4–20 mA Signal – Complete Field Guide
Category: Field Instrumentation · Analog Signals · Control Systems
What Is a 4–20 mA Signal?
The 4–20 mA signal is the global standard analog transmission method used in industrial instrumentation.
Instead of transmitting voltage, the system transmits current. Current is highly immune to cable resistance, voltage drop, and electrical noise, making it ideal for long-distance plant wiring.
- 4 mA = 0% (Live Zero)
- 20 mA = 100%
- Standard for PLC and DCS analog inputs
If your plant has transmitters, it runs on 4–20 mA loops.
Why 4 mA Instead of 0 mA?
The system uses 4 mA as the minimum value to allow fault detection. This design philosophy is known as the Live Zero concept.
- 0 mA = loop open / broken wire
- 4 mA = valid 0% measurement
- Power is maintained for loop-powered transmitters
Basic 4–20 mA Loop Wiring
The current flows through all devices in series. Any break in the loop immediately stops current flow.
Types of 4–20 mA Loops
1. Loop-Powered (2-Wire)
- Power and signal share same wires
- Most common transmitter type
- Requires minimum loop voltage
2. 4-Wire (Externally Powered)
- Separate power supply
- Stronger signal drive capability
- Used for analyzers and high-power devices
Scaling Concept
The PLC converts current into engineering units using linear scaling.
Formula:
% of Range = (Measured mA − 4) ÷ 16 × 100
- 4 mA = 0 bar
- 20 mA = 100 bar
- 12 mA = 50 bar
Incorrect scaling causes mismatch between field and control room readings.
NAMUR Fault Indication
- < 3.6 mA = Sensor fault / underrange
- 4–20 mA = Valid process signal
- > 21 mA = Electronics fault
For full fault logic explanation see: Live Zero & NAMUR NE43 Detailed Guide
Common Field Problems
- Loop open (0 mA)
- Constant 3.6 mA or 21 mA (fault condition)
- Ground loop noise
- Loose terminal causing intermittent signal
- PLC input card failure
Basic Troubleshooting Logic
- Measure loop current in series
- Verify power supply voltage
- Check transmitter output locally
- Inspect wiring continuity
- Confirm PLC scaling configuration
Always isolate field, wiring, and control system step-by-step.