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Safety Manual
Application of the B
to DIN EN ISO 13849-1:
B
10d
MTTF
=
d
0,1× n
op
B
= Mean number of cycles before 10 % of the test pieces
10d
have failed to danger.
MTTF
= Mean time to failure
d
n
= Mean number of actuations per year
op
d
= Mean operation time in days per year
op
h
= Mean operation time in hours per day
op
T
= Mean operation time until 10 % fail to danger
10d
PL
= Performance level
Sample calculation:
The JUMO exTHERM thermostat (STB - 605056) is used to monitor the maximum temperature inside a dryer
system.
The dryer system is operated in three shifts. The system is operated for 220 days a year. As the threshold tem-
perature range is only around 10 % above the operating temperature range, the calculation is based on
switches per hour.
B
= 250,000 cycles
10 d
d
= 220 days
op
h
= 3 x 8 hours = 24 hours
op
d
× h
× 3600
op
op
=
n
op
t
cycles
B
10d
=
MTTF
d
0,1× n
op
B
250000 cycles
10d
T
=
=
10d
n
5280
op
According to Table 5 of DIN EN ISO 13849-1, the MTTF
assumption remains valid for 47.35 years. The JUMO exTHERM thermostat corresponds to category 1, mean-
ing that, according to Table K.1. of DIN EN ISO 13849-1, the maximum PL will be c.
value to calculate the MTTF
10d
d
× h
op
n
=
op
s
220d × 24h × 3600
h
=
3600s
250000 cycles
=
cycles
0,1 × 5280
a
= 47,35 years
cycles
a
value according
d
s
× 3600
op
h
t
cycles
s
h
= 5280 cycles/year
= 473,48 years
value calculated is to be classified as high. This
d
B
10d
T
=
10d
n
op
[cycles]
[years]
[cycles/year]
[days/year]
[hours/day]
[years]
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