Answer Posted / carl_ellis

Part I.
Let's assume the question involves a 2 wire, loop powered
transmitter powered by a 24Vdc power supply in series with
the transmitter and a load resistor across the analog input
of the receiver device.

The loop current is nominally 4-20mA.
The power supply negative is wired to the analog input
negative.
The short puts a positive 24Vdc on the (+) side of the
analog input.

The load resistor for the analog input is across the (+)
and (-) terminals, so the resistor now 'sees' 24Vdc across
its 250 ohms.

Part II. extra credit. Calculate the wattage spec needed
for a 250 ohm resistor to survive the short circuit in the
2 wire, loop powered cable.

Carl Ellis
Measure First

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