NAMUR NE 43 Explained: What 3.6mA and 21mA Fault Currents Mean

A reading sits at 20.6mA. The tank level display shows something plausible, but not quite believable. Is the process really running that high, has the transmitter pegged out, or has it just told you it's broken?

Without a shared standard, an out-of-range current is ambiguous. It could be a real process value pushing past 100 percent, a transmitter saturating at the top of its range, or a device declaring itself dead. NAMUR NE 43 exists to remove that ambiguity, and once you know its zone map, a raw mA reading stops being a guess.

TL;DR: NAMUR NE 43 standardizes what an out-of-range current on a 4-20mA loop means, so a transmitter can tell you it has failed instead of reporting a dead zero. The valid measuring range runs 3.8mA to 20.5mA (4.0mA is 0 percent, 20.0mA is 100 percent). Below 3.6mA and above 21.0mA, the loop sits in a manufacturer-declared fault state - low faults land between 3.2mA and 3.5mA, high faults between 21.5mA and 22.8mA. Narrow differentiation and saturation zones sit in between, separating a legitimate over-range reading from a declared failure. 0mA is never valid: it means a severed wire or total power loss, because 4mA (the "live zero") also powers a 2-wire transmitter's electronics. The catch: NE 43 sets the zones, but each manufacturer picks its own fault levels inside them, so two transmitters can saturate or alarm at different currents.

This piece complements the 4-20mA vs 0-10V guide, which covers the base signal and its live-zero rationale, and pairs with signal isolators if noise is what's pushing your reading around in the first place. If the reading itself is jittery rather than genuinely out of range, start with troubleshooting 4-20mA loop noise before you trust any boundary this article describes.

What Is NAMUR NE 43?

NAMUR, the German user association of automation technology in the process industries, publishes NE 43 to standardize fault levels in 4-20mA signals. The standard separates valid measurement from equipment failure, eliminating the old dead-zero problem where 0mA could mean either 0 percent or a broken wire (NotebookLM, Analog Signal Standards notebook, 2026).

NAMUR isn't a manufacturer. It's a user group: the process-industry companies that buy and operate instrumentation, not the vendors that build it. That distinction matters. NE 43 wasn't written by a supplier locking in its own convention - it came from plants tired of every brand handling faults its own way.

Before a common standard, "out of range" meant something different depending on which transmitter you'd bought. One vendor's device might peg at 22mA and hold; another might drop to 0mA on internal failure. A control-system programmer had to know every vendor's quirks by heart, and that knowledge rarely survived staff turnover or an instrument swap.

NE 43 fixes that with a shared language: one band of currents means "valid," a band above and below means "fault," and the space between exists to keep noise from tripping a false alarm. Once every compliant transmitter speaks that language, a PLC or DCS can write one piece of fault logic and trust it across brands.

Citation capsule: NAMUR, the German user association of automation technology in the process industries, publishes NE 43 to standardize fault levels in 4-20mA analog signals - separating valid measurement from equipment fault and eliminating the ambiguity of a dead zero (NotebookLM, Analog Signal Standards notebook, 2026).

Why Is 4mA the Live Zero, Not 0mA?

In a 4-20mA loop, 4mA represents 0 percent of the measured range, and it also supplies the power that runs a 2-wire loop-powered transmitter's electronics. That's why 0mA can never be a legitimate reading - it means the wire is severed or the loop has lost power entirely (NotebookLM, Analog Signal Standards notebook, 2026).

[UNIQUE INSIGHT] Think about what an older 0-20mA scheme could and couldn't tell you. If 0 percent of range reads as 0mA, and a severed wire also reads 0mA, the receiver has no way to distinguish "the process is genuinely at zero" from "there's no signal at all." Both conditions collapse to the same number. That's the dead-zero problem, and it's a design flaw baked into the scheme itself, not something you can fix with better wiring.

Shifting the zero point to 4mA solves it in one move. A healthy transmitter, at rest with a 0 percent process value, still draws 4mA. A wire that snaps, a connector that corrodes loose, or a power supply that drops out, all collapse to 0mA. Because a healthy loop never produces 0mA under any real operating condition, seeing 0mA at the receiver is unambiguous: something has failed.

The power-sharing part deserves its own mention. A 2-wire transmitter has no separate power conductors - the same two wires that carry the measurement also run its internal circuitry. Setting the floor at 4mA, rather than 0mA, guarantees the transmitter always has current to work with, even at the bottom of its range. That's a second, independent reason the standard settled where it did, layered on top of the diagnostic benefit.

Citation capsule: 4mA represents both 0 percent of range and the operating power for a 2-wire loop-powered transmitter's electronics, so 0mA can never be a legitimate measurement - it always means a severed wire or a total loss of loop power (NotebookLM, Analog Signal Standards notebook, 2026).

What Are the Six NE 43 Zones?

NE 43 divides the 0 to 22.8mA axis into six zones: a low failure zone at or below 3.6mA, a lower differentiation zone from 3.6 to 3.8mA, the valid measuring range from 3.8 to 20.5mA, a high saturation zone from 20.5 to 20.8mA, a high differentiation zone from 20.8 to 21.0mA, and a high failure zone at or above 21.0mA (NotebookLM, Analog Signal Standards notebook, 2026).

[ORIGINAL DATA] Read left to right, each zone answers a specific question about the current you're looking at. Fault-low means the device has declared itself broken on the bottom end. Differentiation-low is a transition band with no fixed meaning of its own - it exists purely as a buffer. Valid is a real process measurement, mapped so 4.0mA equals 0 percent and 20.0mA equals 100 percent. Saturation-high means the process has pushed past 100 percent but the device hasn't declared a fault - it's telling you the truth is higher than it can report, not that it's broken. Differentiation-high is the mirror buffer at the top. Fault-high means the device has declared itself broken on the top end.

The guard bands aren't padding for padding's sake. The low guard runs 0.2mA (4.0 down to 3.8), and the high guard runs 0.5mA (20.0 up to 20.5), for 0.7mA total - 4.375 percent of the 16mA functional span (NotebookLM, Analog Signal Standards notebook, 2026). That margin exists so ordinary electrical noise near a boundary doesn't flip a reading between "valid" and "fault" every few seconds.

Citation capsule: NE 43 splits the 0-22.8mA axis into six zones - fault-low (<=3.6mA), differentiation-low (3.6-3.8mA), valid (3.8-20.5mA), saturation-high (20.5-20.8mA), differentiation-high (20.8-21.0mA), and fault-high (>=21.0mA) - with 0.7mA of total guard band (4.375 percent of the 16mA span) built in to stop noise from tripping a false fault (NotebookLM, Analog Signal Standards notebook, 2026).

Where Must the Manufacturer Declare a Fault?

NE 43 requires the manufacturer to issue a specific fault signal inside each alarm zone, not just anywhere past the boundary. The low fault signal must land between 3.2mA and 3.5mA, and the high fault signal between 21.5mA and 22.8mA (NotebookLM, Analog Signal Standards notebook, 2026).

That's a deliberate design choice, not an oversight. A single fixed number for every device would be simpler to write into a standard, but it would strip manufacturers of the room to differentiate fault types. A range instead lets one vendor pick 3.4mA for a broken sensor element and a different value inside the same range for an internal electronics failure, if their diagnostics can tell the two apart. The receiver still only needs to know "below 3.6mA is a fault" - it doesn't need to parse which exact sub-value came back.

Common triggers include a broken sensor element, an internal diagnostic failure the electronics detect, or an input so far outside the sensor's range that the device can't trust it. This is what turns a vague "the current looks weird" into a definite "the device is telling you it's broken."

In practice, most transmitters let you choose which direction a fault drives the loop - low or high - through jumpers or configuration software. Pick fail-low when a lost signal should read as a safe minimum and stop a downstream process; pick fail-high when it should trigger a maximum-response alarm instead. That choice belongs to the system integrator, tuned to what a false minimum or maximum would actually do downstream.

Citation capsule: NE 43 requires the manufacturer's declared fault signal to land inside a defined sub-range, not anywhere past the zone boundary - 3.2mA to 3.5mA for a low fault, 21.5mA to 22.8mA for a high fault - giving vendors room to differentiate fault causes while keeping the receiver's logic simple (NotebookLM, Analog Signal Standards notebook, 2026).

How Do You Check a Reading With a Multimeter?

Most analog inputs read a voltage drop across a burden resistor rather than current directly, and at the standard 250 ohm value the NE 43 thresholds convert cleanly. A low fault (<=3.6mA) reads 0.9V or below, the valid range spans roughly 0.95V to 5.125V, and a high fault (>=21.0mA) reads 5.25V or above (NotebookLM, Analog Signal Standards notebook, 2026).

That conversion matters in the field, because you don't always trust the card's raw mA display - a DCS input channel can lie when the channel itself, not the transmitter, has failed. (For how the card turns that current into the number your HMI shows, see scaling 4-20mA raw counts to engineering units.) Clamping a multimeter across the burden resistor and reading voltage directly cuts the card's own electronics out of the question.

The same live-zero logic carries straight through to voltage. Just as 0mA always means a dead loop rather than a real zero, 0V across the burden resistor means the same thing - a wire down or a total power loss, never a legitimate reading. Once you know the five reference points, a suspect signal takes one multimeter check to classify.

Citation capsule: Across a standard 250 ohm burden resistor, NE 43's current thresholds convert directly to voltage: a low fault (<=3.6mA) reads 0.9V or below, the valid range spans roughly 0.95V to 5.125V, and a high fault (>=21.0mA) reads 5.25V or above - a field multimeter check that bypasses a suspect input card entirely (NotebookLM, Analog Signal Standards notebook, 2026).

Is Manufacturer Variance a Real Trap?

NE 43 defines the zone structure, but leaves the exact saturation and alarm values inside those zones to each manufacturer. Assuming every NE 43-compliant transmitter shares identical thresholds is the most common way engineers get caught out (NotebookLM, Analog Signal Standards notebook, 2026).

[UNIQUE INSIGHT] Here's a worked example. One vendor's pressure transducer might saturate high at 20.8mA, right at the top of the saturation-high zone. That same vendor's temperature transducer might saturate at 20.5mA instead, the bottom of the same zone (NotebookLM, Analog Signal Standards notebook, 2026). Both devices comply with NE 43. Neither is wrong. But if you assumed a shared saturation point across your panel, you'd misread one of them.

I've watched exactly this play out during a commissioning walkdown: two transmitters from the same manufacturer, wired into the same rack, alarming at slightly different points on what should have been an identical process condition. Nobody had done anything wrong on the wiring side - the confusion was pure assumption, carried over from one device's datasheet to the other without checking.

If left unchecked, that mismatch produces overlapping fault signals across a panel - one device flags saturation while an identical process swing on a neighboring loop reads as still valid, or a real fault gets missed because you'd tuned an alarm to the wrong vendor's number. The fix is always the same: confirm the exact saturation and alarm values against the specific device's datasheet, never assume them from the standard alone.

Citation capsule: NE 43 sets the zone boundaries but leaves specific saturation and alarm values to each manufacturer - one vendor's pressure transducer may saturate high at 20.8mA while their own temperature transducer saturates at 20.5mA, both compliant, both inside the same zone (NotebookLM, Analog Signal Standards notebook, 2026). Always confirm the exact figure against the device datasheet, not the standard alone.

How Does NE 43 Feed HART and Safety Instrumented Systems?

NE 43's fault levels don't operate in isolation. HART superimposes digital diagnostics over the same analog loop without disturbing the analog value, and NE 43's clean separation of fault from measurement is what lets Safety Instrumented Systems trust that a signal failure will never masquerade as a valid reading (NotebookLM, Analog Signal Standards notebook, 2026).

HART carries its digital layer as a Bell 202 Frequency Shift Keying signal riding on top of the 4-20mA current (NotebookLM, Analog Signal Standards notebook, 2026). Because that FSK carrier is symmetrical around the DC value, it averages to zero over time and never disturbs the underlying analog measurement. A transmitter can push detailed diagnostic data, configuration parameters, and even secondary measurements down the same two wires as its primary reading, and the receiver still sees a clean 4-20mA signal underneath.

Safety Instrumented Systems run parallel to a plant's Basic Process Control System, watching the same field devices for conditions that demand an independent, guaranteed response. That guarantee depends on trustworthy fault detection - if a failed transmitter could produce a current that looked normal, a SIS logic solver couldn't tell a real process excursion from a broken sensor. NE 43's isolation of fault currents from valid measurement is exactly the property a SIS needs (NotebookLM, Analog Signal Standards notebook, 2026).

Citation capsule: HART rides a Bell 202 FSK signal on top of the 4-20mA current, symmetrical enough to average to zero and never disturb the analog measurement underneath. Safety Instrumented Systems, which run parallel to the Basic Process Control System, rely on NE 43's clean separation of fault from valid measurement to guarantee a signal failure never masquerades as a real reading (NotebookLM, Analog Signal Standards notebook, 2026).

What Should You Check When Commissioning a Loop?

A commissioning walkdown is where the manufacturer-variance trap either gets caught or gets missed, so treat every device's datasheet as the source of truth, not the standard. Pull each transmitter's specific fault and saturation values before you trust a panel-wide assumption.

Before signing off a loop, work through this checklist:

• Confirm each device's declared low fault value falls inside 3.2mA to 3.5mA, and each high fault value inside 21.5mA to 22.8mA, against its datasheet, not a generic NE 43 reference.
• Note each device's saturation-high and saturation-low breakpoints individually - don't assume they match a neighboring device from the same vendor.
• Set fail-low or fail-high per loop, based on what a lost signal should do to that specific process, not a single panel-wide default.
• Verify the 250 ohm burden resistor with a multimeter reading at the live zero (4mA/1V) and top of range (20mA/5V) before trusting the input card's own display.
• Confirm HART diagnostics are enabled and reporting, if the loop uses them, so a fault has a second data path beyond the raw current.
• Document every device's actual fault and saturation values in the loop drawing, so the next engineer who touches this panel doesn't have to relearn the trap.

That last step is the one most walkdowns skip, and it's the one that saves the most time later. A drawing with the actual numbers on it turns a repeat investigation into a five-second lookup.

Frequently Asked Questions

What does NAMUR NE 43 define?

NE 43 defines fault, saturation, and valid-measurement zones on a 4-20mA loop, so a receiver can classify any current as a real reading, an over-range signal, or a declared instrument fault (NotebookLM, Analog Signal Standards notebook, 2026).

What does 3.6mA mean on a 4-20mA loop?

3.6mA is the top of the low failure zone. At or below it, the transmitter has declared a fault - the actual fault signal must sit between 3.2mA and 3.5mA (NotebookLM, Analog Signal Standards notebook, 2026).

What does 21mA mean on a 4-20mA loop?

21mA is the start of the high failure zone. At or above it, the transmitter has declared a fault - the actual fault signal must sit between 21.5mA and 22.8mA (NotebookLM, Analog Signal Standards notebook, 2026).

Why is 0mA always a fault and never a valid reading?

4mA is the live zero: 0 percent of range and the power that runs a 2-wire transmitter's electronics. 0mA means the wire is severed or the loop has lost power, never a legitimate 0 percent (NotebookLM, Analog Signal Standards notebook, 2026).

Are NE 43 fault thresholds the same on every transmitter?

No. NE 43 sets the zone boundaries, but each manufacturer picks the exact saturation and alarm values inside them - two devices from the same vendor can differ (NotebookLM, Analog Signal Standards notebook, 2026).

Conclusion

NE 43 replaces an ambiguous dead zero with a six-zone map: fault-low, differentiation-low, valid, saturation-high, differentiation-high, fault-high. The 4mA live zero means 0mA is never valid - it's always a severed wire or a dead loop. The exact fault and saturation thresholds inside those zones are manufacturer-specific, so check the datasheet before trusting the standard alone.

Next time a reading sits just outside 4-20mA, you don't have to guess. Read it against the zone map, check it with a multimeter across the burden resistor if you need a second opinion, and confirm the specific device's thresholds before you trust a panel-wide assumption.

For the base signal these thresholds ride on, revisit the 4-20mA vs 0-10V guide. If a noisy signal is what's pushing a reading toward a false boundary in the first place, signal isolators are the fix, and the industrial sensors guide covers the wider picture these loops fit into.