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Sample calculation:
The JUMO heatTHERM thermostat (STB - 602031) is used to monitor the maximum temperature in a drying
plant.
The drying plant is operated in three shifts. The plant is in operation 220 days a year. Because the limit
temperature range is only about 10 % above the operating temperature range, switching once per hour is
assumed.
B
= 250,000 cycles
10d
= 220 days
d
op
h
= 3 x 8 hours = 24 hours
op
d
h
3600
op
op
n
op
t
cycle
B
10d
MTTF
d
0.1 n
op
B
250000 cycles
10d
T
10d
n
5280
op
Based on table 5 of DIN EN ISO 13849-1, the calculated MTTFd value is classified as high. This assumption
applies for 47.35 years. The JUMO heatTHERM (STB) thermostat corresponds to category 1 and therefore
according to table K.1. of DIN EN ISO 13849-1, the maximum achievable PL is c.
Consideration as defined by DIN EN 61508 (DIN EN 61062)
The following formula is used to calculate the PFH or
Because the system has one channel and no diagnostics (1oo1), the following relationship applies:
1
PFH
d
MTTF
= Probability of a failure resulting in danger in FIT [1x10
PFH
= Rate of failure with resulting danger in FIT [1x10
d
= Failures in time (number of failures in 10
FIT
The following relationship applies for calculating PFH and
0.1
PFH
d
250000
This corresponds to 241.1 failures in 10
s
220d
24h
h
3600s
250000 cycles
cycles
0.1 5280
a
47.35 years
cycles
a
0.1
n
op
B
d
10d
1
10
5280
a
d
365
a
9
hours
s
3600
h
5280 switching cycles per year
473.48 years
based on DIN EN 61508.
d
-9
/hour]
-9
/hour]
9
hours)
in FIT in the example above:
d
9
h
241.1 FIT
h
24
d
Safety Manual
a = year
d = days
h = hours
s = seconds
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