IEC 60381-1 / NAMUR NE 43
Fundamentals
Industrial Instrumentation
4-20mA Current Loop Transmitter & Line Resistance
Simulate 2-wire smart transmitters, cable loop resistance drops, and NAMUR fault alarms.
Calculates the 4-20mA linear current representation of process variables. Inspects transmitter terminal compliance voltage, total loop resistance load lines, and burnout trip levels (3.6mA / 21.0mA).
Governing Physical Law & Equations
I_{loop} = 4\text{mA} + 16\text{mA}\left(\frac{PV - LRV}{URV - LRV}\right), \quad V_{term} = V_s - I_{loop}(R_{wire} + R_{load})
Loop current proportional formula and transmitter terminal voltage compliance.
Law: Ohm’s Law & Kirchhoff’s Voltage Loop | Standard Reference: IEC 60381-1 / NAMUR NE 43 / ISA-50.1 (Analogue Signals for Process Control Systems)
Adjustable System Parameters
| Parameter |
Nominal Value |
Dynamic Range |
Physical Role |
| Process Pressure (PV) |
6.5 bar |
0 to 10 bar |
Measured process variable (0 to 10 bar range) |
| Field Cable Resistance (R_wire) |
25 Ω |
0 to 80 Ω |
Total round-trip lead wire resistance |
| DCS / PLC Burden Resistor (R_load) |
250 Ω |
100 to 500 Ω |
Analog input receiving channel impedance |
| Loop Power Supply (V_s) |
24 VDC |
18 to 30 VDC |
DC power supply powering 2-wire loop |
Analytical Proof & Derivation
The live-zero 4-20 mA standard maps physical process variable range [LRV, URV] linearly to loop current: I_loop = 4 mA + 16 mA × (PV - LRV)/(URV - LRV). Applying Kirchhoff’s Voltage Law around the 2-wire series loop: V_supply - I_loop R_wire - I_loop R_load - V_transmitter = 0. The transmitter terminal voltage V_transmitter = V_supply - I_loop (R_wire + R_load) must remain strictly above the transmitter minimum operating compliance voltage V_min (typically 11.5 VDC at 20 mA).
Verification Benchmark
IEC 60381-1 benchmark: PV = 6.5 bar on 0-10 bar range: theoretical I_loop = 4 + 16 × (6.5/10) = 14.40 mA. With V_s = 24 VDC, R_wire = 25 Ω, R_load = 250 Ω: total loop drop = 14.40 mA × 275 Ω = 3.96 V; terminal voltage V_term = 24 - 3.96 = 20.04 VDC > 11.5 VDC compliance threshold (PASSED).
Field Engineering Insights
NAMUR NE 43 standardizes diagnostic failure states: current < 3.6 mA indicates sensor burnout or wire-break fail-low, while current > 21.0 mA signals sensor fail-high. Normal measuring span is 3.8 mA to 20.5 mA.