The live zero

In a 4–20 mA loop, 4 mA represents the bottom of the range and 20 mA the top. Because even zero is a flowing current, a broken wire or dead instrument reads 0 mA and is instantly recognisable as a fault.

Powering the loop

Two-wire transmitters take their operating power from the loop itself. A 24 V supply, a few hundred ohms of load and a short cable run are usually enough.

Adding HART

HART superimposes a low-level digital signal on the same wires, carrying configuration, diagnostics and secondary variables without disturbing the analogue reading. NAMUR NE 43 defines the out-of-range levels used to signal failures.