Answer Posted / carl_ellis

The minimum required loop voltage depends on the voltage needed to drive 20ma (or 21.5mA if fail-safe high is used) through the total loop resistance.

Even if the power supply voltage sags/drops slightly, it is still possible that the loop will function properly, because it depends on the actual loop resistances and the lift-off voltage required by the transmitter

If there is not sufficient loop voltage, it will become apparent when the transmitter fails to go above a certain milliamp output, because there is insufficient voltage to drive more current through the loop's total resistance.

In some cases of low loop voltage, a field transmitter will cycle on and off, re-initializing each time it fails to push sufficient current because it is voltage-starved.

If the loop wiring is sized properly, then a voltage drop is likely due to
- a bad connection in junction box
- in some cases a ground loop from damaged wire insulation
- adding too many additional loads - more transmitters than the power supply is rated for
- failing power supply
- adding another device in series in the loop, like an indicator, recorder, or I/O module input that pushes the total loop resistance too high. Carl Ellis
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