Answer / carl_ellis

A 4-20mA control output is always "active", powered by the
device from which it is sourced.

If a 4-20mA control output is shorted at its output
terminals, the loop current will go to whatever value within
the 4-20mA range that the controller is calling for, because
that is what current analog outputs are designed to do -
regulate the current.

The controller's analog output is not damaged by a short
circuit across its terminals because the loop current is
regulated by the analog output.

On the other hand, a short circuit in the loop of the 2-wire
analog output of a field transmitter (sensor) that is loop
powered by an external DC power supply can be a problem.

A short circuit across the transmitters' terminals removes
the transmitter's current regulation so the loop current is
determined by the remaining load resistance; wire
resistance, receiver's load resistance, power supply
internal resistance.

In the vast majority of field instrument current loops, the
bulk of the loop resistance is the resistor across the
receiver's analog input, typically 250 ohms. A typical
24Vdc power supply will drive 95 mA through a 250 ohm
resistor, which is 2.3 watts. 2.3 watts will usually burn
up the analog input resistor, disabling the circuit.

Carl Ellis
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