Glossary · Process instrumentation and analytical measurement
Dead zero
Also known as: True zero, Zero-based signal
German: Nullpunkt ohne Offset
In analog signal transmission, dead zero means that the lower range value of a signal is represented by zero current or voltage, as in 0–20 mA or 0–10 V signals, so a broken wire cannot be distinguished from the minimum value.
- Process instrumentation
In one sentence
Dead zero means the lowest signal value is 0 mA or 0 V, so a broken wire looks like a valid minimum value and cannot be detected.
Example
A 0–20 mA level signal shows 0 % after a cable is cut, and the control system cannot tell whether the tank is empty or the wire is broken.
How it applies
- Engineering: Dead-zero signals use the full signal range for the value, but lose the ability to detect wire breaks and cannot power two-wire devices. They are common for voltage signals and in older installations.
- Safety: If a zero value can mean either a safe or an unsafe process state, a dead-zero signal can hide a fault. Signal choice should follow the fault analysis of the function.
- Commissioning: Check that the controller does not interpret a failed signal as a valid value that leads to an unsafe action.
- Documentation: State in interface descriptions whether a signal is dead zero or live zero and what the receiving system does on signal loss.
Dead zero vs. live zero
With a Live zero, the minimum value is represented by a non-zero current or voltage, for example 4 mA. Anything clearly below it indicates a fault. With dead zero, this distinction is impossible.