Why is the range of analogue (current-) input modules specified as 0(4)..20 mA?

FAQ #100675

 

Why do the SBC input modules have a range of 0..20 mA?
One advantage of measuring the whole range of 0..20 mA is that this way also a cable break (or a defective sensor) can be detected. If a value between 0..4 mA is measured, the conclusion is that either the cable is broken or the sensor is not working correctly.
Another advantage is that one module type can be used for both ranges (0..20 mA and 4..20 mA).

Does the specified resolution apply for the 0..20 mA or for the 4..20 mA?
The specified resolution does always apply for the full range of 0..20 mA. The same resoultion is used if the input module is used for measuring 4..20 mA.

Example
If a 12 bit input card is used, we'll have a resolution of 4095 DV (Digital Value) over the whole range 0..20 mA. One DV corresponds to 4.88 µA.
In case a range of 4..20 mA is measured the resolution will remain the same (4.88µA per DV) and the measured range of 16 mA will be divided by 3276 DV.
Of course it is still possible detecting a sensor/cable fault because the whole range 0..20 mA is measured by the module.