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SAFETY MANUAL SIL KFD0-RSH-1.4S.PS2
Safety Recommendation
3
Safety Recommendation
3.1
Interfaces
The device has the following interfaces. For corresponding terminals see data
sheet.
■
Safety relevant interfaces: input, output I (ETS), output II (DTS)
■
To avoid contact welding in DTS application we recommend to use a serial
fuse in the load circuit. This can be the internal fuse F1 or any external fuse of
max. 5 A nominal value.
■
Test input interface may not be used during normal operation (only for proof
test)
3.2
Configuration
A configuration of the device is not necessary and not possible.
ETS, DTS and DPS can be selected by using the referring terminals. See data
sheet. The fuse in delivery status (2.5 A) can be changed to max 5 A. Please note
the temperature derating according to the data sheet.
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.
