The most common setup where the transmitter is powered by the loop itself. Two wires carry both power and the signal. Benefit: Saves cable costs and simplifies installation. 3-Wire Connection (Active)
The sensor has its own dedicated power source (often AC or high-voltage DC). shema tokovaja petlja 4 20 ma
If the wire snaps, the current hits 0 mA, immediately triggering a "fault" alarm. The most common setup where the transmitter is