Overall Safety Accuracy; Restricted Modes And Parameters; Behavior For Undetected Faults; Environmental Limits - ABB LLT100 Functional Safety Manual

• During simulation
• During proof test of the safety function.
The fraction of failures without the potential to put the device into a dangerous function status is given
by the SFF value shown in "Safety parameters" on page 15.

Overall safety accuracy

The defined value for the safety threshold of the safety function of this device is: 2% of 4–20 mA span
(2% of 16 mA).

Restricted modes and parameters

Restriction 1
The combination of the following instrument configuration parameters shall not be used for a Safety
Instrumented Function:
Configuration Parameter
Measurement mode
Drain rate
Vessel height
This restriction is only related to the combination of clear liquids measurement mode, vessel height
smaller or equal to 1 m and drain rate enabled. In this situation, the 4–20 mA output will remain at a
constant value between 4 mA and 20 mA, regardless of actual level variations.
All other instrument configurations can be used for a Safety Instrumented Function.

Behavior for undetected faults

If a LLT100 fault occurs and is not detected through its internal diagnostics, the safety function
may output a current between 4 and 20 mA, which exceeds its specified safety accuracy or is not
proportional to the level, volume or ullage as defined by the operator.

Environmental limits

The designer of a SIF must check that the product is rated for use within the expected environmental
limits. Refer to the Operating Manual (AD1) or data sheet AD2, for accurate information about
environmental limits. Some limits depend on the model and accessories installed on it. Additional
accessories can be used to help reduce the impact of dust or fog environment on the LLT100
measurement.
The LLT100 measurements can be affected by dust or fog between the instrument and the surface to
be measured. Dust and fog between the instrument and the surface to be measured should be kept to
a minimum. The user shall perform verifications to ensure that proper measurements can be made in
the target application environment.
14
User Guide
Value
Clear Liquids
Enabled
≤1 m