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) in accordance with 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 surroundings and the plant that can emanate from critical processes must therefore be reduced to an acceptable minimum. IEC 61508 supplies the necessary specifications because of this. These are defined in various 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. At the same time, it provides criteria for the possibility of controlling and avoiding systematic errors.
When 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. These include measuring instruments that do not want external power and for that reason work reliably, even in the event of a power failure. A SIL approval for these does mean an obvious statement on functional safety. In the case of 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 up with 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 arrive at a SIL assessment: ?Route 1H? states that the instruments or components involved have 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 in the case of 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 an authentic basis: For each element of a device, at the very least 100 million operating hours must 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 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 and temperature switches of the T series ? can be found on the WIKA website. Catastrophe can be available as a PDF file there. Should you have any further questions, your contact will gladly assist you to.
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