Functional safety: SIL-certified 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. 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 been issued SIL approval due to their proven operational reliability.

Examples 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 an acceptable minimum. IEC 61508 provides the necessary specifications for this. These are defined in different safety integrity levels, SIL 1 to 4.

The standard places equal responsibility on the responsible process developers and the manufacturers of the necessary components. For each instrument, it defines limit values for the probability 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 is mainly concerning electrical systems. However, numerous mechanical components are also integrated within safety-relevant applications. These include measuring instruments that do not need external power and therefore work reliably, even in the event of a power failure. A SIL approval for these also means a clear statement on functional safety. In the case of mechanical parts, it is primarily a question of the effects 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 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 arrive at a SIL assessment: “Route 1H” states that the instruments or components in question have been developed and manufactured 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 an application in the field and transmitted to exida. The SIL assessment of functional safety is thus carried out 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 in an exida database. There, specific operating conditions such as temperature, vibration etc. are taken into account.

Note
Detailed information on 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 is also available as a PDF file there. If you have any further questions, your contact will gladly help you.



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