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Definition
Time at which statistically
10%
of test specimens have failed
(determined per DIN EN ISO
19973).
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Per definition, 10% of the test
specimens have failed at this
time.
A component can also fail
before the B10 value is reached.
The service life cannot be
guaranteed.
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Dangerous failures:
In relation to the safety of
machines/the EC Machinery
Directive/ISO 13849-1, only
dangerous failures are relevant.
It depends on the respective
application whether the failure is
a dangerous failure. If no
information is possible/available
on the number of dangerous
failures, ISO 13849 permits the
assumption that every second
failure is dangerous. Therefore,
it can be assumed that
B10 d = 2*B10.
B10:
Statistical probability
of failure
B10 d :
Statistical probability
of failure due to
dangerous faults
For which products do I require
a B10 d value?
For all products which are
subject to wear, are used in
safety-related parts of a
controller and directly contribute
to the execution of a safety
function, such as valves,
clamping cartridges, for
example.
This does not apply to fittings,
tubes, angle brackets, fixtures,
etc.
For which products do I need an
MTTF d value?
For all products which are used in
safety-related parts of a controller
and directly contribute to the
execution of a safety function,
such as controllers, fieldbus
nodes which serve to detect
dangerous situations, sensors
(test channel Category 2).
Do I need an MTTF d value or
B10 d value for components
which are used for monitoring
purposes in safety-related parts
of controllers?
No, for SRP/CS Category 3 and 4.
Yes, for SRP/CS Category 2 in the
test channel.
B10 value
Defining the MTTF d
The MTTF d value is application-dependent and describes the mean period to a dangerous failure of a system part.
Formula for determining the
MTTF d value for a mechanical
element in a channel
Mean number of annual
actuations n op for the
mechanical element
Calculating the total MTTF d
for two different channels
MTTF d =
B10 d
0.1 • n op
n op =
d op • h op • 3600s/h
t cycle
MTTF d =
MTTF dC1 + MTTF dC2 –
1
1
MTTF dC1
+ 1
MTTF dC2
2
3
Where:
B10
d
[cycles] = Mean number of cycles,
up to 10% of the components fail dangerously
B10
d
= 2xB10
hop [h/d]: Operating hours/day
dop [d/anno]: Operating days/year
tcycle [s]: Cycle time
MTTF
dC1
and MTTF
dC2
:
Values for two different,
redundant channels.
If the MTTF d of a channel is more
than 100 years, 100 years is used
for further calculation.