The 4-20mA standard outlived rival signals by making 'zero' a fault

In process automation, a transmitter turns a physical value (pressure, temperature, flow) into an electrical signal sent to a PLC or controller. Early systems used 0mA to 20mA or 0V to 10V, but those maps had a hidden flaw: if the wiring broke, the signal dropped to 0, which looked exactly like a legitimate reading of 'nothing'.

The fix was the 'live zero'. Start the measuring range at a non-zero value — 4mA rather than 0mA, or 2V rather than 0V — so an intact loop never reads 0. Now a drop to 0mA or 0V is unambiguously a fault: a broken wire, a dead transmitter, or an open circuit. The operator is alerted to a problem instead of quietly trusting a wrong number.

Current signaling won over voltage because a current loop is effectively lossless over long distances (Kirchhoff's law says the current is the same everywhere in the loop), is far less affected by adjacent loads switching on and off, and can be run in a two-wire configuration that also powers the instrument. The result, standardized as DIN/IEC 60381, became the default analog interface in nearly all industrial applications.

Reserve a non-zero floor so a fault is unambiguous

A non-zero floor means a broken wire can never masquerade as a valid minimum reading

Current loops are insensitive to line resistance, so accuracy survives long runs

Current signals resist electromagnetic interference from neighbouring equipment

A 4-20mA loop can power the instrument over the same two wires that carry the signal

A standard that builds failure detection into the representation itself — where the lowest value is impossible to confuse with a broken connection — becomes cheap to adopt and hard to replace.

4-20mA remained the de facto analog process signal for decades and is still built into virtually every industrial transmitter, PLC input card, and instrument. Its live-zero logic is now a standard textbook principle, and the same 'reserve a floor for faults' idea shows up in other safety-oriented signaling designs. Newer digital fieldbuses have layered over it rather than replaced it.