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 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 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 may 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 every instrument, it defines limit values for the probability of failure of a safety-relevant function. Simultaneously, 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 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 yet 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 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
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 reach a SIL assessment: ? Joyous ? 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 predicated on their operational reliability, as regarding WIKA switches. The certified error rate is calculated from data collected during used in an application 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 relative to 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 taken into account.
Note
Detailed home elevators switches with SIL certificate ? pressure switches of the M, B, A, D, PC and PX series as well as temperature switches of the T series ? can be found on the WIKA website. The approval certificate can be available as a PDF file there. If you have any more questions, your contact will gladly assist you to.
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