Glossary
© 2010 Wieland Electric GmbH | BA000256 | 12/2012 (Rev. I)
49
samos
and IEC/EN 61508/EN 62061
The international norm IEC/EN 61508/EN 62061 is the new standard for the specification,
design and operation of safety systems.
The norm takes the whole system into consideration. It expands the safety
requirements of single complex devices to cover the whole safety installation from sensor
through control electronics and communications systems to actuators, and also includes
possible errors by operators. The norm provides the framework for future developments
and is decisive for manufacturers and users alike, especially as it also touches on questions
of product liability.
The safety considerations concentrate on analyzing dangers and defining risks. The goal
of the assessment is to reduce the risk involved in a safety system to an acceptable level of
residual risk by calculating failure probabilities for components, system and design from
development right through to disposal.
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Safety integrity level
The safety function of the samos system is shutting down or preventing a dangerous
process. All the system functions described in this manual are safety functions.
A safety integrity level defines the range for failure probability of a safety function in
relation to operating mode and request frequency. IEC/EN 61508/EN 62061 describes two
modes for safety functions:
A safety function in demand mode is only executed on demand. It brings the
monitored system into a defined safe state and has no influence on the system in normal
operation (example: emergency stop monitoring).
A safety function in continuous mode continuously maintains the monitored system in
its normal safe state (example: speed monitoring).
This division means there are two different demand rates for safety functions:
Mode with low demand rate when the demand rate is less than once per year;
Mode with high demand rate or continuous demand when the demand rate is more
than once per year.
The demand rates are considered in relation to failure probabilities:
Low demand rate in relation to probability of failure on demand (PFD),
High demand rate or continuous demand in relation to probability of failure per hour
(PFH).
According to IEC/EN 61508/EN 62061, the samos safety function can be used in modes
with low and high demand rates.
With low demand rates the PFD for SIL 3 is
10
-4
to <10
-3
(e.g. emergency stop components) for the whole safety chain.
With high or continuous demand rates the PFD is
10
-8
to <10
-7
(e.g. two-hand application) for the whole safety chain.
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Other safety parameters used in this manual
SFF - Safe Failure Fraction
The percentage of the full number of errors that do not critically affect safety. Quantitative
parameter from IEC 61508-2. It characterizes the safety structure and defines whether a
component for executing the safety function can be single-channel or must be redundant
in the safety chain of the plant/machine.
DC - Diagnostic Coverage
Quantitative parameter from IEC 61 508, prEN ISO 13849. Identifies the proportion of
dangerous faults that will be detected.
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