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) in accordance with the IEC 61508 standard. WIKA switches have already been issued SIL approval because of their proven operational reliability.
Types 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 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. At the same time, it offers criteria for the possibility of controlling and avoiding systematic errors.
Once the topic of ?functional safety? is discussed in Excellent regarding the SIL approval, it is mainly concerning electrical systems. However, numerous mechanical components are also integrated within safety-relevant applications. Included in these are measuring instruments that not want external power and therefore work reliably, even yet in the event of an electrical failure. A SIL approval for these does mean an obvious 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. 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 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 been developed and stated in accordance with the respective SIL specifications. ?Route 2H?, alternatively, 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 an authentic basis: For each element of a device, at least 100 million operating hours will need to 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 for example temperature, vibration etc. are considered.
Note
Detailed Astonishing with SIL certificate ? pressure switches of the M, B, A, D, PC and PX series and temperature switches of the T series ? are available on the WIKA website. The approval certificate is also available as a PDF file there. In case you have any further questions, your contact will gladly assist you to.
Also read our articles
SIL pressure switch: Safety in tyre manufacturing

Leave a Comment