BERTHOLD TECHNOLOGIES Uni-Probe LB 490, User Manual

Page: 65 / 730

Uni-Probe LB 490
BERTHOLD TECHNOLOGIES GmbH & Co. KG
1 – 65
Volume 1 9 Functional Safety
1
9.5
Project Planning
General instructions and restrictions
• Please make sure that the measuring system will be used in
accordance with its designated function.
• The application-specific limits have to be observed and the
specifications must not be exceeded. See also the technical
data and ambient conditions in the User’s Manual.
• The fault tolerance time of the overall system must be greater
than the reaction time of the measuring system.
• The digital inputs 1 and 2 must not be closed in case of a
safety-related application.
• The multidrop mode must be turned off. Set the polling address
in the parameter
POLL ADDR
to “0” (see section "2.40 HART
I
NTERFACE
", page 3–336).
• The error handling function must be set to STOP.
• The 4–20mA current output has been defined as signal circuit
for safety-related applications.
• 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 welding
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.
• To ensure the proper function of the Level Gauge Uni-Probe
LB 490, the avoidance of interference radiation and the avoid-
ance of disturbances due to parallel radiometric measurements
have to be taken into account.
Assumptions
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 device.
• The following is not taken into consideration:
–
external power supply failure rates
– multiple errors
• The mean ambient temperature during the operating time is
40°C.
• The environmental conditions correspond those of an average
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).
• In the operating mode with low demand rate the reaction time
of the LB 490 to dangerous detected faults is max. 15 minutes.