Pilz PNOZsigma, Applications Manual

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1.1-6
1.1
Standards and Directives
Safety-related parts of control systems –
General principles for design in accordance with EN ISO 13849-1
Safety-related parts of control systems
– General principles for design in accor-
dance with EN ISO 13849-1
As the successor standard to EN 954-1,
EN ISO 13849-1 is based on the familiar ca-
tegories. Equally, it examines complete safety
functions, including all the components in-
volved in their design. EN ISO 13849-1 goes
beyond the qualitative approach of EN 954-1
to include a quantitative assessment of the
safety functions. A performance level (PL) is
used for this, building upon the categories.
Components/devices require the following
safety parameters:
• Category (structural requirement)
• PL: Performance level
• MTTF
d
: Mean time to dangerous failure
• DC: Diagnostic coverage
• CCF: Common cause failure
The standard describes how to calculate
the performance level (PL) for safety-related
parts of control systems, based on designa-
ted architectures. EN ISO 13849-1 refers any
deviations to IEC 61508.
Parameters S, F and P are used on the risk
graph to determine the required performance
level (PLr) for a safety function. The selection
of parameters is no different to the procedure
used in EN 954-1 (1996). However, the result
is no longer a category but a PL.
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Risk assessment in accordance with EN
ISO 13849-1
Risk assessment is an iterative process, i.e. it
will need to be carried out more than once. The
risk must be estimated and the performance
level defi ned for each hazard on which the risk
is to be reduced through control measures.
The risk is estimated through consideration
of the severity of injury (S), the frequency and
duration of exposure to the hazard (F) and the
possibility of avoiding or limiting the harm (P).
Determination of the required Performance Level (PL
r
)
S – Severity of injury
S
1
= Slight (normally reversible injury)
S
2
= Serious (normally irreversible injury, including death)
F – Frequency and/or exposure to a hazard
F
1
= Seldom to less often and/or the exposure time is short
F
2
= Frequent to continuous and/or the exposure time is long
P – Possibility of avoiding the hazard or limiting the harm
P
1
= Possible under specific conditions
P
2
= Scarcely possible
Low risk
High risk
Starting point
for evaluation
of safety
function’s
contribution to
risk reduction
Requir
ed Performance Level PL
r