A.4 Terms - Siemens SINUMERIK 840D sl Function Manual

Appendix

A.4 Terms

A.4 Terms
Category
Used in EN 954–1 to "Classify safety–related parts of control with reference to their
immunity to faults and their behavior when a fault condition exists as a result of the
structural arrangement of the parts/components and/or their reliability".
Channel
Element or group of elements that execute function(s) independently of one
another.
2–channel structure
This is a structure that is used to achieve fault tolerance.
For instance, a 2–channel protective door control can only be implemented if at
least two enable circuits are available and the main circuit is redundantly shut
down or a sensor (e.g. Emergency Stop switch) with two contacts is interrogated
and these are separately routed to the evaluation unit.
Fail–safe
The ability of a control system, also when faults occur (failure), to maintain a safe
condition of the controlled equipment (e.g. machine, process), or to bring the
equipment into a safe condition.
Failure/Fault
Failure
A piece of equipment or device can no longer execute the demanded function.
Fault
Undesirable condition of a piece of equipment or a device, characterized by the
fact that it is unable to execute the demanded function.
Note:
Fault tolerance
Fault tolerance N means that a piece of equipment can still execute the required
task even if N faults are present. For N+1 faults, the equipment can no longer exe-
cute the required function.
Final controlling element
Converter that converts electrical signals into mechanical or other non–electrical
quantities.
Performance Level (PL)
The revision of ISO 13849–1 (with EN designation EN 954–1) includes guidelines
to simply develop, test and certify safety–related machine controls. Deterministic
and probability requirements are combined with one another in a practical fashion.
The control category (defined e.g. using redundancy and testing) and probability
aspects (failure rate of the components and quality of the tests, expressed in the
form of MTTFd and DC, as well as common cause faults) are used as basis to
define the so–called "Performance Levels" (PL). Markov models, based on typical
A-592
SINUMERIK 840D sl/SINAMICS S120 SINUMERIK Safety Integrated (FBSI sl) – 05.2008 Edition
"Failure" is an event and "fault" is a condition.
© Siemens AG 2008 All Rights Reserved
05.08