What happens if polarity is reversed on a 2-wire transmitter?

• A. The transmitter will emit a higher voltage.
• B. The transmitter will power up but read negative values.
• C. The transmitter will still operate normally.
• D. The transmitter will not power up.

Answer: (D)

Polarity matters because a 2-wire transmitter is powered from the loop and relies on correct orientation to complete the power path. When the wires are reversed, the internal protection path blocks the loop current, so the transmitter doesn’t receive power. Without power, it cannot regulate or send the 4–20 mA signal, so nothing in the loop changes and no measurement is output.

The loop current is inherently positive (4–20 mA); reversing polarity doesn’t create a normal negative reading, and since the device isn’t powered, it won’t operate or produce a signal. In short, reverse polarity cuts off power to the transmitter, preventing operation.