SIL CALCULATIONS

Jose-

I presume you are referring to Safety Integrity Level. If that is the case, IEC 61508 defines each SIL level, but it sounds like you may have the cart before the horse, so to speak.

Normally, the SIL required for any given control circuit is determined after a risk reduction analysis is performed for the task governed by the control circuit. The circuit is then designed to conform to the standards defined by the task-specific SIL requirement.

For example: say you have a situation where an operator needs to periodically enter a robotic cell to effect a tooling change. The severity of injury that could be incurred due to an unexpected movement of the robot would most likely require a SIL 3 system to meet the required risk reduction for this task. The control circuit that allows the operator to shut down the robot and enter the cell would then need to be designed to meet SIL 3 requirements.

Meeting any given SIL is accomplished through circuit design and selection of properly rated components. If this is an in-house one-off design, it should be certified by a licenced safety professional prior to construction. At the same time, validation protocols must be written to test each safety function of the control circuit. These protocols must be executed immediately after the circuit is installed, prior to it's commissioning for use. The safety circuit also must be inspected/tested regularly after it is put in service.

Your documentation will consist your initial risk assessment showing the required SIL, your certified design, the signed-off validation protocols and your ongoing inspection/testing log.

Does that fill in some of the blanks? If you're interested, I do consulting on this type of work and would be happy to help out with some of the specifics. Feel free to send me a message. Good luck!