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Reliability, Availability and Safety of Electronic Control Systems
9.1.1
Failure Characteristics of Electronic Devices
The failure-rate-versus-time curve can be broken down roughly into three periods of time.
Early
Failures
0
Figure 9-1. Failure Characteristics of Electronic Devices ("Bathtub" Curve)
(1) Early failures are caused by material and manufacturing defects and the failure rate falls
steeply during the initial period of operation.
(2) The random failure phase is characterized by a constant failure rate. Provided the systems are
used in accordance with the specifications, only random failures occur during this period.
This period covers the normal behaviour of system components and is the basis for the
calculation of all reliability parameters.
(3) The failure rate increases with time. Wear-out failures become more frequent, indicating that
the end of the useful life is approaching. The transition to this phase is gradual. There is no
sudden increase in the failure rate.
9.1.2
Reliability of SIMATIC S5 Programmable Controllers and Components
A very high degree of reliability can be achieved by taking the following extensive and cost-
intensive measures during the development and manufacture of SIMATIC S5 systems.
•
The use of high-quality components;
•
Worst-case design of all circuits;
•
Systematic and computer-controlled testing of all components supplied by subcontractors;
•
Burn-in of all LSI circuits (e.g. processors, memories etc.);
•
Measures to prevent static charge building up when handling MOS ICs;
•
Visual checks at different stages of manufacture;
•
In-circuit testing of all components, i.e. computer-aided testing of all components and their
interaction with other components in the circuit;
•
Continuous heat-run test at elevated ambient temperature over a period of several days;
•
Careful computer-controlled final testing;
•
Statistical evaluation of all failures during testing to enable the immediate initiation of
suitable corrective measures.
9-2
Random
Failures
(1)
4
10
Wear-out
Failures
(2)
6
10
S5-115F Manual
(3)
t in h
EWA 4NEB 811 6148-02
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