What is Live Zero in 4–20 mA?
One of the smartest features of the 4–20 mA signal is something called Live Zero 
What is Live Zero?
In a 4–20 mA loop:
4 mA = 0% Process Value
20 mA = 100% Process Value
Even when the measured value is zero, the transmitter still sends 4 mA, not 0 mA.
This is called Live Zero.
Why not use 0–20 mA?
Because with 0–20 mA:
Is 0 mA a valid measurement?
Or is the wire broken?
You cannot tell the difference.
Advantage of Live Zero
With 4–20 mA:
4 mA = Valid process value (0%)
0 mA = Fault condition
This makes fault detection easy.
Example
Pressure Range = 0–100 bar
4 mA → 0 bar
12 mA → 50 bar
20 mA → 100 bar
If PLC reads:
0 mA → Wire break / power loss
2 mA → Loop fault or transmitter issue
Why industries love Live Zero
Detects open-circuit faults
Detects transmitter power loss
Improves troubleshooting
Increases system reliability
Common alarm settings
Many PLCs/DCS systems use:
< 3.6 mA → Under-range / Fault
> 21 mA → Over-range / Fault
(As per NAMUR NE43 recommendations)
Key takeaway
4 mA is not “zero current”
4 mA means “I am alive and measuring zero”
Simple understanding
Live Zero = The transmitter sends 4 mA at zero measurement so the control system knows the instrument is healthy and the wiring is intact.
Video Tutorial:
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