Typically 24V DC to drive the current through the loop.
Current signals are less affected by electromagnetic interference (EMI) than voltage signals. shema tokovaja petlja 4 20 ma
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) Typically 24V DC to drive the current through the loop