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. That is always the case if the instruments fulfil the 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 compact 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 may emanate from critical processes must therefore be reduced to a satisfactory minimum. IEC 61508 provides the necessary specifications for this. These are defined in various safety integrity levels, SIL 1 to 4.
The typical places equal responsibility on the responsible process developers and the manufacturers of the necessary components. For every instrument, it defines limit values for the probability of failure of a safety-relevant function. Simultaneously, it offers criteria for the chance of controlling and avoiding systematic errors.
When the topic of ?functional safety? is discussed in expert groups in connection with SIL approval, it is mainly concerning electrical systems. However, numerous mechanical components are also integrated within safety-relevant applications. These include measuring instruments that not need external power and therefore work reliably, even yet in the event of a power 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. According to its assessment, the instruments, when installed individually, meet up with the requirements of SIL 2, and in redundant combination, they even meet those of the particular 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? states that the instruments or components involved have already been developed and stated in accordance with the respective SIL specifications. ?Route 2H?, on the other hand, confirms that the assessment was additionally based on their operational reliability, as regarding WIKA switches. Humility certified error rate is calculated from data collected during used in a credit card applicatoin in the field and transmitted to exida. Speedy of functional safety is thus completed on an authentic 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 relative to 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 taken into account.
Note
Detailed info on switches with SIL certificate ? pressure switches of the M, B, A, D, PC and PX series and temperature switches of the T series ? can be found 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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