What is the most likely cause if a 4-20 mA transmitter is powering off at 100% output?

When a 4-20 mA transmitter is reaching 100% output and powering off, it is typically indicative of an issue with the loop's electrical characteristics. The correct answer is grounded in the concept of loop resistance.

In a 4-20 mA current loop, the transmitter operates within a specified range of output current (4 mA to 20 mA) to signify measurement values. If there is too much resistance in the loop, it can impede the current flow, leading to a condition known as voltage drop. This voltage drop can potentially reduce the supply voltage that the transmitter receives, causing it to shut down if it can no longer operate within its required voltage and current range.

Excessive resistance in the loop could be a result of long cable runs or using inappropriate wire gauge, both of which can lead to excess voltage drop across these resistances. Therefore, if the transmitter is reaching full output and subsequently powering off, it is sensible to conclude that the loop is unable to maintain the required electrical conditions for proper operation, leading to a shutdown situation.

This broader context informs you of the importance of ensuring correct loop resistance to maintain optimal performance within the transmitter's specifications.