- •Input/Output (I/O) capabilities
- •Discrete I/O
- •Analog I/O
- •Network I/O
- •Logic programming
- •Relating I/O status to virtual elements
- •Memory maps and I/O addressing
- •Ladder Diagram (LD) programming
- •Contacts and coils
- •Counters
- •Timers
- •Data comparison instructions
- •Math instructions
- •Sequencers
- •Structured Text (ST) programming
- •Instruction List (IL) programming
- •Function Block Diagram (FBD) programming
- •Sequential Function Chart (SFC) programming
- •Human-Machine Interfaces
- •How to teach yourself PLC programming
- •Review of fundamental principles
- •Analog electronic instrumentation
- •4 to 20 mA analog current signals
- •Relating 4 to 20 mA signals to instrument variables
- •Example calculation: controller output to valve
- •Example calculation: temperature transmitter
- •Example calculation: pH transmitter
- •Example calculation: PLC analog input scaling
- •Graphical interpretation of signal ranges
- •Thinking in terms of per unit quantities
- •Controller output current loops
- •Troubleshooting current loops
- •Using a standard milliammeter to measure loop current
- •Using shunt resistors to measure loop current
- •Troubleshooting current loops with voltage measurements
- •Using loop calibrators
- •NAMUR signal levels
- •Review of fundamental principles
- •Pneumatic instrumentation
- •Pneumatic sensing elements
- •Self-balancing pneumatic instrument principles
- •Pilot valves and pneumatic amplifying relays
- •Analogy to opamp circuits
- •Analysis of practical pneumatic instruments
- •Proper care and feeding of pneumatic instruments
- •Advantages and disadvantages of pneumatic instruments
- •Review of fundamental principles
13.7. TROUBLESHOOTING CURRENT LOOPS |
915 |
13.7.7NAMUR signal levels
One of the intrinsic benefits of a “live zero” analog signal standard such as 4-20 mA is that a wire break (open fault) can immediately be detected by the absence of current in the circuit. If the signal scale started at zero (e.g. 0-20 mA), there would be no way to electrically distinguish between a broken wire and a legitimate 0% signal value. In other words, the “live” LRV point of a 4-20 mA signal provides us with a way to indicate a certain type of circuit fault in addition to indicating an analog measurement during normal operation.
The NAMUR signal standard takes this philosophy one step further by defining specific diagnostic meaning to values of current lying outside the 4-20 mA range:
Signal level |
Fault condition |
Output ≤ 3.6 mA |
Sensing transducer failed low |
3.6 mA < Output < 3.8 mA |
Sensing transducer failed (detected) low |
3.8 mA ≤ Output < 4.0 mA |
Measurement under-range |
21.0 > Output ≥ 20.5 mA |
Measurement over-range |
Output ≥ 21.0 mA |
Sensing transducer failed high |
NAMUR-compliant transmitters are designed to limit their output signals between 3.8 mA and less than 21 mA when functioning properly. Signals lying outside this range indicate some form of failure has occurred within the transmitter or the circuit wiring.
NAMUR-compliant control systems will recognize these errant milliamp values as fault states, and may be programmed to take specific action upon receiving these signal values. Such actions include forcing controllers into manual mode, initiating automatic shutdown procedures, or taking some other form of safe action appropriate to the knowledge of a failed process transmitter.