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3 Useful information
Useful information
3
3.1
Basics
3.1.1
Basic terminology
Cross-circuit
Cross-circuit detection is a diagnostic function of an evaluation unit that detects
short-circuits or cross-circuits between two input channels (sensor circuits).
A cross-circuit can be caused, for example, by a sheathed cable being pinched. In
devices without cross-circuit detection, this can mean that a two-channel
emergency stop circuit does not trip even though only one NC contact is faulty
(secondary error).
Feedback circuit
A feedback circuit is used for monitoring controlled actuators (e.g., relays or load
contactors) with positive-action contacts or mirror contacts. The outputs can only
be activated when the feedback circuit is closed. When using a redundant
shutdown path, the feedback circuit of both actuators must be evaluated. These
may also be connected in series.
Positive opening operation
Positive opening switches are designed in such a way that the actuation of the
switch inevitably causes the contacts to open. Welded contacts are broken by
actuation (EN 60947-5-1).
Positively-driven contacts
With a component that has positively-driven contacts, it is guaranteed that the NC
and NO contacts are never closed at the same time (EN 60947-5-1).
3.1.2
Functional safety
From the perspective of the object to be protected, safety is indivisible. The causes
of hazards and, in turn, the technical measures to avoid them can vary significantly.
This is why a differentiation is made between different types of safety (e.g. by
specifying the cause of possible hazards). We therefore speak of "electrical safety"
when referring to protection against hazards caused by electricity, or "functional
safety" when safety depends on correct functioning.
To ensure the functional safety of a machine or plant, the safety-related parts of the
protection and control devices must function correctly. In addition, the systems
must behave in such a way that either the plant remains in a safe state or it is
brought into a safe state if a fault occurs.
In this case, it is necessary to use specially qualified technology that fulfills the
requirements described in the relevant standards. The requirements for achieving
functional safety are based on the following basic goals:
•
Avoidance of systematic faults
•
Management of systematic faults
•
Management of random faults or failures
The measure for the achieved functional safety is the probability of dangerous
failures, the fault tolerance and the quality that is to be guaranteed as a result of
Inductive clocking sensor
Entry ID: 109818998,
V1.0,
05/2023
23
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