BigJohn1023 said:
However the PLC still shows a number when I would assume it should be a 0 if it is below 4mA.
Generally, no. To give you a more specific answer, we'd need to know part numbers of your analog input card, but most analog input cards that handle 4-20mA actually read somewhere in the range of 0-21mA. That's deliberate - it's to take advantage of the "raised zero" inherent in a 4-20mA signal.
Imagine you have a 0-10V analog input. Your input is zero. Is the process just at minimum scale, or has your transmitter failed, or had a wire break? You can't tell. If it's the latter, nobody will know until they deduce it from secondary or tertiary issues that arise out of the initial problem. Likewise, what if your analog input indicates full scale (10V)? Is the process variable
actually at 100%, or has someone shorted 24V to your input? Or has the sensor failed to a high-scale position? Same thing again - you have no way of knowing.
On the other hand, consider a 4-20mA sensor, wired into an input that measures 0-21mA. Your input is 4mA. You know the sensor is healthy, and the process variable is at 0%. If your input is 3.5mA, you know that your sensor is connected, but has probably gone to a fault condition (as many sensors do when they fail). So you have a prompt to go and look in the right place to start with. If your input is zero, chances are you have a wiring problem, or your sensor is completely kaput. Likewise, at the other end of the scale, if you're getting 20mA back you know that the sensor is healthy and your process variable is at 100%. If you have 21mA, again, your sensor has probably failed, and is set to fail to 21mA.
tldr; if your analog input can measure from 0-21mA you can to a large degree distinguish between normal operation, abnormal operation, and a wiring problem
within the PLC. You just need to scale the input from 4mA to 20mA as 0-100%, and then any readings outside that range are cause for concern.