Underflow (InValue below the envelope)

What does FB ENVELOPE Value 1 mean?

The input value (InValue) has fallen below the defined lower limit of the envelope in the FB Envelope function block. This indicates a deviation from the expected operating range, potentially causing the safety function to trip or behave unpredictably, leading to an unsafe state.

• Analog sensor (e.g., pressure, flow, temperature) reading below its expected operating range due to calibration drift or process upset.
• Incorrect lower limit parameter (e.g., P1-05) set too high within the FB Envelope function block configuration.
• Signal conditioning error causing the input value to be attenuated below the envelope's lower threshold (e.g., faulty isolator).
• Loss of input signal (e.g., 4-20mA sensor wire broken, resulting in 0mA) interpreted as an underflow condition.

Click steps to track your progress.

1. 1

   1. Measure the actual process variable (PV) and compare it to the 'InValue' displayed in the safety logic.

2. 2

   2. Verify the analog sensor's output (e.g., 4-20mA, 0-10V) directly at the input terminals of the safety module (e.g., EL3xxx terminal X1).

3. 3

   3. Check the 'Lower Limit' parameter (e.g., P1-05) within the FB Envelope configuration for correct scaling and value.

4. 4

   4. Calibrate the associated analog sensor according to manufacturer specifications.

5. 5

   5. Inspect the signal wiring from the sensor to the safety module for damage or loose connections, particularly ground and shield integrity.