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5 Functional safety (SIL)
SOLITRAC 31 • Four-wire 4 … 20 mA/HART
62085-EN-211203
5 Functional safety (SIL)
5.1 Objective
In case of dangerous failures, processing facilities and machines can
cause risks for persons, environment and property. The risk of such
failures must be judged by the plant operator. Dependent thereon are
measures for risk reduction through error prevention, error detection
and fault control.
The part of plant safety depending on the correct functioning of
safety-related components for risk reduction is called functional
safety. Components used in such safety-instrumented systems (SIS)
must therefore execute their intended function (safety function) with a
defined high probability.
The safety requirements for such components are described in the
international standards IEC 61508 and 61511, which set the standard
for uniform and comparable judgement of instrument and plant (or
machine) safety and hence contribute to worldwide legal certainty.
We distinguish between four safety levels, from SIL1 for low risk to
SIL4 for very high risk (SIL = Safety Integrity Level), depending on the
required degree of risk reduction.
5.2 SIL qualification
When developing instruments that can be used in safety-instrument-
ed systems, the focus is on avoiding systematical errors as well as
determining and controlling random errors.
Here are the most important characteristics and requirements from
the perspective of functional safety according to IEC 61508 (Edi-
tion 2):
•
Internal monitoring of safety-relevant circuit parts
•
Extended standardization of the software development
•
In case of failure, switching of the safety-relevant outputs to a
defined safe state
•
Determination of the failure probability of the defined safety func-
tion
•
Reliable parameterization with non-safe user environment
•
Proof test
The SIL qualification of components is specified in a manual on func-
tional safety (Safety Manual). Here, you can find all safety-relevant
characteristics and information the user and the planner need for
planning and operating the safety-instrumented system. This docu-
ment is attached to each instrument with SIL rating and can be also
found on our homepage via the search.
5.3 Application area
The instrument can be used for point level detection or level measure-
ment of liquids and bulk solids in safety-instrumented systems (SIS)
Background
Plant safety by risk
reduction
Standards and safety
levels
Properties and require-
ments
Safety Manual
