Why 4–20 mA Uses 4 mA Instead of 0 mA

A simple design decision that prevents dangerous misinterpretation in industrial plants.

The Problem With 0 mA

If 0 mA represented 0% measurement, how would we detect a broken cable?

• 0 mA could mean zero process value
• 0 mA could mean transmitter failure
• 0 mA could mean wiring open

This ambiguity is unacceptable in industrial control systems.

The Live Zero Solution

• 4 mA = valid 0% measurement
• 0 mA = confirmed loop fault
• < 3.6 mA = NAMUR low fault
• > 21 mA = electronics fault

This design allows instant fault detection without extra wiring.

Real Plant Scenario

A pressure transmitter shows 0 mA. Operator assumes zero pressure. Pump trips due to safety logic.

But if 4 mA were live zero, the system would immediately recognize 0 mA as a fault instead of valid measurement.

👉 For full technical explanation, read: Live Zero & NAMUR Explained