MTTFD , Mean Time to Dangerous Failure , refers to the expected mean time to dangerous failure.
The MTTFD for each channel is given in three levels (see Table 1 ).
Table 1. Mean Time Between Critical Failures ( MTTFD ) for each channel
For Category 4 safety control systems, the maximum MTTFD value per channel is increased to 2500 years.
Whether it is a single component, such as a transistor, valve, or contactor, or a module, channel, and control system as a whole, it possesses MTTFD .
For estimating the MTTFD of a component, the data should be looked up in the following order:
a ) Using manufacturer data;
b ) Use the methods in Annex C and Annex D of GB/T 16855.1 ;
Appendix C of GB/T 16855.1 is for the calculation or evaluation of the MTTFD value of a single component.
Appendix D of GB/T 16855.1 provides a simplified calculation method for the MTTFD values of each channel.
c ) Selected as 10 years.
In the design of safety control systems, the main parameters for obtaining accurate PLA values are MTTFD , category, and DCavg . These three parameters can be used to determine a unique PLA value based on Appendix D of GB/T 16855.1 . In traditional safety control systems, the MTTFD needs to be calculated for electromechanical components such as emergency stop buttons, mechanical safety door interlock switches, or contactors .
Here is an example of calculating MTTFD:
This is a channel consisting of a mechanical position switch SW1B and a contactor K1B .
Assuming the manufacturer provides values of B10D,SW1B = 20,000,000 times and B10D,K1B = 400,000 times, and taking a working period of 300 days / year, 16 hours/day, and a cycle time of 4 minutes ( the position switch is operated every 4 minutes, and the corresponding contactor also operates every 4 minutes), we obtain MTTFD,SW1B = 2778 years (Formula 2 ) and MTTFD,K1B = 56 years. According to the component counting method in D.1 , the MTTFD for this channel is calculated to be 55 years (Formula 1 ).
B10D — Number of cycles until 10% of components are at risk of failure (for pneumatic and electromechanical components).
nop — average number of operations per year
According to Table 1, the MTTFD of this channel is "high".
For this channel, the resulting MTTFD = 55 years or MTTFD "high" can be used to determine the precise PLA value or to evaluate a simplified procedure for PLA .
