Mechanical pressure and temperature switches perform sensitive tasks in safety-relevant applications, e.g. monitoring limit values. Their functional safety must be guaranteed accordingly. This 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 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 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 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 chance of controlling and avoiding systematic errors.
When the topic of ? Cool ? is discussed in expert groups 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 do not want external power and therefore work reliably, even yet in the event of an electrical failure. A SIL approval for these also means a clear statement on functional safety. Regarding 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 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, you can find two ways to reach a SIL assessment: ?Route 1H? states that the instruments or components in question 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 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 completed on a realistic basis: For each component of a device, at least 100 million operating hours must have been proven.
Empirical values in a database
The SIL assessment in accordance with route 2H refers to the empirical values from field use, which are stored within an exida database. There, specific operating conditions such as for example temperature, vibration etc. are taken into account.
Note
Detailed home elevators switches with SIL certificate ? Hypnotic 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. Exquisite is also available as a PDF file there. In case you have any more questions, your contact will gladly help you.
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