Safety Function; Alarm Response And Current Output - ABB LLT100 Functional Safety Manual

Safety function

The laser level transmitter produces an analog signal, between 4 mA and 20 mA, proportional to the
level, volume or ullage, as defined by the operator. The analog signal is fed to a downstream logics
unit such as a PLC or a limit signal generator, and is monitored for exceeding a specified maximum or
minimum value. All safety functions refer exclusively to this analog output. The total valid range of the
output signal shall be configured to a minimum of 3.8 mA and a maximum of 20.5 mA (factory defaults).
The LLT100 provides the following Type-B safety functions:
• Computes a distance/level measurement.
• Outputs a 4–20 mA signal of the transformed measurement.
• Measurement transformations are user-defined (math functions). These functions can be either on
or off. Multiple parameters can be set related to:
– Damping
– Linearization function
– Filtering
The safety related function of the transmitter is the safe monitoring of the level within an accuracy of
2% of span (2% of 16 mA). The safe state output current can be configured to be ≤3.6 mA (low alarm) or
≥21.0 mA (high alarm), with the exception of CPU faults where the current output is in low alarm mode
(≤3.6 mA).
There are other functionalities of the LLT100 that are not considered as part of the safety function:
• Digital input (optional, external)
• HART communication
• HMI (optional)
• Remote display (optional accessory)
• Other accessories: lens heater, dust tube

Alarm response and current output

In case of detected critical faults, the configured alarm current will be produced and fed to a
subsequent logic unit, e.g., a DCS, and monitored for violation of a defined maximum value.
Selectable alarm current modes
There are two selectable modes for this alarm current:
• HIGH (max alarm current) which is the factory default setting;
or
• LOW (min alarm current).
Configurable alarm currents
The low alarm current is configurable from 3.5 to 3.6 mA with a factory default setting of 3.6 mA. The
high alarm current is configurable from 21.0 to 21.6 mA with a factory default setting of 21.0 mA. The
reaction time after the occurrence of a critical error until the output of the alarm current amounts to
≤1 min.
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User Guide