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Safety Manual SIL KCD2-SR-(Ex)*(.LB)(.SP), HiC282*
Safety Recommendation
3
Safety Recommendation
3.1
Interfaces
The device has the following interfaces. For corresponding terminals see data
sheet.
Safety relevant interfaces:
KCD2-SR-(Ex)1.LB(.SP), HiC2821: input I, output I, output II
KCD2-SR-(Ex)2(.SP), HiC2822: input I, input II, output I, output II
Non-safety relevant interfaces: output ERR
3.2
Configuration
The device must be configured through the user accessible DIP switches for the
required output function before the start-up. During the functionality any change
of the operating function (DIP switch modification) can invalidate the safety
function behavior and must be avoided.
The KCD2 devices provide a suitable cover to protect against accidental changes
while on the HiC devices the access to the DIP switch is permitted only through a
small window on the side and by a small screw driver.
3.3
Useful Life Time
Although a constant failure rate is assumed by the probabilistic estimation this
only applies provided that the useful life time of components is not exceeded.
Beyond this useful life time, the result of the probabilistic calculation is
meaningless as the probability of failure significantly increases with time. The
useful life time is highly dependent on the component itself and its operating
conditions
–
temperature in particular (for example, the electrolytic capacitors can
be very sensitive to the working temperature).
This assumption of a constant failure rate is based on the bathtub curve, which
shows the typical behavior for electronic components.
Therefore it is obvious that failure calculation is only valid for components that
have this constant domain and that the validity of the calculation is limited to the
useful life time of each component.
It is assumed that early failures are detected to a huge percentage during the
installation period and therefore the assumption of a constant failure rate during
the useful life time is valid.
However, according to IEC 61508-2, a useful life time, based on experience,
should be assumed. Experience has shown that the useful life time often lies
within a range period of about 8 ... 12 years.
As noted in DIN EN 61508-2:2011 note NA4, appropriate measures taken by the
manufacturer and operator can extend the useful lifetime.
Safety Manual SIL KCD2-SR-(Ex)*(.LB)(.SP), HiC282*
