2 - Schneider Electric CZ, s.r.o.

Presentation
Machine safety
Functional Safety of Machinery
Standard EN ISO 13849-1
Standard EN ISO 13849-1
Machinery safety - Safety-related parts of control systems
Process (continued)
Stage 4 - Evaluate the performance level PL for each safety-related part
Typically the data needed for the calculation of the performance level is being
provided by the components supplier. For safety processing devices the MTTF d
,
DC and performance level values are provided. For other non-safety components
such as contactors, limit switches, etc. which wear primary as a result of their
mechanical actuation, B10 d
values are provided by the supplier in some cases.
When the B10 d
values are not available, the annex C from the 13849-1 standard
has to be used.
Example
B 10d
(where 10% of the population fail MTTF d
DC
to dangerous failure mode)
SRP/CS a
: Limit switch
2 000 000 (the B10 d
is coming from the 284 –
typical components values table from
Annex C 13849-1)
SRP/CS b
: XPS AXE safety module - 457 99,99%
SRP/CS c
: TeSys contactor 1 369 863 194 99%
> > For estimating the performance level of a safety function, the condition is that the
MTTF d
, the DC and the category from each component are known. The
procedure to follow:
> > Calculation of MTTF d
and DC of the complete system
> > Analysis of the category.
> > For electromechanical products, the MTTF d
is calculated on the basis of the total
number of operations that the product can perform, using B 10d
values:
In our case, the machine operates for 220 days per year, 8 hours per day with a
cycle of 90 s
N = 220 x 8 x (3600 / 90) = 70 400 operations/year
MTTF d
= B 10d
/ (0.1 x N)
> > For the limit switch: the MTTF d
= (2 000 000) / (0.1) x 70 400 = 284 years
> > For the contactor:
> > The MTTF d
= (1 369 863) / (0.1) x 70 400 = 194 years
The MTTF d
for each channel will then be calculated using the formula:
i.e. 284 years.
A similar formula is used to calculate the diagnostic capability
1
2
3
4
5
6
The DC is our example is < 60%, e.g. nil.
Stage 5 - Checking that required performance level is achieved
The result of the above calculations is summarised below:
> > An architecture: category 1
> > A mean time to failure > 30 years: high MTTF d
> > A Diagnostic Capability < 60% (nil)
Looking at this table, we confirm that PL level c is achieved:
a
7
8
b
c
d
9
e
Cat. B Cat. 1 Cat. 2 Cat. 2 Cat. 3 Cat. 3 Cat. 4
DCavg =
0 (nil)
DCavg =
0 (nil)
DCavg =
low
DCavg =
medium
DCavg =
low
DCavg =
medium
DCavg =
high
Checking the PL
Stage 6 - Validation of the required performance level
The design of SRP/CS must be validated and must show that the combination of
SRP/CS performing each safety function satisfies all the applicable requirements
of EN/ISO 13849.
10
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