Getting started with SIL...

[DLMUK]

Hi all,

We are being asked by companies to quote for systems meeting various SIL levels. Can somebody point me in the right direction or offer advice for someone who needs to start figuring out how to do it correctly.

I have had a scout around the net and found some general info but was after something more specific, i.e. a process which you follow start to finish. Or a step-by-step of how you calculate the SIL level of your system.

Thanks,

 

[CapinWinky]

Call a safety device vendor and get them to do a quick training for you. Schmersal did it for free for one OEM I know of with the understanding that they got exclusivity safety sensors/switches.

The hard part is deciding what SIL & PL to design to, once you know that, you just choose products that meet that requirement and follow the suggested wiring. Guard door design and if they have to be locking doors or not is the only thing you might have to consult a specification on. That and if you have hot stuff, like molten glass.

 

[mk42]

I think the official standard for SIL is IEC/EN 62061. Reading the standard is quite dense, but it is really the only way to understand what the requirements are. The requirements to meet safety standards are at least as much paperwork and making sure you've covered all your bases as they are about actual technical requirements.

There are two different parts involved with calculating Safety Integrity Levels (SIL). The first thing you need to do is a risk assessment, which tells you what risks and hazards a machine how, and how reliable a safety system you need. Second, you need to do reliability calculations to prove that the safety system that you designed actually meets that requirement. Most safety vendors have tools that can greatly assist with those calculations.

I've seen courses from TUV that train and certify people on the standard. They also offer services (for a fee, obviously) to assist with risk assessments, and safety system verification.

 

[D.Chris]

The overall standard for industrial functional safety is the IEC 61508 and 61511..

If you need to calculate the SIL of a system (or in this case you need to design a system which have to meet a certain SIL), you can do it by calculating the overall probability of failure on demand (PFD) of the loop which is depending on several things such as the configuration of the system (redundancy, etc), common mode failure, proof test interval, component's failure rate, etc.. The thing is, meeting a certain SIL is not just a matter of putting several SILX approved hardware and connect them together..

I suggest some google search with terms like "SIL verification methods".. Or if you can get it, try to get some training in the matter..

 

[JonAW]

If you regularly need to carry out new calculations there is a free software designed to calculate the mttdf and safety levels. Download it from here.

http://www.dguv.de/ifa/Praxishilfen/Software/SISTEMA/index-2.jsp

It takes a bit to get your head around it and is very involved but once set up and loaded with details of your favourite safety gear it will quickly and accurately write a safety report to go along with your system.

Hope this helps

Jonny