NAMUR NE 43 is a standard that defines the operating AO-LL and AO-UL values. The transmitter outputs a 4-20 mA current. However, as per NAMUR NE 43, the transmitter has it upper and lower current values.
NAMUR NE43 compliant transmitters are designed to output signals more than a 4 to 20 mA base signal. This tolerance is kept for a small amount of Signal Saturation.
The transmitter compliant with NAMUR NE 43 has an AO-LL and AO-UL of 3.8 mA and 20.5 mA respectively. These values can not be changed once the transmitter is designed as per the standard.
NAMUR NE 43
As per API Recommended Practice 551. we can clearly distinguish the following things if the signal levels are properly defined.
- When the process variable value increases beyond the transmitter calibrated range and the transmitter goes into saturation
- When the transmitter goes into saturation because of a faulty transmitter.
- In the case of the transmitter, the wires are short-circuited.
- When the transmitter wire is open circuit.
1. Transmitter Failure Identification
The transmitter has the feature to output a LOW or High current on the failure of the transmitter. Thus, it is possible to configure the transmitter according to the HIGH and LOW values.
| Current Signal | Current Range |
| High | 3.6 to 3.8 mA |
| LOW | 20.5 to 22 mA |
Thus, we can configure the transmitter output as per NAMUR NE 43 between 3.6 to 3.8 mA or 20.5 to 22 mA on the failure of the transmitter.
2. When Process Variable goes under or Over Range
Over-Range
The transmitter output between 20 to 20.5 mA shows the transmitter is functional, but the process variable value is above the transmitter calibrated range.
As per API 551, the transmitter output signal (mA) under different conditions are as follows.
| Normal | 4 to 20 mA |
| Normal Under Range | 3.8 to 4.0 mA |
| Normal Over Range | 20.0 to 20.5 mA |
| Transmitter Failure | 3.6 to 3.8 mA |
| Open Field Wire | 0 to 3.6 mA |
| Short Field Wire | > 22 mA |
Under-Range
If the transmitter output current is between 3.8 to 4 mA shows the transmitter is functional, but the process variable value is below the calibrated range.
What happens if the connecting wire is open or short circuit?
There are ample chances that despite the transmitter being functional and the process variable being within the range, the transmitter output current may not be the true value of the process variable. This happens when the connecting wires are open or short circuit. Now, we will discuss the wire open and short circuit scenario.
3. Open Wire
In the case of open-wire, the transmitter output is 4 mA and not 0 mA. if the wire is partially cut or completely cut is the case of an open field. In open field conditions, the output is between 0 to 3.6 mA.
4. Short Circuited wire
As per NAMUR NE 43, when the transmitter is functional and the process variable is within the limit, the short circuit will cause a transmitter output of more than 22 mA(As per API 551).
Advantages of Namur NE43
It is possible to detect the sensor healthiness in PLC and DCS systems. The Instrumentation technician/ engineer can repair or replace the transmitter without stopping the plant. Thus, it increases the plant upkeeping time and reduces the production loss.
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