Mitsubishi Electric MELSEC iQ-R Series Safety Manual page 26

Mean Time to Dangerous Failure (MTTFd)
Failures can be divided into safe failure triggering the fail safe after the failure occurs and dangerous failure not triggering
safety functions because of an inability to detect the failure. Mean Time to Dangerous Failure (MTTFd) means a mean time
regarding dangerous failure.
The following shows the classifications of the MTTFd.
Notation
Low
Medium
High
(For details, refer to ISO13849-1: 2006)
MTTFd can be calculated based on component failure rate and average number of operations per year.
If the values of devices used (average number of movements until 10% of the device undergoes dangerous failure) are
known, calculate the MTTFd value using the following formula.
If the values are not known, refer to ISO13849-1: 2006 or ask the manufacturer.
B
10d
MTTF =
d
0.1×n
d ×h ×3600
op op
n =
op
t
cycle
• n : average number of operations per year (times/year)
op
• d : average number of days operated per year (days/year)
op
• h : average number of hours per day (hours/day)
op
• t : average hour per cycle (seconds/time)
cycle
For details on the calculation of the MTTFd value, refer to ISO13849-1: 2006.
Diagnostics coverage (DC) and average diagnostics coverage (DCavg)
The scope of the diagnostics coverage is the diagnostic ratio of the diagnostic method used for safety system components. It
is defined based on the ratio of probabilities of dangerous failure to be detected at diagnostics and all dangerous failures. The
standard specifies classification rules for how to determine which diagnostic scope shall be classified into either low, medium,
or high as representative diagnostic methods, and also numerical values within the scope of diagnostics for classifications
(low, medium, and high) For details on the DC estimation method, refer to ISO13849-1: 2006.
This indicates classification table for diagnostic scope.
Notation
None
Low
Medium
High
(For details, refer to ISO13849-1: 2006)
When the safety system uses multiple components, use average diagnostics coverage (DCavg) as a mean value for
diagnosis coverage (DC) for these components. The DCavg can be estimated based on the ratio of the sum total of the
probability of dangerous failures to be detected at component diagnostics and the sum total of probability for all dangerous
failures using the following formula.
DC DC
1
+
MTTF MTTF
d 1
DC =
avg
1
+
MTTF MTTF
d 1
• DCi: A component configuring safety system i (i = 1, ... , N)
• MTTFdi: A component configuring safety system i (i = 1, ... , N)
The relationship between low, medium, and high for the DCavg and figures shall follow the table above.
3 RISK ASSESSMENT AND SAFETY LEVEL
24
3.3 PL
op
DC
2 N
+
MTTF
d 2 dN
1 1
+
MTTF
d 2 dN
Scope of MTTFd
3 years  MTTFd < 10 years
10 years  MTTFd < 30 years
30 years  MTTFd  100 years
Scope of DC
DC < 60%
60%  DC < 90%
90%  DC < 99%
99%  DC