SIL, MTTF and B10D - A User's Guide
In order to evaluate the functional safety of a machine, developers and users must consider the implemented parts and components individually but also in their interaction with each other.
The SIL calculation1) should quantify the failure of electrical, electronic and programmable electronic components (E/E/PE systems) due to random errors. For purely mechanical components and parts such as our mechanical pressure switches, a SIL calculation is not possible due to the SIL definition.
For mechanical components, EN ISO 13849-1:2008 Annex C (Safety of machinery - Safety-related parts of control systems) can be used as the basis for the safety assessment.
Based on this, we specify a B10d value with 1 million switching cycles for our mechanical pressure switches, from which the MTTFd value2) ableiten kann.
The MTTFd value is then included in the SIL calculation of the entire system.
B10d and MTTFd
The B10d value specifies the service life for which failures must be expected with a probability of 10 %. For SUCO pressure switches, the B10d value applies with 1 million switching cycles with the boundary conditions at room temperature for an ohmic load and currents < 1 A and a pressure rise rate of <= 1000 bar/s within the permissible overpressure resistance.
It is not useful to specify an MTTFd time for mechanical pressure switches, because the individual user-specific switching conditions have to be known. However, the MTTFd time can be easily determined from the B10d value as follows:
|MTTFd = ————|
nop = number of cycles per year
B10d = number of cylces until 10% of components have failed
Mechanical Pressure Switches
According to DIN EN ISO 13849, most mechanical pressure switches from SUCO can be used in safety-related parts of a control system. These pressure switches have been developed and manufactured according to fundamental and proven safety principles.
The design principle of the mechanical SUCO pressure switches hex 24 normally open (design A) and normally closed (design B) according to DIN EN 60947-5-1 (single break, switching element with two connections) or hex 27 with change-over contact design C according to DIN EN 60947-5-1 (single break, switching element with three connections) is considered as positively driven.
Our Pressure switches with integrated connectors hex 24 fulfill the requirements of design A, our Pressure switches with change-over contact hex 27 and 30 A/F fullfill the requirements of design C.
In order to control systematic failures "fail-safe", the user has to consider the closed-circuit current principle, i.e. the safe state is always the closed contact, regardless of whether normally open or normally closed contact. The user must select the suitable contact system depending on the function to be monitored.
The Pressure switch "PLUS" series relieves the user of some of the responsibility, since the integrated NAMUR resistor circuit directly implements the closed-circuit principle. With the NAMUR circuit, both cable breaks (no current) and short circuits between the conductors (current too high) can be detected.
1) SIL = Safety Integritiy Level
2) MTTFd = Mean time to (dangerous) failure