To assess the functional safety of a machine, developers and users must consider the individual parts and components used, as well as how they interact with one another.
The SIL calculation (in accordance with IEC 61508 and IEC 62061) quantifies the failure of electrical, electronic and programmable electronic components (E/E/PE systems) due to random faults.
For purely mechanical or electromechanical components such as pressure switches, the safety assessment is generally carried out using the Performance Level (PL) in accordance with EN ISO 13849-1, as their failure behaviour can be represented in a practical manner using B10d/MTTFd. SIL and PL are correlated by means of standardised conversion tables. SUCO is also currently working on independent SIL certification in accordance with IEC 61508 for its mechanical pressure switches.
For SUCO pressure switches, EN ISO 13849-1:2008, Annex C (Safety of machinery - Safety-related parts of control systems), may serve as the basis for the safety assessment.
On this basis, we specify a B10d value of one million switching cycles for our mechanical pressure switches. This value can be used to derive the MTTFd value (mean time to dangerous failure). The MTTFd value is then used to determine the PL of the overall system and also for SIL classification in accordance with IEC 62061.
Specifically, the B10d value defines the service life at which 10% of the components can be expected to have failed dangerously. For SUCO pressure switches, the B10d value of one million switching cycles applies under the following boundary conditions: room temperature, resistive load, currents < 1 A, and a pressure rise rate of <= 1,000 bar/s within the permissible overpressure resistance.
SUCO cannot specify a general MTTFd value because it depends on the switching frequency of the specific application. However, the MTTFd time can be easily calculated from the B10d value as follows:
MTTFd = B10d / 0,1nop
nop : average number of switching cycles per year
B10d : average number of cycles until 10% of the components have failed
In accordance with DIN EN ISO 13849-1, SUCO mechanical pressure switches may be used in safety-related parts of a control system. These pressure switches have been designed and manufactured in accordance with basic and well-tried safety principles.
The SUCO SW24 mechanical pressure switches with normally open contact (type A) and normally closed contact (type B), as well as the SW27 with changeover contact (type C), comply with the single-break principle in accordance with DIN EN 60947-5-1 (SW24: switching element with two terminals; SW27: switching element with three terminals).
As a measure for controlling systematic failures in a fail-safe design, the user must apply the closed-circuit principle, meaning that the safe state is the closed contact in each case, regardless of whether a normally open or normally closed contact is used. Depending on the function to be monitored, the user must select the appropriate contact system.