06 apr 2021

All kinds of machines have become part of our daily lives for years now – we wash our dishes in a dishwasher, rely on cars and trains for our daily commute to work and even more applications exist for a plethora of different types of machinery. They all have one thing in common, however. They all must pass safety requirements.   

It sounds like a scene straight out of a Laurel & Hardy comedy skit, the clumsy factory worker who has to enter a machine to repair a broken part, meanwhile his colleague, not knowing why production was halted, turns it back on not knowing his friend is still inside. Although in the world of cinema all kinds of silly shenanigans occur due to this unfortunate situation, in the real world these scenarios would have a far more sinister and disastrous ending. To prevent accidents like these from happening, you need a Safety Integrity Level (SIL) or Performance Level (PL).  

Every machine or product you design will have to get a CE-mark in order to be sold within the European Union. In most cases the producer of these machines can apply these themselves, in this way you show that you commit to the European Directive, which shows that your device is safe for use. Some standards include the fact that you cannot get stuck in the machine, the thickness of your cables and where the openings are. When designing a machine, you should therefore always do a safety risk analysis.  

Let us take a gander at the following example: a sawing machine. What is a safe way to replace the sawblade? The operator has to reach it in order to be able to swap it, so there has to be a safety-protocol that nobody can use the saw while the blade is being swapped.  

SIL stands for ‘Safety Integrity Level’, while PL stands for ‘Performance Level’ and both are marks used as a certificate to prove the safety of your machine. Let’s take a look at the differences. Firstly, both have different levels of safety. PL goes from level PLA to PLB all the way to PLE. SIL has only four levels, SIL1, 2, 3 and 4 – where the latter is the strictest.   

With SIL you look at the architecture of the operating system. This results in a SIL-claim-limit, which reflects the maximum SIL-level you can achieve with your design. This is not the same with PL, with PL you have a small chance you have to go back and redesign your architecture when you don’t achieve your desired level.  

PL has its advantages though, its levels offer less choices than those of SIL, the latter having more wiggle room between them. With PL you either pass or you do not, SIL requires much more calculative research. 

When you assign your SIL or PL-mark your machine is set for the rest of its lifetime. Even if the standard changes, your machine is still allowed to carry its certification, only new designs need to match new standards.  

If you change components or other things in your design, however, you have to redo your calculations to see if your machine is still viable to carry its certification. Mostly it’s a case of reworking some formulas, but if changes create possible dangerous situations you have to go back to see if everything van be reworked to guarantee safety.   

Both norms can be used for any machine, but you mostly only go for one of the two. They aren’t connected either, conversion-graphs exists, but they are not entirely accurate. Most engineers use the one they are used to most, but we have already explored some interesting, subtle differences.  

We at PDM-Group are always thinking about the safety of our designs when creating machines, but also check designs our customers made to see if a SIL- or PL-mark is applicable. We always strive for Industrial Excellence, which includes the longevity of machinery and the people who operate them. By making sure every design is safe to use in operation and during maintenance we can ensure the sustainability of your operations for years to come and are always ready to assist in making your designs optimized for performance and safety.   

In short, the safety of people operating machinery is the most important thing in the workplace – which is why machines need to be safe to operate, SIL- and PL-marks exist as certificate to show that your machine has taken all sorts of safety-protocols into consideration. Which one to use is mostly which one your lead-designer prefers, but there are some subtle differences that make one preferable to the other. We at PDM-Group are always ready to assist in grading your designs, if you want more detailed information about SIL, PL or simply want us to help you, please contact Peter Timmermans at peter.timmermans@pdm-group.com .