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 certain requirements of the corresponding safety integrity level (SIL) in accordance with the IEC 61508 standard. WIKA switches have 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 can emanate from critical processes must therefore be reduced to a satisfactory minimum. IEC 61508 provides 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 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 in connection with SIL approval, it really 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 also means an obvious statement on functional safety. In the case of mechanical parts, it really is primarily a question of the effects of aging and wear.
Individually installed: SIL 2, combined redundantly: SIL 3
The SIL certificate for WIKA? Valuable 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
As well as 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: ?Route 1H? states that the instruments or components involved have been developed and manufactured in accordance with the respective SIL specifications. ?Route 2H?, however, 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 component 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 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 ? pressure switches of the M, B, A, D, PC and PX series together with 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 further questions, your contact will gladly assist you to.
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