Radar 4-20 mA Wiring: Loop Power, HART and Receiver Tests

Radar 4-20 mA wiring needs a complete loop drawing. The supply, transmitter, cable, input card and any barrier or isolator must work together over the required current range. A lit local display proves neither adequate voltage margin at high output nor a usable HART communications path.

Identify Who Powers the Loop

For a two-wire transmitter, the same circuit carries supply current and the analogue signal. An active input or separate loop supply may provide that power; a passive receiving input does not automatically do so. Follow the transmitter and input-card drawings rather than connecting two power sources because both devices have positive terminals.

The METRAVON MV-RD12 offers a 24 V DC two-wire 4-20 mA/HART configuration. Separately powered RS485 electronics require a different connection arrangement. Confirm the supplied version before laying cable or reusing an old terminal schedule.

METRAVON MV-RD12 radar available with an ordered two-wire 4-20 mA HART circuit — Match the instrument order code, loop supply and receiver before connecting the MV-RD12.

Radar 4-20 mA Wiring: Account for Every Voltage Drop

At the highest current used for operation or configured fault signalling, enough voltage must remain at the transmitter. The design check is: available supply voltage minus cable and series-device drops must meet the instrument's specified terminal requirement. Use the lowest credible supply and the actual devices in the loop.

Part	Obtain	Check under load
Transmitter	Required terminal voltage for the supplied electronics.	The remaining voltage stays adequate through the intended current range.
Cable	Total conductor resistance for the installed circuit.	The round-trip drop is included, not just one cable leg.
Series device	Barrier, isolator or indicator requirements and voltage drops.	Each device is compatible with the selected loop and communications path.
Input	Input type, burden and any built-in HART impedance.	No unnecessary resistor duplicates an impedance already provided.

Do not treat every series device as an ideal resistor; use its stated circuit requirements. Record the calculation and measured terminal voltage during commissioning. A supply measurement with the circuit unloaded can miss a problem that appears only as current rises.

Check HART Without Disturbing the Analogue Measurement

HART requires an appropriate communications path and compatible loop impedance. Confirm the transmitter, modem and interface requirements instead of adding a fixed resistor to every installation. An isolator may pass the analogue value without providing the required HART path, so check its specification explicitly.

A successful HART connection and a correct PLC analogue value are separate test results. Compare the configured output variable and range with the receiver. Where digital communication shows distance while the analogue channel represents level, label the two values clearly rather than expecting their numbers to match.

Commission the Loop With Controlled Test Values

Confirm the circuit, polarity and required isolation before energising or connecting test equipment.
Use supported output simulation or a suitable calibrated source to check low, intermediate and high current values at the receiver.
Compare raw input data with the PLC engineering scale and HMI display. Remove any temporary simulation before returning to process measurement.
Test the specified fault indication and receiver response separately from normal range points, then retain the as-left configuration.

Electrical tests and disconnection belong to qualified personnel following the site's procedures, particularly where barriers or classified areas are involved. Do not place a current meter directly across a supply. Use the approved test point and instrument method for the circuit.

For intermittent noise, record the current and input diagnostic while nearby equipment operates. Inspect cable routing, entry seals, terminals and the designed shielding arrangement. Adding a second shield bond or increasing software damping without identifying the cause can create a new problem or conceal the original one.

Loop-Wiring Questions

Q1. Can a good reading at mid-scale prove the voltage budget?

No. The voltage consumed by resistance increases with current. Verify the demanding end of the configured range and relevant fault-current condition against the actual terminal requirement. Do not infer margin from a single convenient operating point.

Q2. Is a HART resistor always an additional component?

No. The needed impedance may already exist in the input or interface. Check the full circuit before adding anything, then verify communication and analogue performance together. An unnecessary resistor consumes voltage margin.

Q3. What should accompany a wiring-support request?

Send the delivered electronics code, loop drawing, input-card details, barrier or isolator model and measured terminal voltage and current. Identify when the fault occurs. These details allow the circuit to be reviewed without guessing from wire colours or a housing photograph.

Summary

Verify power, load and communications as one circuit, then test the receiver over its useful range. Submit the MV-RD12 loop drawing for an interface review before replacing components or changing grounding. Preserve the measured results with the final wiring and scaling settings.