Live Zero vs Dead Zero Signals
Why 4–20 mA uses 4 mA as a live zero instead of 0 mA, how a live zero lets 0 mA flag a broken wire or lost power, and what under-range and over-range readings tell you.
Definition
A live zero is a signal standard where 0% of the measured range corresponds to a non-zero signal value. In a 4–20 mA loop, 0% of span is 4 mA — the zero is 'live' because a healthy instrument at the bottom of its range still produces a measurable current. A dead zero is the opposite: 0% of range corresponds to a zero signal (0 mA, or 0 V), so there is no way to distinguish a genuine low reading from a dead loop.
Why it matters
The live zero is one of the most useful diagnostic features in process control, and it is the main reason 4–20 mA became the analogue standard. Because 0% of span sits at 4 mA, a reading of 0 mA cannot be a valid measurement — it can only mean a broken wire, a disconnected transmitter, or loss of loop power. With a dead-zero (0-based) signal, that same 0 would be ambiguous: is the tank empty, or is the cable cut? The 4 mA offset turns the absence of signal into an unambiguous alarm, and leaves room below 4 mA and above 20 mA for transmitters to flag specific fault conditions.
Formula
Units involved
- •mA — loop current; 4 mA at 0% of span, 20 mA at 100% of span
- •live zero — the 4 mA signal representing 0% of the calibrated span
- •dead zero — a 0 mA (or 0 V) signal at 0% of range, with no fault distinction
- •% span — position within the calibrated range, 0% to 100%
- •fault current — a defined out-of-range value (e.g. < 3.6 mA or > 21 mA) some transmitters use to signal a failure
Concept diagram
Worked example
A level transmitter is ranged 0–5 m. Compare what the loop shows when the tank is genuinely empty versus when the signal cable is cut, on a 4–20 mA live-zero loop.
- 01Tank empty (0% of span): mA = 4 + 16 × 0 = 4 mA — a valid, healthy low reading
- 02Cable cut or power lost: current falls to 0 mA
- 030 mA is below the live zero of 4 mA, so it cannot be a real measurement
- 04The control system reads 0 mA as a loop fault, not as an empty tank
The live zero lets the system tell "genuinely empty" (4 mA) apart from "loop dead" (0 mA) — a distinction a dead-zero signal cannot make.
Common mistakes
- •Reading 0 mA as 0% of span. On a live-zero loop, 0% of span is 4 mA. A 0 mA reading is a fault, not a low value.
- •Assuming under-range or over-range always means a process problem. A reading just below 4 mA or just above 20 mA can be a real out-of-range process value, but defined fault currents (e.g. 3.6 mA or 21 mA) are deliberate failure signals — check the transmitter and site convention.
- •Forgetting the convention is configurable. Downscale-burnout versus upscale-burnout, and the exact fault-current values, vary by vendor and by site standard. Do not assume; confirm how a given loop is configured.
- •Confusing live zero with a suppressed zero. Live zero is about the signal (4 mA at 0% of span). A suppressed zero is about the range (LRV above zero). They are independent concepts.
When to use the calculator
Use the mA to Process Value calculator to see what a given loop current represents and whether it is under-range or over-range, and the Process Value to mA calculator to find the expected signal for a process value. Both flag readings outside the 4–20 mA window, which is where live-zero diagnostics matter most.