What’s the difference between live zero and dead zero?

Live zero is a non-zero signal value, like 4mA in a 4-20 mA loop, that represents the lowest measurement confirming the signal is live, and the system is operational. Dead zero occurs when a zero-based signal like 0 mA or 0 V indicates the lowest measurement, making it impossible to distinguish between a true zero reading and a system failure.

NAMUR NE 43, from the German User Association of Automation Technology in Process Industries, is a widely adopted industry standard for defining fault levels in the 4-20 mA analog signals used in process automation, enabling the use of live zeros.

Defining fault signals is important in basic process control systems (BPCS), sometimes called a distributed control system (DCS), and in safety instrumented systems (SIS) (Figure 1). The BPCS controls and optimizes daily operations in continuous process industries like petrochemical refining, chemical manufacturing, paper and pulp production, and power generation. The SIS is a parallel system that protects personnel, the environment, and equipment from damaging and dangerous events, thereby enhancing safety and reducing risk.

Figure 1. NAMUR NE 43 can support both BPCS and SIS applications. (Image: AutomationBlog)

NAMUR NE 43 basics

Most digital transmitters are compliant with NE 43. The standard establishes a basic structure for transmitter failure signals (A). The structure enables users to separate fault signals from process measurements (M). It’s designed to provide early warnings of faults to support preventative maintenance and safety, and keep the process operating for increased productivity.

Instead of 4-20 mA, NE 43 uses a 3.8 to 20.5 mA signal range for M with ≥21 mA or ≤3.6 mA, indicating some type of instrument failure. That provides guard bands of 0.5 mA at the top end and 0.2 mA at the bottom end, separating measurement information from failure alerts (Figure 2).

Figure 2. The basic elements in NE 43 are the measurement range (M) and the fault information zones (A). (Image: Lesman)

NAMUR nuances

NE 43 is intended to solve the dead zero problem, but it doesn’t define specific values for alarm and saturation levels; that’s left to individual equipment manufacturers. In fact, NE 43 defines a saturation zone, not a saturation level. That zone sits between 20.0 mA for nominal high values and establishes a buffer zone between 20.5 mA (minimum high saturation) and 20.8 mA (maximum high saturation) and a differentiation zone between 20.8 and 21.0 mA before reaching the high fault zone (Figure 3).

Figure 3. NE 43 sets a series of saturation zones, buffer zones, and differentiation zones that designers can use when integrating sensors and transducers. (Image: Orion Technical Solutions)

A similar set of ranges is established for low readings, with 4.0 mA being the nominal 0% reading. The minimum low fault is set at 3.8 mA, with the maximum low fault set at 3.6 mA, which establishes the lower differentiation zone. The maximum low fault measurement is 3.6 mA.

A key to NE 43 is the flexibility to accommodate different manufacturers, different types of transducers, and different sensor technologies. Note that while 3.6 mA is the low fault demarcation line, device makers are required to issue a low fault signal between 3.5 and 3.2 mA. And while 21.0 mA is the high fault threshold, device makers are required to issue a high fault signal between 21.5 and 22.8 mA.

There can be variations in definitions for different transducer types, even from the same manufacturer. For example, one manufacturer of both pressure and temperature measurement instruments defines the various (and very similar, but not identical) limit and threshold levels for specific models as follows:

Pressure transducer: low saturation limit 3.9 mA, high saturation limit 20.8 mA, low alarm (fault) indication 3.5 mA, and high alarm (fault) indication 22.5 mA.
Temperature transducer: low saturation limit 3.9 mA, high saturation limit 20.5 mA, low alarm (fault) indication 3.6 mA, and high alarm (fault) indication 22.5 mA.

Equipment designers need to understand the nuances of NE 43 to ensure proper operation. Using NE 43 compliant devices ensures that it will be possible to identify values that will work in each application without overlapping fault signals on the high or low ends. It does not relieve equipment designers from the requirement to ensure proper instrument integration into the final solution.

Summary

A dead zero occurs in a measurement system when the zero value corresponds to a reading of 0 mA from the sensor or transducer. That makes it impossible to distinguish between zero reading and system faults. The NAMUR NE 43 standard was developed to eliminate the dead zero problem in process control and safety systems. NE 43 is a tool designers can use when integrating instrumentation solutions.