In brief
Under IEC 61508, SIL (safety integrity level) is a four-level measure of the reliability with which a safety function reduces a risk.
Contents
What is SIL?
The safety integrity level originates from the IEC 61508 series of standards (DIN EN 61508) on the functional safety of electrical, electronic and programmable electronic systems. There are levels SIL 1 to SIL 4, with SIL 4 imposing the highest requirements. What is assessed is not an individual device but a safety function, such as switching off a pump in the event of overpressure, consisting of sensor, logic and actuator.
To determine the required SIL, the risk is first analysed. The level achieved then depends on the probability of failure, expressed as PFD for low demand and as PFH for high or continuous demand, as well as on the hardware fault tolerance, the safe failure fraction and systematic requirements for development and software. Derived standards are IEC 61511 for the process industry and IEC 62061 for machinery.
For maintenance and spare parts procurement, what counts is this: components in safety functions may only be replaced by parts with the same or higher SIL capability and comparable parameters, and proof test intervals must be observed. The text of the standard and the manufacturer’s safety manual are authoritative.
In practice: maintenance and procurement
For SIL-relevant spare parts, request the certificate or manufacturer’s declaration of SIL capability and the safety manual. After replacement, the safety function must be tested and documented before the plant is put back into operation.
Synonyms and abbreviations: safety integrity level · IEC 61508 · SIL rating · SIL capability
Frequently asked questions
What is the difference between SIL and performance level?
Both describe the reliability of safety functions but originate from different standards. SIL is based on IEC 61508 and is used in the process industry and, via IEC 62061, also for machinery. The performance level comes from EN ISO 13849-1 and is widespread in mechanical engineering, including for pneumatic and hydraulic control systems. The standards provide guidance on how the two relate.
What does IEC 61508 cover?
IEC 61508 is the basic standard for functional safety. It describes how electrical, electronic and programmable electronic safety systems are developed, assessed, operated and modified throughout their entire life cycle. This includes risk analysis, determination of the SIL, hardware and software requirements, and verification and validation. Sector standards such as IEC 61511 build on it.
Can a SIL-certified device be replaced by a different one?
In principle, yes, if the replacement device is suitable for the required SIL and its parameters, such as failure rates and proof test intervals, fit into the calculation of the safety function. A different model may require the safety function to be reassessed. After replacement, a functional test and documentation are required. Responsibility lies with the operator or the competent person.