Industrial Instrumentation Selection Guide
Selecting a 4-20mA surge protector is not only a question of surge current. The signal SPD must match the loop voltage, two-wire or four-wire circuit, transmitter power arrangement, PLC or DCS input, HART communication and the voltage available across the complete current loop.
Table des matières
Quick answer: what SPD should be used for a 4-20mA loop?
For a typical 24V two-wire, loop-powered transmitter, use a signal SPD designed for one floating two-conductor current loop.
Confirm that the SPD maximum continuous operating voltage is above the highest normal loop voltage. Then check the series resistance or voltage drop at maximum loop current, leakage current, protection level, grounding method and HART compatibility.
A four-wire transmitter normally has a separate power input and 4-20mA output. Its power conductors and signal conductors must be evaluated as different circuits. One two-wire signal SPD may not protect the complete instrument.
Recommended protection configurations
Use this matrix for initial planning. Final model confirmation still requires basic loop and equipment information.
2-wire, 24V transmitter
Use one matched signal SPD for the floating current-loop pair.
Confirm: UC, series resistance, leakage current and line-to-line/earth protection.
2-wire HART loop
Use a 4-20mA signal SPD with explicit HART compatibility.
Confirm: capacitance, frequency response, loop impedance and voltage drop.
4-wire transmitter
Evaluate the instrument power input and 4-20mA output separately.
Confirm: power voltage, output type, isolation and signal reference.
Outdoor or long cable
Evaluate field-side and cabinet-entry protection for both ends.
Confirm: cable route, LPZ boundary, enclosure, shield and bonding.
Ex i instrument loop
Use only an SPD approved and coordinated for the intrinsically safe circuit.
Confirm: Ex approval, entity parameters, barrier, earthing and installation zone.
Why there is no universal “24V 4-20mA” model
Two loops with the same nominal voltage may have different power sources, input burdens, grounding references, HART requirements and hazardous-area conditions. A final model should not be selected from “24V, 4-20mA” alone.
How to select a 4-20mA signal SPD
Start with the loop drawing rather than the SPD catalogue. Two systems described as “24V 4-20mA” may use different power sources, grounding references and conductor arrangements.
Identify the complete circuit
Confirm whether the instrument is two-wire loop-powered, three-wire or four-wire self-powered.
Find the source of loop power
Determine whether the PLC/DCS input, an external supply or the field instrument provides loop power.
Determine the highest normal voltage
Include supply tolerance and the maximum voltage that can remain continuously on the protected conductors.
Match the SPD circuit topology
Choose protection for a floating pair, common-reference circuit, or separate signal and power circuits.
Check signal transparency
Review series resistance, voltage drop, leakage current, capacitance and HART compatibility.
Confirm installation conditions
Specify field or cabinet mounting, grounding, terminal size, hazardous-area approval and environmental requirements.
Two-wire, three-wire and four-wire 4-20mA loops
The number of conductors changes how power, signal and surge protection are arranged. A two-wire loop normally uses the same pair for power and measurement. A four-wire transmitter normally separates its power input from its analog output.
Two-wire loop-powered
Power and signal use the same conductor pair.
- Usually requires one protected floating pair.
- SPD voltage drop forms part of the voltage budget.
- Confirm active or passive AI arrangement.
- Check HART when a smart transmitter is used.
Three-wire transmitter
The instrument normally has supply positive, supply common and a separate output conductor.
- The signal may share the supply reference.
- A floating-pair SPD may not match the circuit.
- Confirm which conductors carry normal current.
- Provide a wiring diagram before final selection.
Four-wire self-powered
The instrument has a separate power input and an independent 4-20mA output.
- Evaluate power and signal as separate surge paths.
- The signal output may be active or isolated.
- Power voltage may differ from signal voltage.
- One signal SPD may not protect all conductors.
Do not select the SPD only from terminal count
A four-wire transmitter does not automatically require one four-channel signal SPD. Confirm the power input, analog output, internal isolation and grounding reference first.
Active and passive 4-20mA loops
“Active” and “passive” describe which device supplies loop power. This changes the normal voltage and reference present on the conductors.
| Arrangement | Who supplies loop power? | Typical connection | What must be confirmed? |
|---|---|---|---|
| Active AI | PLC or DCS analog input | AI card powers a passive two-wire transmitter. | AI supply voltage and circuit reference. |
| Passive AI | External loop power supply | Supply, transmitter and passive AI are connected in series. | Supply tolerance, AI burden and grounding reference. |
| Active transmitter output | The field instrument | A self-powered instrument drives a passive AI. | Output voltage, common-mode limits and isolation. |
| Analog output loop | PLC/DCS AO card or external supply | AO controls a valve positioner or actuator. | Whether the AO is sourcing or sinking. |
A request that only says “24V, 4-20mA SPD” may not be enough for a final model. A basic wiring diagram, terminal photo or equipment model can usually resolve the missing information.
What voltage rating should the 4-20mA SPD have?
Select the signal SPD from the highest voltage that can remain continuously on the protected conductors—not from the 4mA or 20mA signal values.
The SPD maximum continuous operating voltage must be above the highest normal circuit voltage, including supply tolerance, while remaining low enough to provide a useful protection level.
Lower UC is not automatically better
If the SPD continuous working voltage is below the actual loop voltage, it may conduct during normal operation, increase leakage or fail prematurely.
Will an SPD cause voltage drop in a 4-20mA loop?
Yes. A series-connected signal SPD normally introduces resistance or another series impedance. At 20mA, that impedance creates a voltage drop.
A small voltage loss does not normally create a proportional current measurement error. The transmitter regulates the requested current while sufficient voltage remains across its terminals.
Include the following series loads:
- PLC or DCS analog-input burden;
- cable loop resistance;
- cabinet-side and field-side SPDs;
- intrinsic-safety barriers or isolators;
- loop-powered displays or test terminals;
- engineering safety margin.
4-20mA loop voltage budget calculator
Use the minimum supply voltage and maximum expected loop current for a conservative preliminary check.
The example retains positive voltage margin. Confirm the actual equipment data before final selection.
This calculator is a preliminary engineering aid and does not replace the actual loop drawing or equipment datasheets.
Example interpretation
If two SPDs each drop 0.3V at 20mA, they consume 0.6V together. This may be acceptable in a simple loop but important in a long circuit that also contains a 250Ω input and a safety barrier.
Can a signal SPD affect 4-20mA measurement accuracy?
A properly selected SPD should remain electrically transparent during normal operation.
| Caractéristique | Possible effect | Supplier data to check |
|---|---|---|
| Series resistance | Consumes loop voltage at high current. | Resistance per conductor and voltage drop at 20mA. |
| Leakage current | May add offset in high-accuracy circuits. | Maximum leakage at UC and operating temperature. |
| Conductor imbalance | May reduce common-mode noise rejection. | Matched protection paths for the signal pair. |
| Capacitance and bandwidth | May affect HART communication. | HART compatibility and frequency response. |
Important distinction
A small SPD voltage drop does not normally create a proportional scaling error. It reduces the voltage available to the transmitter. If the margin becomes insufficient, the loop may fail to reach the expected high-end current.
How to select a HART-compatible surge protector
HART superimposes a digital communication signal on the conventional 4-20mA loop. An SPD may pass DC current but still attenuate HART communication.
Should the SPD be installed at the field instrument or control cabinet?
The correct position depends on cable exposure, lightning protection zones, building entry points and the equipment requiring protection.
| Installation condition | Recommended consideration | Raison |
|---|---|---|
| Outdoor transmitter with long cable | Evaluate field-side and cabinet-entry protection. | Both ends contain sensitive electronics. |
| Cable between buildings | Protect at each building entry and coordinate bonding. | The cable crosses different equipotential zones. |
| Tank, mast or pump station | Use a field-compatible SPD near the instrument. | Remote outdoor locations face higher exposure. |
| Short wiring inside one cabinet | Two SPDs are not automatically required. | The complete circuit may remain in one protected zone. |
| Field junction box | Choose the cable entry or equipment boundary. | The SPD should be close to the protection-zone boundary. |
Shielding, DIN-rail grounding and equipotential bonding
A signal SPD requires a low-impedance discharge path. The PE terminal, grounding foot or bonded DIN rail must connect to the local equipotential bonding system with the shortest practical route.
4-20mA SPD selection for hazardous areas and Ex i loops
A standard industrial signal SPD should not be placed in an intrinsically safe loop without verification.
- Confirm ATEX, IECEx or locally required approval;
- confirm the installation zone and protection concept;
- check maximum voltage and current parameters;
- check SPD capacitance and inductance;
- coordinate the SPD with the barrier or isolator;
- confirm the permitted earthing method;
- check HART or SIL requirements where applicable.
Approval of the SPD alone is not enough
The transmitter, cable, SPD, barrier and installation arrangement must be evaluated as one intrinsically safe circuit.
What the buyer provides and what the SPD supplier confirms
The buyer only needs to provide information already available. The SPD supplier should use it to confirm the detailed product parameters.
| Selection item | Buyer or engineer provides | SPD supplier confirms |
|---|---|---|
| Loop topology | Two-wire, three-wire or four-wire information, drawing or terminal photo. | Matching protection modes, terminal arrangement and circuit reference. |
| Power arrangement | PLC/DCS card, transmitter or power-supply model where available. | Suitability for the normal circuit voltage and current path. |
| Working voltage | Nominal loop voltage, for example 24V DC. | Nominal voltage and maximum continuous operating voltage. |
| Voltage margin | Equipment models or available loop data. | Series resistance and voltage drop at maximum current. |
| Measurement accuracy | Special accuracy requirement, when applicable. | Leakage current at UC and over the operating temperature range. |
| Protection performance | Indoor, outdoor, interbuilding or other exposure information. | Protection levels, test waveform and surge-current values. |
| HART communication | HART required or not required. | HART compatibility, capacitance and frequency response. |
| Hazardous area | Hazardous-area requirement and barrier model where available. | Approval scope, entity parameters and permitted earthing. |
| Installation | Field, cabinet or both-end installation. | Mounting, terminal, temperature and grounding arrangement. |
Model confirmation before a project or OEM order
LEEYEE can start from basic information and request additional data only when it is necessary for final model confirmation.
Four-step confirmation process
Check the loop type, voltage and connected equipment.
Confirm UC, load current, protection level and voltage drop.
Check HART, Ex i, mounting, terminals and environment.
Approve the datasheet, label, packaging and quantity.
Typical industrial applications
Level, pressure, flow, pH and conductivity transmitters connected to remote PLC or SCADA panels.
Outdoor pressure and level instruments with long signal cables.
DCS instrument loops in chemical, food, oil, gas and manufacturing facilities.
Remote level and temperature transmitters in exposed locations.
Analog sensors outside the main control cabinet or near large machinery.
Long analog loops between plant rooms and monitoring panels.
Meteorological and auxiliary process instruments on exposed cable routes.
Repeatable analog-input protection for standardized cabinet projects.
Common 4-20mA SPD selection mistakes
Products with the same label may have different UC, leakage, resistance and protection levels.
A four-wire transmitter may have separate power and signal surge paths.
A high surge rating does not prove suitability for a loop with limited voltage margin.
HART also depends on frequency response, capacitance and series impedance.
A remote field instrument can also be damaged by induced surge energy.
Good laboratory parameters cannot compensate for a long or unbonded discharge path.
Approval and entity parameters must be verified for the complete circuit.
What to send for a 4-20mA SPD recommendation
Send any information you already have. You do not need to prepare every item before contacting us.
Six details are usually enough to start
No loop drawing? Send an equipment datasheet, terminal photo or existing SPD model. We will help identify the circuit.
2-wire, 3-wire or 4-wire
For example: 24V DC
Required or not required
Transmitter or PLC/DCS card model
Field, cabinet or both ends
Quantity, logo and packaging needs
Need help selecting a 4-20mA signal SPD?
Send a wiring diagram, terminal photo, transmitter model or existing SPD model. LEEYEE will help identify the loop and confirm the suitable protection configuration.
Questions fréquemment posées
Can the same SPD be used for all 4-20mA loops?
No. Two-wire, three-wire and four-wire circuits may use different power sources, conductor arrangements and grounding references.
Is a 24V signal SPD always correct for a 24V loop?
Not automatically. Compare the SPD maximum continuous operating voltage with the highest normal loop voltage and supply tolerance.
Does SPD resistance change the 4-20mA reading?
Normally, SPD resistance consumes loop voltage rather than creating a proportional scaling error. If the voltage margin becomes insufficient, the loop may fail to reach full-scale current.
Should a 4-20mA loop be protected at both ends?
Both-end protection is often useful for outdoor, long-distance or interbuilding circuits. It is not mandatory for every short internal connection.
Can a standard 4-20mA SPD pass HART communication?
Only when its capacitance, frequency response and series impedance are suitable. Explicit HART compatibility should be confirmed.
Does a four-wire transmitter need two SPDs?
It may require separate protection for its power input and signal output. The final arrangement depends on the wiring and internal isolation.
Can the cable shield replace the SPD earth connection?
No. Shield termination and SPD protective earthing perform related but different functions.
Which standard applies to 4-20mA signal SPDs?
IEC 61643-21 covers requirements and tests for signalling-network SPDs. IEC 61643-22 provides selection and application principles.
Technical references
- IEC, IEC 61643-21:2025 — Requirements and test methods for SPDs connected to telecommunications and signalling networks .
- IEC, IEC 61643-22:2015 — Selection and application principles .
- FieldComm Group, HART Application Guide .
- Texas Instruments, Low-Cost Loop-Powered 4-20mA Transmitter Reference Design .
- LEEYEE, PLC Surge Protection Guide .
