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Safety using S7 PLC

Created by: gcavallotti at: 5/6/2008 10:41 AM (9 Replies)

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5/6/2008 10:41 AM	Rate (0)
gcavallotti Posts: 5 Rating: (0)	Hello together, My question is to know if is possible to reach safety level requested by category 2 using redundant PLC´s (315 DP). Idea is to perform this function from two PLC's and the signals are derived from two separate sources to provide redundancy in the circuit. Do the standard PLC 315DP has a SIL classification? Any advice is welcome. Regards.
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5/6/2008 11:10 AM	Rate (0)
dchartier Grandmaster Joined: 9/27/2006 Last visit: 11/4/2024 Posts: 12293 Rating: (2690)	Hello gcavallotti; First point, the CPU 315-2DP cannot be used in any certified safety application. Second point, Siemens offers alternatives that do offer safety certification, with redundancy (F/H systems) or stand-alone (F-CPUs + F-I/Owith Distributed Safety package). See the following: 2.3 Fail-Safe Systems in SIMATIC S7 Which Fail-Safe Systems Are Available in SIMATIC S7? Two fail-safe systems are available for integrating safety engineering into SIMATIC S7 automation systems: 1. The S7 Distributed Safety system is used to implement safety concepts for protection of machines and personnel (for example, emergency OFF devices for operation of machine tool and production equipment) and in the process control industry (for example, for the execution of protection functions for measurement and control protection equipment and burners). 2. The fail-safe and, in particular, fault-tolerant S7 F/FH systems are available for automation of plants in the process control industry and in the oil industry. Fail-Safe, Fault-Tolerant S7 FH Systems To increase availability of an automation system and thus to prevent process failures due to errors in the F-system, S7 F-Systems can be optionally equipped with a redundancy feature (S7 FH Systems). Increased availability is achieved through component redundancy (power supply, central processing unit, communication and I/O). Achievable Safety Requirements S7 Distributed Safety and S7 F/FH systems can satisfy the following safety requirements: • Requirement class AK1 to AK6 in accordance with DIN V 19250/DIN V VDE 0801 • Safety class (Safety Integrity Level) SIL1 to SIL3 in accordance with IEC 61508 • Category 2 to Category 4 in accordance with EN 954-1 In the attached file you will have a list of available CPUs for either F or F/H applications. If you would like to readmore on the subject, download the files Safety Engineering in Simatic S7and Distributed safety: Configuring and programming Hope this helps, Daniel Chartier Attachment S7F_FH_CPUs.jpg (950 Downloads)
	Last edited by: dchartier at: 10/26/2010 6:33 PM a bit late, perhaps... Last edited by: dchartier at: 5/6/2008 11:14 AM
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5/6/2008 11:43 AM	Rate (0)
gcavallotti Posts: 5 Rating: (0)	Hello dchartier, thank you for your detailed answer. I agree that from literature Siemens provides wide possibility to ensure safety basing on new plc series. Actually I´m in a crucial position to accept safety protection made with standard PLC. To refuse it I would need to proof by numbers that the SIL level that it would be achievable is not acceptable (lower than SIL2). Thank you for any further indications.
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5/6/2008 1:33 PM	Rate (0)
dchartier Grandmaster Joined: 9/27/2006 Last visit: 11/4/2024 Posts: 12293 Rating: (2690)	Hello gcavallotti; Before you react to my comments, i would suggest that you look carefully at Chapters 5 and 6 of the Safety Engineering manual I suggested earlier. First point: all safety CPUs and I/Os proposed in the Siemens safety offering are TUV certified. Second: a lot of development was incorporated by Siemens in the safety systems based on international standards and guidelines for functionnal safety, machine and electrical equipmentsafety, and personnel safety. Third: the requirements of safety design level are based on standardized risk analysis and risk charts ; they introduce calculations of the effect onselected level or risk avoidance in the precise enviromnent it is used in. Basically, Siemens offers the user a certifiable safety system, that can be used as part of a risk abattement process in a plant or on a machine. By the way, they are not the only ones to offer such designs. Pilz is famous for the quality of their safety relays. As-iSafe is another venue that can be used for safe shutdown systems. Many other PLC manufacturers offer safety PLCs also (Rockwell, HIMA, Triconex...), all based on the same standards and certificates. Thia removes a great deal of pressure from the system provider, as without those standards and certificates he is engaging his own responsability in case of failure of the safety system. So maybe you could determine if in your case the safety system could be implemented outside of the PLCs, using Pilz safety relays maybe, and leaving the non-safety aspects of the control system in the 315-2s. Hope this helps, Daniel Chartier
	Last edited by: dchartier at: 5/6/2008 1:53 PM
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5/29/2008 9:44 AM	Rate (0)
Mario Braganca Posts: 124 Rating: (17)	Hello all DHChartier is right. I guess it's impossible to get SIL3 certification with Standard PLCs. I didn't do the calculations but I guess you cannot get the SIL3 Level because standard PLCs cannot achieve the so low level of Failures on Demand requested for a simple Fail-Safe Loop. Standard PLC alone has more Failures than the whole safe loop shouldhave. It's a long explanation but if you visit Exida site, they have a online calculation tool (you have to pay to use one month, for instance) with the tests of hundreds of equipments. You must calculate each single Fail-Safe loop, from sensors, through logic to control final elements. Depending on your hardware you can have even a Safety PLC or logic but the quality of your sensors and control elements do not enable your loop to be a legal SIL3. It's not a availability question but a realibilty question - the probability of a safe loop failure in a dangerous situation. Remember that these system are static. That is you cannot test them for months (years). And normally with reverse (negative) logic. So you have valves that are ON for months - and if you test them your process stops (and it costs money - maybe hundreds of thousands, depending on process). How do you know that this valve will work, that is open the contacts, after months? How do you know that the coil is not burned and the contacts melded. The SM 326 modules for instance send a 1 ms pulse to see the continuity of the installation. But only with a F-CPU. Ok. It's a long matter. Use Safety PLCs. It's safer. For all. Regards Mario
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5/29/2008 12:21 PM	Rate (0)
gcavallotti Posts: 5 Rating: (0)	Hello Mario, Thank you for your post, now we come nearer to my point. There is no doubt that safety plc are to be used, it is also common sense. The question is to demonstrate with number (therefore with evidences) that the use of common PLC in safety loop is degrading the SIL to a level that is not acceptable. Very interesting the Exida site you mentioned. Worth further investigation.
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5/29/2008 3:44 PM	Rate (0)
dchartier Grandmaster Joined: 9/27/2006 Last visit: 11/4/2024 Posts: 12293 Rating: (2690)	Hello gcavallotti; I had seen a SIL-assistant software on the net a while back, it is used to calculate the SIL level of a safety loop. Have a look at their manual, it gives a presentation of the calculations and the evaluationsinvolved n such a procedure. Maybe it will help ou in your research. http://www.tmartin.de/SIL-Assistant%20Manual.pdf Hope this helps, Daniel Chartier

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5/30/2008 6:34 AM	Rate (0)
gcavallotti Posts: 5 Rating: (0)	Hello Daniel, thank you for the information. I´ll study it. Their approach is quite clear, and surely it will help to semplify the item. Regards, Gabriele Cavallotti
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5/30/2008 10:29 AM	Rate (0)
gcavallotti Posts: 5 Rating: (0)	Hello Daniel, Interesting stuff. I spent a while looking for software on net. It seems it´s not available. Do you know anything? Thank you. Regards, Gabriele
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5/30/2008 1:20 PM

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Mario Braganca

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Hello again

Please read

Safety Engineering in SIMATIC S7 System Description

specially item 4.8

You can calculate the safety requirements using Picture 4.1 and Table 4.1 for each single loop.

After this, you can approach the managers with the following issues:

Safety Certified PLCs (TÜV) can achieve SIL2 or SIL3 Certification for the Safety Loops.
It's impossible to get these numbers with Standard PLCs. For instance: SIL 3 requires
probability of a dangerous failure per hour ≥ 10-8 to < 10-7.
If we cannot achieve these Certifications (and with Standard PLCs you can't) the consequences are:

Potential deaths, injuries, material losses, image losses, prison, etc.

You can search news about Bhopal in India, and other catastrophic accidents that could be avoid with the right Risk analysis.

In my country, I worked in a major company (Auto branch), many years ago, where 92 fingers were statisticaly lost for year.
And I worked also many year in a French Chemical Group. And the Group Lawer always said us that there were Engineers in Prison and Managers too, due to not fulfill the legal regulations.

The law in many countries says that the work must be done without risk. Every work has it's own risk even driving a taxi.
But the Judge does not consider this. So if there is an accident, it means that there is risk. And if there is risk someone is guilty because the law says that the work must be without risks.

So the final equation is MONEY X LIFES.

This is a qualitative argumentation, but you can calculate also how much it costs for a Company Union Carbide for example,an accident like Bhopal.

Good Luck

Created by: gcavallotti at: 5/6/2008 10:41 AM (9 Replies)

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