During loop check you check the integrity of the 4-20 mA loop and the indication in the DCS
The reason for the loop check of each signal associated with each device is to ensure that:
Validate that each signal of the device is really connected to the correct terminals and that each and every signal associated with the device is wired correctly through the terminals in the field junction box, to the field termination assembly in the marshalling cabinet, to the I/O card and to the correct tag in the DCS database
Validate that each signal of the device is not limited in the high end for example due to high resistance caused by poor connection or corrosion on terminal at the device or field junction box
Validate that each signal of the device is not biased due to a ground loop current caused by water or moisture in the device housing
Validate that there are no non-linear elements in the current loops for each signal of the device such as signal conditioner or safety barrier with problem
Validate that the ranging in the DCS for each signal is correct so that the correct engineering unit value is displayed
Loop check for each signal of a transmitter is done by simulating 4/8/12/16/20 mA corresponding 0/25/50/75/100% reading. For instance, if the range is 0-800 kPa, simulating 4/8/12/16/20 mA should result in a display of 0/200/400/600/800 kPa in the DCS.
For FOUNDATION fieldbus
You will know that the device is connected to the correct terminals because the device automatically appears in the system "live list" displayed with device tag, manufacturer, model number, unique identifier and version etc. This is true also for discrete devices like intelligent two-wire on/off valves and electric actuators / motor operated valves (MOV). All the signals associated with that device automatically become correct so you only check the devices is there, you don't have to check signal by signal.
A digital signal value cannot be limited, biased, non-linear or otherwise distorted without being detected. Therefore a 5-point loop check of each signal is not required. Instead use a portable fieldbus tester like Relcom FBT-6 to verify device supply voltage, signal level, and noise level are within limits thus detecting any wiring issues or water ingress etc.
Credits : Jonas