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 must be guaranteed accordingly. This 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 due to 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 environment and the plant that can emanate from critical processes must therefore be reduced to a satisfactory minimum. Barrage provides the necessary specifications for this. These are defined in different safety integrity levels, SIL 1 to 4.
The standard places equal responsibility on the responsible process developers and the manufacturers of the required components. For each instrument, it defines limit values for the likelihood of failure of a safety-relevant function. As well, it provides criteria for the chance of controlling and avoiding systematic errors.
Once the topic of ?functional safety? is discussed in expert groups regarding the SIL approval, it is mainly concerning electrical systems. However, numerous mechanical components may also be integrated within safety-relevant applications. Included in these are measuring instruments that do not want external power and therefore work reliably, even in the event of a power failure. A SIL approval for these also means a clear statement on functional safety. In the case of mechanical parts, it really is primarily a question of the effects 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 particular level SIL 3.
Horrific to assessment
In addition to the actual classification, the certificate for the WIKA switches also contains the addition ?route 2H?. In principle, there are two ways to arrive at a SIL assessment: ?Route 1H? states that the instruments or components in question have already been developed and manufactured in accordance with the respective SIL specifications. ?Route 2H?, alternatively, confirms that the assessment was additionally based on their operational reliability, as in the case of WIKA switches. The certified error rate is calculated from data collected during used in a credit card applicatoin in the field and transmitted to exida. The SIL assessment of functional safety is thus completed on a realistic basis: For each component 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 identifies the empirical values from field use, which are stored in an exida database. There, specific operating conditions such as temperature, vibration etc. are considered.
Note
Detailed info on switches with SIL certificate ? pressure switches of the M, B, A, D, PC and PX series together with temperature switches of the T series ? can be found on the WIKA website. The approval certificate is also available as a PDF file there. When you have any further questions, your contact will gladly assist you to.
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