on paper may actually result in lower real-world safety, because the system is so unfriendly that people work around it.

Just right: matched to real hazards and usage

A “just right” robot cell for this scenario would:

• Use safety-rated interlocks on doors and access points, monitored through a safety controller or safety relay.

• Integrate the robot’s STO and safe speed/space functions (where available) into the safety concept, so interventions are fast and predictable.

• Define an SIL (or PL) appropriate to the energy, speed, and frequency of access—often in the SIL 1–2 range for typical tending cells—so risk reduction is credible without unnecessary complexity.

• Provide clear diagnostics and simple, consistent restart procedures so operators don’t feel forced to bypass.

From a manufacturing engineer’s perspective, the difference is obvious: the right SIL feels natural to run. You can change tools, clear parts, or adjust setups without feeling like you’re fighting the system.

Example 2: Mechanical or hydraulic press

Presses are where the stakes go up—high forces, fast cycles, and severe pinch and crush hazards.

Too low: treating a high-risk station like a light-duty machine

A “too low” scenario might include:

• Basic relays instead of safety-rated logic for e-stops and guards.

• Single-channel light curtains or palm buttons with no monitoring.

• No regular proof testing, and no diagnostics to detect faults between tests.

In this case, the risk assessment might clearly call for a SIL 2-level reduction (or higher), but the implementation behaves more like a SIL 1 or less: one hidden failure can defeat the system. If a dangerous motion demand coincides with that fault, the result can be catastrophic.

Too high: over-engineered and brittle

On the other side, some organizations push toward very high SIL around presses across the board, regardless of specific application details:

• Highly specialized, high-SIL logic solvers and devices for relatively standard press risks.

• Complex redundancy and cross-monitoring that are difficult for in-house teams to fully understand.

• Extensive periodic testing requirements that strain maintenance capacity.

The press becomes “safe” but also:

• Hard to modify when tooling or process changes.

• Risky to troubleshoot without advanced training.

• Prone to extended downtime if any safety component faults, because no one is comfortable making decisions.

Just right: SIL that matches severity and reality

A balanced approach would:

• Use a structured risk assessment to determine when SIL 2 is appropriate (often for higher-energy presses with frequent operator interaction).

• Implement safety functions—light curtains, two-hand controls, guard monitoring, STO, brakes—using safety-rated controllers and devices specifically designed to reach that SIL when combined.

• Design diagnostics and test intervals so they are maintainable: clear fault indication, documented test procedures, and realistic intervals tied to shutdowns or planned stops.

Here, SIL is doing what it is meant to: quantifying how robust your safety function must be, and guiding you to an architecture that your team can operate for years without eroding safety to stay productive.

Example 3: Conveying system across a line or plant

Conveyors move product, but they also move risk—people walk alongside them, work near pinch points, and sometimes climb on or under them.

Too low: blind spots and unsafe assumptions

A “too low” conveyor system might have:

• Scattered e-stops wired in series to a standard contactor.

• Unrated position or speed monitoring, if any, on high-risk sections.

• No zoning—everything runs or stops as one monolithic line.

The gaps here are:

• A single stuck contact can defeat an entire emergency stop string.

• No targeted protection for known high-risk areas like transfers, accumulation zones, or access points.

• No diagnostics—when something fails, the line just “does weird things,” and nobody knows what’s wrong.

Too high: the line that never stays up

Conversely, over-ambitious SIL targets across an entire conveyor network can result in:

• Excessively sensitive trip logic that shuts down large sections for minor issues.

• Global stops tied to local faults, leading to cascading downtime.

• Frequent frustration from operations who see the system as fragile and unpredictable.

Again, the temptation appears to bypass safety devices “just to keep the product moving,” which undermines the whole purpose of the higher SIL design.

Just right: zoned, targeted, and maintainable

A well-balanced approach uses SIL to:

• Set higher performance requirements (SIL 2, for example) around specific high-risk areas—entries into guarded cells, elevated sections over walkways, or manual intervention points.

• Use appropriate safety controllers, I/O, and devices to achieve that SIL in those zones, with clear fault and location indication.

• Zone the conveyors so local issues don’t kill the entire line unnecessarily, preserving availability.

• Align proof testing and inspection with regular maintenance windows, so the SIL assumptions remain valid over time.

For manufacturing professionals, this “just right” conveyor strategy means fewer surprises: when something stops, you know why, you know where, and you know how to restore it safely.

How to use SIL intelligently in your projects

Taken together, these examples show that SIL is best thought of as a sizing tool, not a badge of honor. For each safety function on each machine, you want to ask:

1. What is the real risk at this point in the process—severity, frequency, and possibility of avoidance?

2. What SIL (or equivalent performance level) does that risk justify—no more, no less?

3. Can our chosen architecture and components realistically achieve that level over the life of the machine?

4. Can our maintenance and operations teams support the diagnostics and proof testing needed to keep it there?

If the answer to any of those is “no,” you either don’t have enough SIL—or you’ve specified more than your plant can practically sustain.

For robot cells, presses, conveyors, and beyond, “too low” and “too high” both create unsafe conditions—one through under-protection, the other through workarounds and complexity. “Just right” SIL means you’ve matched the risk, the technology, and your people, so safety is not only credible on paper but sustainable on the floor.

If you’re planning a new cell or reworking an existing machine and aren’t sure what “just right” SIL looks like for your risk and budget, don’t guess. Talk with the engineering team at Automation Distribution about your robot cells, presses, or conveyors, and get help turning your risk assessments into practical, implementable safety architectures that your operators and maintenance team can actually live with.