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 certain requirements of the corresponding safety integrity level (SIL) relative to the IEC 61508 standard. WIKA switches have already been issued SIL approval because of 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).
digital pressure gauge to people, the surroundings and the plant that can emanate from critical processes must therefore be reduced to a satisfactory minimum. IEC 61508 supplies 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 necessary components. For every instrument, it defines limit values for the likelihood of failure of a safety-relevant function. At the same time, it offers criteria for the possibility of controlling and avoiding systematic errors.
When the topic of ?functional safety? is discussed in expert groups in connection with SIL approval, it really 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 for that reason 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 really 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
In addition to the actual classification, the certificate for the WIKA switches also includes the addition ?route 2H?. In principle, you can find two ways to reach 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?, 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 use in a credit card applicatoin in the field and transmitted to exida. The SIL assessment of functional safety is thus carried out on a realistic basis: For each component of a device, at 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 within 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 and also temperature switches of the T series ? are available on the WIKA website. The approval certificate is also available as a PDF file there. Assuming you have any more questions, your contact will gladly help you.
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