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Signals and PLC · Glossary

What is Live Zero?

4 mA at 0 %Signals and PLC

Live zero means the lowest signal value (4 mA) is above zero, so a broken loop (0 mA) can be told apart from a real zero reading.

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Live Zero Explained

With 0 to 20 mA a broken wire looks like a valid zero. With 4 to 20 mA anything well below 4 mA is a fault.

The 4 mA also supplies the operating current that two wire transmitters need.

Live Zero: Typical Values

Normal range4 to 20 mA
Common broken loop indicationbelow about 3.6 mA (NAMUR NE43)
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Calculators for Live Zero

CalculatorNAMUR NE43 CheckerFree online calculator with formula and worked example.CalculatorAnalog Scaling CalculatorFree online calculator with formula and worked example.

Free Downloads

Free download4 to 20 mA Cheat SheetFree printable PDF cheat sheet.

Frequently Asked Questions

What is Live Zero?

Live zero means the lowest signal value (4 mA) is above zero, so a broken loop (0 mA) can be told apart from a real zero reading.

Is 1 to 5 V also live zero?

Yes, 1 to 5 V is the voltage form, often from 4 to 20 mA across 250 Ω.

Can a transmitter go below 4 mA?

Slightly, to show under range, and to about 3.6 mA or less for a fault.

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4 to 20 mA current loop explained

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Related Terms

Signal term4 to 20 mA Current LoopA 4 to 20 mA current loop sends a measurement as a current between 4 and 20 mA, which is not affected by…Signal termNAMUR NE43NAMUR NE43 is a recommendation that defines 4 to 20 mA signal levels for measurement and for transmitter…Signal termTwo Wire TransmitterA two wire transmitter is powered by the same 4 to 20 mA loop it signals on, so it needs only two wires.

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