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Sample calculation:
The JUMO EM-80 thermostat (602026) 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
temperature range is only around 10% above the operating temperature range, the calculation is based on
switches per hour.
B
= 250,000 cycles
10d
d
= 220 days
op
= 3 x 8 hours = 24 hours
h
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
op
5280
According to Table 5 of DIN EN ISO 13849-1, the MTTFd value calculated is to be classified as high. This
assumption remains valid for 47.35 years. The JUMO EM-80 thermostat corresponds to category 1, meaning
that, according to Table K.1. of DIN EN ISO 13849-1, the maximum PL will be c.
Consideration on the basis of DIN EN 61508 (DIN EN 61062)
Using the following formula, the PFH or
Since a single-channel system without diagnosis (1oo1) is concerned, the following relationship applies:
1
PFH =
=
d
MTTF
PFH
= Probability of a critical failure in FIT [1x10
= Critical failure rate in FIT [1x10
d
FIT
= Failures in time (number of failures in 10
For the example above, the following applies to the calculation of PFH and the
0,1
PFH =
=
d
250000
This corresponds to 241.1 failures in 10
s
220d × 24h × 3600
h
=
3600s
250000 cycles
=
cycles
0,1 × 5280
a
= 47,35 years
cycles
a
can be calculated in line with DIN EN 61508.
d
0,1
=
× n
op
B
d
10d
-9
/hour]
1
10
× 5280
×
a
d
365
a
9
hours
s
h
= 5280 cycles per year
= 473,48 years
-9
/hour]]
9
hours)
9
h
= 241,1 FIT
h
× 24
d
Safety Manual
a = year
d = days
h = hours
s = seconds
in FIT:
d
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