GGSTech ToolsWhy current loops are robust
In a series loop, the same current flows through the transmitter and receiving input. Within compliance-voltage limits, moderate wiring resistance has less effect on the represented value than a simple voltage signal.
Loop voltage budget
Check at maximum loop current and worst-case cable resistance/barriers. The transmitter must retain enough voltage to operate.
Shielding
Follow project and manufacturer practices. Cable shield termination strategy depends on system grounding and EMC design; avoid casually grounding shields at both ends when that creates unwanted loop currents.
Fault values
Transmitters may signal underrange/overrange or configured fault currents outside the normal 4–20 mA measurement span. Exact values are device/configuration dependent.
What you should understand first
The 4–20 mA loop combines power and signal in many two-wire transmitters. Understanding current path and voltage budget is more important than memorizing a single wiring sketch.
Signal and energy boundaries
A PLC is a controller, not the power source for a motor. Separate the low-energy logic side from the field power circuit. Digital outputs often command an interposing relay, contactor coil, valve solenoid or VFD input. The motor power still passes through the MCC/MCCB/contactor/VFD power circuit.
Fail-safe thinking
For critical shutdowns, do not rely only on software logic. Emergency-stop, trip and safety functions may require dedicated safety-rated hardware and a risk assessment. A normal PLC output should not be assumed to provide a safety function unless the architecture is specifically designed and certified for it.
Common mistakes
- Polarity reversal
- Connecting a current output into a voltage-only input
- No loop-voltage budget calculation
- Ground loops/shielding mistakes
- Interpreting a fault current as a valid process value
Commissioning checklist
- Check whether transmitter is 2-wire, 3-wire or 4-wire.
- Verify AI input type and commoning.
- Calculate cable + barrier voltage drop.
- Loop-check at 4, 12 and 20 mA.
- Test broken-wire and transmitter-fault handling.
Frequently asked questions
Why use 4 mA as zero?
It is a live zero that can power a two-wire transmitter and helps distinguish valid zero from loss of current.
Can a 4–20 mA loop run very long distance?
Distance is limited by loop voltage budget, cable resistance, EMC and installation conditions—not by one universal length.
Where should the shield be grounded?
Use the equipment/project EMC grounding strategy; there is no safe universal rule for every installation.