4-20 mA Signal Lost Threshold

At what mA value would you say that the analog signal is lost? 3.5 mA? 3.0 mA? 2 mA? 1 mA? 0?

in our standards

we consider anything below 3.8 to be a signal loss

An open circuit (broken wire, disconnected terminal) will be 0.0mA

2 wire loop powered transmitters use up to about 3.5mA to power the sensor electronics. Any controlled value below 3.5mA is unrealistic, since the device needs power for operation.

NAMUR specs define a fault or fail-safe failure mode at something around 3.6-3.8mA. The people who pay attention to NAMUR frequently calibrate an input to see 3.7mA (or 22.3mA on the high side) knowing that that's indication of a serious fault.

Poor calibration or the use of a non-precision dropping (shunt) resistor (say 233 ohms, rather than 250 ohms) on the analog inpt can produce apparent values below 4.0mA.

I use -10% so 4-2 mA is 2 mA as too low.
And above 22 mA same.
I have controllers that i can set to off by setting them on -20% obvious being zero. but to have some slack i use above.
When a sensor is just under 4 mA it should not give alarm just a recalibration is needed.

That's why I prefer to configure analog inputs for 0-20mA, since some I/O will cut off the scaling at 4mA in 4-20mA mode. We discovered this when replacing obsolete GE Field Control I/O with VersaMax for one customer. When set for 4-20mA, Field Control analog inputs scale 0mA as -8000 while VersaMax holds the scaled value at 0 below 4mA input. Some of the programming used -8000 to indicate that the machine driving that input was powered off.

Allen-Bradley 1746-NI8 analog input modules have underrange and overrange bits that trigger at one bit outside of the 4-20mA range. One of our customers has several machines where the OEM used these bits to trigger signal fail alarms. At zero flow, the flow signal alarm cycles on and off hundreds of times every day. We discovered this when the alarm log grew to the point of crashing the iFix VB script. The solution was to move about 40000 alarms into an offline archive.