In a 4-20 mA loop the smallest valid signal is 4 mA, not zero, and that four-milliamp floor is deliberate - it leaves room below the live range to tell a real low reading from no reading at all. When a wire breaks, a terminal works loose, or a transmitter dies, the current collapses toward 0 mA and the input reads under-range. This guide explains how an open-circuit fault differs from a genuine low process value, how NE43 out-of-range detection flags the break, and how a technician localizes where the loop actually opened.
Open-circuit 4-20 mA fault in one line: An open-circuit 4-20 mA fault is a break in the current loop - a cut or loose wire, a failed terminal, or a dead transmitter - that stops current flowing, so the input reads near 0 mA, well below the 4 mA live-zero. Because 4 mA is the lowest valid signal, a reading in the 0-to-under-4 mA band is not a real low process value but a wire-break signature. NE43 out-of-range detection uses that downscale band to raise a wire-break fault distinct from a valid measurement.
The 4-20 mA standard reserves 4 mA as the live zero - the current that corresponds to the bottom of the measured range. A healthy loop therefore always carries at least 4 mA when everything is working, even when the process is at its minimum. That deliberate offset is what gives the loop its self-diagnostic power: any current below 4 mA cannot represent a valid measurement, so it must mean something is wrong with the circuit rather than with the process. When the loop opens, current cannot flow at all, and the input sees essentially 0 mA - unmistakably below the live zero.
This is why an open circuit does not masquerade as a low reading. If 0 mA were a valid signal, a broken wire would look exactly like a process sitting at the bottom of its range, and no one could tell the difference. Because the valid range starts at 4 mA, the gap between 0 and 4 mA is a diagnostic dead band: a reading there means loss of signal. Some transmitters even drive their output to a defined downscale value on internal failure so the receiver sees the same below-range signature for a dead transmitter as for a cut wire. The result is that an open loop announces itself with a reading that is physically impossible for a working measurement.
NE43 is the NAMUR recommendation that defines how transmitters and inputs use the out-of-range bands to signal faults rather than measurements. It designates a downscale band below the live zero and an upscale band above the full-scale 20 mA as failure-indication regions, and a defined margin around the 4-20 mA range where readings are still treated as valid but flagged as marginally out of range. An input configured for NE43 knows that a reading in the downscale band is not a real value; it is a wire-break or transmitter-failure indication, and it drives the tag to a bad or uncertain quality and raises a fault rather than passing along a bogus process number.
This matters because it changes what the operator sees. Without NE43-style detection, an open loop might silently show the bottom of the range and be mistaken for a real low condition, prompting the wrong response. With it, the same event produces an explicit wire-break or signal-fault alarm, and the tag is marked bad so downstream logic and displays do not trust the value. Wire-break detection is the general capability; NE43 is the widely-used convention that standardizes the downscale band so different vendors' devices agree on what below-range means. Together they convert a dangling wire from an ambiguous low reading into a clear, actionable fault.
Once the fault is recognized, the field task is to find where the loop opened, and the loop's series nature makes that a systematic search. Because the same current flows through every element in a single loop, a break anywhere - at the transmitter terminals, along the field wiring, at a junction or terminal block, at a barrier, or at the input card - produces the same downscale reading. The technician works the series path: confirm the transmitter has power and is alive, then measure the loop current at successive points to find where current stops flowing, since upstream of the break current is present and downstream it is not. Checking the obvious suspects first - loose terminal screws, corroded connections, a tripped or blown loop element - often locates it quickly.
A cloud SCADA platform shortens the front half of this work by making the fault visible instantly and remotely. When the input reports a downscale, NE43-driven quality, the platform can raise a wire-break alarm the moment it happens rather than waiting for someone to notice a tag reading zero, and it can distinguish that signal-fault state from a genuine low value on the display so an operator does not misread it. For a remote well, tank, or facility, that early, unambiguous alert is what dispatches a technician to the right site with the right expectation - a broken loop to trace, not a process to investigate. The platform cannot walk the wiring, so the physical localization stays a field job, but framing the open circuit as an explicit fault rather than a low number is what turns it into a work order instead of a mystery. This complements the wire-break detection and NE43 pages by presenting the open circuit as the symptom a technician actually chases down.
A reading near 0 mA, below the 4 mA live-zero, means the current loop is open - a broken or loose wire, a failed terminal, or a dead transmitter has stopped current from flowing. Because the lowest valid signal is 4 mA, anything below that band cannot be a real low process value; it is a wire-break signature. Treat it as a signal fault to be traced in the field, not as a genuine low measurement.
NE43 defines a downscale band below the 4 mA live-zero as a failure-indication region, so an input configured for it treats any reading there as a wire-break or transmitter-failure signal rather than a measurement. The tag is driven to bad or uncertain quality and a fault is raised, instead of a bogus low value being passed downstream. This is what lets the same below-range signature reliably mean loss of signal across different vendors' devices.
Because a single loop carries the same current through every element in series, a break anywhere produces the same downscale reading, so you trace the series path to find it. Confirm the transmitter has power, then measure loop current at successive points - upstream of the break current flows, downstream it does not - to bracket the open. Loose terminals, corroded connections, and failed loop elements are the usual suspects to check first.
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