Glossary · Process instrumentation and analytical measurement
Live zero
Also known as: Elevated zero, Offset zero
German: Angehobener Nullpunkt
In analog signal transmission, live zero means that the lower range value of a signal is represented by a non-zero current or voltage, such as 4 mA in a 4–20 mA signal or 2 V in 2–10 V, so that a signal near zero indicates a fault rather than a valid value.
- Process instrumentation
In one sentence
Live zero represents the minimum value with a non-zero signal, e.g. 4 mA, so a broken wire (0 mA) is recognized as a fault.
Example
When a cable to a flow transmitter breaks, the input reads 0 mA, well below the 4 mA live zero, and the control system raises a wire break alarm.
How it applies
- Engineering: Live zero enables wire break detection and gives two-wire transmitters a minimum current for their own supply. It slightly reduces the usable signal range.
- Diagnostics: The receiving system needs defined thresholds for underrange and failure. NAMUR NE 43 provides common values for 4–20 mA signals.
- Safety: Live zero supports detection of some faults, but it does not detect all of them, for example a transmitter stuck at a valid value. Signal diagnostics are one element of a safety concept, not the whole of it.
- Documentation: State the signal range, failure thresholds and the configured reaction in the interface description and alarm list.
Live zero vs. dead zero
With Dead zero, 0 mA or 0 V is a valid value and a broken wire goes unnoticed. With live zero, zero signal is recognizable as a fault.