What is the Safety Integrity Level (SIL)?

If you have ever worked in a factory then you are probably familiar with lockout cages, buzzers and warning lights at every machine. These are typical devices in production facilities that are meant to provide a safe environment. There are regulations that must be followed to ensure the safety of workers, such as those stipulated by the Occupational Safety and Health Association (OSHA) of the U.S. Department of Labor. And these are important indeed; however, there are additional risks that industrial product manufacturers must contend with as well. These include risks to manufacturing process, equipment, and the environment.

Industrial machine warning lights

Industrial machine warning lights

Successfully contending with these potential factory contingencies means performing risk analysis and instituting a risk management plan, similar to what is required for the aerospace and medical devices industries. Although not mandated, IEC-61511 provides a framework for not only evaluating industrial risks, but also in creating a structured layer-based approach to mitigating them. At the root of this approach is the ability to define the type or level of safety protection required. In this article, we take a look at the four safety integrity levels (SILs), as defined by IEC-61508, and discuss why these are important to your industrial PCB development.

What is the Safety Integrity Level (SIL)?

Today, most industrial production is performed by automated product manufacturing systems. Although this means there are fewer opportunities for injuries to people, they still exist. Moreover, process failures can result in damage to equipment, their subsystems (including electronics and PCBs), the facility, and, if severe enough, the environment. To avoid situations that can escalate into contingencies such as these, many industrial operations institute safety-related systems that incorporate functional safety or electrical and/or electronic and/or programmable electronic (E/E/PE) devices.

Functional safety systems are active, not passive, and are designed to perform specific functions when activated. For each risk that cannot otherwise be mitigated or reduced to a tolerable level, a Safety Instrumented System (SIS) needs to be implemented. The International Electrotechnical Commission standard IEC-61508 Functional safety of electrical/electronic/programmable electronic safety-related systems provides guidelines for these systems. This includes specifying a safety integrity level (SIL) that must be assigned to each SIS. There are four SILs defined by IEC-61508 that are determined by first calculating a hazard Risk Reduction Factor (RRF). The ratio of the effective frequency of a hazard occurring without a SIS to the acceptable frequency for the hazard provides the minimum RRF and indicates what SIL is appropriate. An example of this relationship is shown below.

Safety Integrity Level (SIL)Risk Reduction Factor (RRF)
SIL 4≥ 10,000  and < 100,000
SIL 3≥ 1000 and < 10,000
SIL 2≥ 100 and < 1000
SIL 1≥ 10 < 100

As shown above the frequency of occurrence is a significant factor in determining whether an electronics based SIS is warranted. Often for industrial manufacturing processes, these E/E/PE systems are necessary and IEC-61511 provides the manufacturer with guidance on how this should be done.

Why Is the Safety Integrity Level Important to Industrial PCB Development?

As stated above, a primary factor of the SIL associated with an industrial process risk or hazard is how often it may occur. However, just as risk management for PCB manufacturing requires that controls be instituted to respond when risk thresholds are exceeded, action must be taken to return industrial processes to an acceptable risk level. For the production environment, these functional safety controls are systems comprised of electronics and circuit boards. These include switching devices for valves, motor controllers, communications, signal processing and logic circuitry, connectors, interfaces, and sensors.

Depending upon the likelihood of occurrence, which is directly related to the SIL, the quality and reliability of the PCBs for these systems can be critical. Failure could mean downtime, loss of productivity, equipment damage, or even loss of life. When developing PCBs for Industrial SISs, you should ensure that the following considerations are included in your process:

  • Performance classification

Safety devices are not intended to operate continuously. Instead, they need to respond reliably when needed and usually for a short period of time. You need to make sure that your board is build to the IPC classification, typically class 2, that aligns with this performance objective.

  • Component quality

Obviously, choosing components that meet your operational requirements is the priority; however, there are concerns for industrial environments. For example, repetitive motion or vibrations and high temperatures are common in factories. For these and other reasons, you should optimize your component selection process.

  • Structural integrity

One of the most important concerns for any PCBs that will be used in an industrial environment is making sure that your boards will not suffer from fatigue or any other failure due to the harsh industrial environment. Thus, ensuring your board’s structural integrity should be a priority.

  • Surface coating

Not only can industrial environments for embedded electronics be hot and vibratory, but there can also be debris, including liquids, that could disrupt or halt the operation of your board. One way to mitigate this is to ensure that your contract manufacturer (CM) adds a surface coating to your boards.

Tempo‘s Industry 4.0 Custom PCB Manufacturing Services
  • ISO-9001, IPC-600 and IPC-610 commitment to quality certifications.
  • Agile manufacturing process to support risk management.
  • Accurate quote in less than a day.
  • DFM support from Day 1 of design.
  • Entire turnkey PCB manufacturing in as fast as 3 days.
  • High precision, complex PCB fabrication and assembly capabilities.
  • Sources components from the most reputable suppliers in the industry to reduce procurement time and help with component security.
  • Specialization in fast prototyping and low-volume production.
  • Standard quality testing; including X-ray and inline AOI.
  • Advanced QC testing; including flying probe, time domain reflectometry and ROSE.

By utilizing these suggestions and other good tips for your industrial PCB manufacturing process, you can make sure that your boards will meet the requirements of any SIL deployment. Quality and reliability are critical for industrial boards charged with performing SIS actions to protect equipment and personnel. At Tempo Automation, we are fully committed to building the highest quality PCBs and partnering with you to institute the best methods to ensure your boards perform reliably.

And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.

If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on the safety integrity level (SIL) and developing industrial PCBs for production facility safety systems, contact us.

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