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. This is always the case if the instruments fulfil certain requirements of the corresponding safety integrity level (SIL) in accordance with 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 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. Concurrently, it provides criteria for the chance of controlling and avoiding systematic errors.
Once the topic of ? เพรสเชอร์เกจ ? is discussed in expert groups regarding the SIL approval, it really is mainly concerning electrical systems. However, numerous mechanical components may also be integrated within safety-relevant applications. These include measuring instruments that not want external power and for that reason work reliably, even yet in the event of a power failure. A SIL approval for these does mean an obvious statement on functional safety. Regarding mechanical parts, it 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 level SIL 3.
Two routes to assessment
As well as the actual classification, the certificate for the WIKA switches also contains 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 in question 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. 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 relative to route 2H refers to 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 home elevators switches with SIL certificate ? pressure switches of the M, B, A, D, PC and PX series together with temperature switches of the T series ? are available on the WIKA website. The approval certificate can be available as a PDF file there. When you have any further questions, your contact will gladly assist you to.
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