Wiring 4–20 mA/HART Radar Level Meters: Power, Load and Shielding

A 4–20 mA radar level loop is simple only when its complete voltage and resistance budget is checked. The transmitter, barrier, cable, display, input card and test resistor all share the same loop. Incorrect polarity, insufficient voltage, excessive resistance or poor shielding can produce dropouts, a fixed current or unstable control-system values even when the radar display is healthy.

Identify two-wire or four-wire architecture

A two-wire transmitter uses the same pair for power and signal. A four-wire device has separate power and output terminals and may require an isolated or correctly referenced input. Follow the terminal diagram for the exact model; do not infer wiring from wire colors or another instrument family.

Record supply type, nominal voltage, analog input type and whether a safety barrier, isolator or surge protector is installed. Confirm polarity before energizing and use the site's isolation and hazardous-area procedures.

Calculate the loop voltage budget

At the highest current, the supply must provide the transmitter's minimum terminal voltage plus the drop across all series resistance. Cable resistance, input resistance, barrier resistance and any HART resistor contribute. For example, 20 mA through 500 ohms drops 10 V before the transmitter requirement is considered.

Measure voltage at the transmitter terminals while the loop is operating, especially during a fault or high-output condition. A supply that looks correct with the loop open may fall below the required voltage under load. Include the lowest expected supply voltage and the highest cable temperature when calculating design margin, because both can reduce available loop voltage.

Provide a valid HART path

HART communication needs suitable loop impedance and a compatible connection point. Many systems use approximately 250 ohms of resistance, but the allowed range and device requirements must be confirmed. A low-impedance power source connected without the required resistance can carry analog current while preventing reliable HART communication.

Connect the communicator at an approved point and observe hazardous-area restrictions. HART communication does not replace verification of the analog value received by the PLC; test both paths.

Control shielding and grounding

Use the specified twisted, shielded instrumentation cable and route it away from variable-frequency drives, motor leads and high-current switching. Follow the site's grounding design. A shield commonly bonds at one controlled end to avoid circulating current, but the correct arrangement depends on barriers, equipotential bonding and local standards.

Keep cable glands sealed and maintain shield continuity through junction boxes where required. Do not use the signal conductor as a protective earth. If the reading changes when motors run, compare raw loop current and transmitter diagnostics before increasing software filtering.

Commission normal and fault outputs

Verify 4 mA, 20 mA and at least one intermediate point using transmitter simulation or a controlled reference level. Confirm PLC engineering units, range direction and input-card configuration. Measure current in series or through an approved test terminal; placing a current meter directly across the supply can damage the meter or loop.

Agree how the transmitter signals a fault, such as a configured low or high alarm current, and ensure the PLC distinguishes it from a valid process value. Test power loss, open circuit and device fault behavior, then record the as-left current, displayed value and PLC value.

Commissioning checklist

• Confirm two-wire or four-wire terminals and polarity.
• Calculate supply voltage against every series resistance.
• Measure transmitter terminal voltage under load.
• Provide the required HART loop impedance and connection point.
• Route and ground shielded cable according to the site design.
• Test 4 mA, 20 mA, an intermediate point and configured fault current.

Frequently asked questions

Why is the local display correct but the PLC value is low?

Check loop resistance, input configuration and PLC scaling. Measure actual current and transmitter terminal voltage to separate electrical loss from scaling error.

Can HART work without a resistor?

Some interfaces include suitable impedance internally, but many loops require added resistance. Confirm the communicator, power source and transmitter requirements.

Should both ends of the shield be grounded?

Not by default. Use the approved site grounding and barrier design; uncontrolled multiple bonds can create noise or unsafe current paths.

Related measurement solutions

80 GHz radar level meters · 26 GHz radar level meters