BERTHOLD TECHNOLOGIES LB4710-050, User Manual

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Chapter 3 Functional Safety
Mini-Switch LB 471
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
Interfering radiation, e.g. due to welding seam tests, is largely
identified and signaled by the measurement. However, in some
situations it is conceivable that the intensity of the interfering
radiation will increase the radiation level at the detector only
slightly, so that no alarm is triggered or not in due time. There-
fore, the facility always has to be informed as soon as a weld-
ing seam test is carried out in the environment of the facility in
which the measurement is employed. In this case, suitable
safety precautions have to be taken.

Interfering radiation from adjacent measuring points has to be
avoided.
The FMEDA (Failure Mode Effects and Diagnostics Analysis) is
based on the following assumptions:

The failure rates are constant over the service life of the de-
vice.

The following is not taken into consideration:
external power supply failure rates
multiple errors
operating mode as minimum level switch

The mean ambient temperature during the operating time is
40°C.

The environmental conditions correspond to those of an aver-
age industrial environment.

The working life of the components is between 8 and 12 years.

The time to repair (replacement of the measuring system) after
a fault protected from interference is eight hours (MTTR = 8h).
If the demand rate is not more than once a year, the measuring
system may be operated as a safety-relevant sub-system in the
operating mode with low demand rate (IEC 61508-4, 3.5.12).
Numerical values see section “Safety-Technical Data”.
The fail-safe state is reached when the current output indicates the
following values.
A safe failure is defined as a failure that causes the measuring sys-
tem to go to the defined fail-safe state without a demand from the
process.
A dangerous undetected failure is present if the measuring system,
following a demand from the process, does not go to the defined
fail-safe state.
Assumptions
Safe state and fault
description