Siemens SIMATIC S5-115U, User Manual

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S5-115U Manual Reliability, Availability and Safety of Electronic Control Equipment
14 Reliability, Availability and Safety of Electronic
Control Equipment
The terms reliability, availability and safety of electronic control equipment are not always clear
and sometimes even misinterpreted. This can be explained on the one hand by the different
failure characteristics of electronic control systems compared with conventional systems. On the
other hand, some of the safety regulations have been made considerably more stringent in a
number of application areas in the course of the last few years. The following chapter is intended
to familiarize the large number of users of SIMATIC electronic control systems with the basics of
this problem complex.
The information given is of a predominantly fundamental nature and applies regardless of the
type of electronic control system and its manufacturer.
14.1 Reliability
Reliability is the capability of an electronic control system to satisfy, over a specified period and
within the specified limits (i.e. technical data), the requirements placed upon it by its application.
Despite all the measures taken to prevent failures, there is no such thing as 100 % reliability.
The failure rate is a measure of the reliability:
=
n
N
o
x t
and
n=Number of failures during time t
N
o
=Remaining components
EWA 4NEB 811 6130-02b
14-1