SIL-certified: Functional safety with mechanical switches

Mechanical pressure and temperature switches perform sensitive tasks in safety-relevant applications, e.g. monitoring limit values. Their functional safety should be guaranteed accordingly. That is always the case if the instruments fulfil certain requirements of the corresponding safety integrity level (SIL) relative to the IEC 61508 standard. WIKA switches have been issued SIL approval because of their proven operational reliability.
Examples of WIKA switches whose functional safety is SIL-certified: a concise temperature switch with Ex d flameproof enclosure (model TCA; left), a Bourdon tube pressure switch (model BA/BAX; top right) and a miniature pressure switch (model PXS; bottom right).
Hazards to people, the surroundings and the plant that can emanate from critical processes must therefore be reduced to a satisfactory minimum. IEC 61508 provides the necessary specifications for this. These are defined in different safety integrity levels, SIL 1 to 4.
The typical places equal responsibility on the responsible process developers and the manufacturers of the required components. For every instrument, it defines limit values for the probability of failure of a safety-relevant function. At the same time, it provides criteria for the chance of controlling and avoiding systematic errors.
When the topic of ?functional safety? is discussed in expert groups regarding the SIL approval, it really is mainly concerning electrical systems. However, numerous mechanical components are also integrated within safety-relevant applications. Included in these are measuring instruments that do not need external power and therefore work reliably, even yet in the event of an electrical failure. A SIL approval for these does mean a clear statement on functional safety. Regarding mechanical parts, it is primarily a question of the consequences of aging and wear.
Individually installed: SIL 2, combined redundantly: SIL 3
The SIL certificate for WIKA?s pressure and temperature switches was issued in 2019 by exida, a globally recognised testing company. In accordance with its assessment, the instruments, when installed individually, meet the requirements of SIL 2, and in redundant combination, they even meet those of the level SIL 3.
Two routes to assessment
As well as the actual classification, the certificate for the WIKA switches also includes the addition ?route 2H?. In principle, you can find two ways to arrive at a SIL assessment: ?Route 1H? Gleeful that the instruments or components involved have already been developed and stated in accordance with the respective SIL specifications. ?Route 2H?, however, confirms that the assessment was additionally predicated on their operational reliability, as regarding WIKA switches. The certified error rate is calculated from data collected during used in an application in the field and transmitted to exida. The SIL assessment of functional safety is thus completed on a realistic basis: For each element of a device, at the very least 100 million operating hours will need to have been proven.
Empirical values in a database
The SIL assessment in accordance with route 2H refers to the empirical values from field use, which are stored within an exida database. There, specific operating conditions such as for example temperature, vibration etc. are considered.
Note
Detailed home elevators switches with SIL certificate ? pressure switches of the M, B, A, D, PC and PX series along with temperature switches of the T series ? are available on the WIKA website. The approval certificate can be available as a PDF file there. Should you have any more questions, your contact will gladly help you.
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