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Consider a simple example:

  • there's a vessel with water and a continuos level sensor (4-20 mW signal)
  • there's a water line with a magnetic valve, feeding the vessel
  • When the water level is below a certain treshold, the valve opens - this is realized via a PLC unit somewhere

Now, consider the same scenario, the water level is critical for human safety so the whole refilling function needs to be SIL II. Now I - not an electrical or control systems engineer - need to understand how much more complications, cost, etc. will arise due to the SIL II requirement.

p.s. To give you an idea how much or little I know: My understanding is that I either need a safety PLC or a SIL II certified measurement amplifier with an additional potential free contact set to the correct level, hardwired to the relais controlling the valve.

Since SIL are describesd in an international IEC norm, I assume the answer will not be highly dependent on local codes.

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    $\begingroup$ The SIL risk assessment should apply to the complete system, not to individual components (ref: IEC 61508) Also "SIL" strictly speaking only applies to electrical components. As a trivial (but not unrealistic) example, if your system could be exposed to temperatures below 0C, your "SIl II certified PLC" etc won't prevent failure from burst pipes when the temperature rises again! IEC62061 may be more relevant than IEC 61508 for the complete system. $\endgroup$ – alephzero Jan 25 '19 at 11:40
  • $\begingroup$ This is of course correct. I'm mostly asking about control systems because that's the part I understand the least. $\endgroup$ – mart Jan 25 '19 at 12:21
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    $\begingroup$ You don't know what reliability you need from the components till AFTER you have assessed the complete system. For example, consider what happens if the only purpose of the sensor and controller is the stop the tank filling, and there is an adequate overflow system. So far as the water level is concerned, the reliability of the sensor and controller is now irrelevant. Their only function is to prevent wasting water. $\endgroup$ – alephzero Jan 25 '19 at 19:23
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Within the U.S. the standard is ISA 84.00.01 and internationally it is IEC 61511 which is the process industry standard for functional safety. Typically some type of risk analysis is driving the SIL requirement. Often this is a risk analysis of some type providing a quantitative failure rate of a safety function. This helps determine the level of SIL protection based on some risk tolerance criteria which is often company specific.

A SIL 2 function will require devices which have a SIL rating which includes the safety relay or PLC or DCS. A calculation should be done according to the requirements of the standard to determine the SIL reliability based on the probability of failure on demand for the input devices and control elements. Often a more reliable function requires multiple input or control devices. If you truly have a SIL requirement, I suggest to hire a qualified engineering firm to help design it.

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