4–20 mA current loops

A 4–20 mA current loop carries a measurement as an electrical current: 4 mA means the bottom of the range, 20 mA means the top, and everything in between is proportional. It has been the standard analog signal in instrumentation for decades because it is simple, travels long distances without losing accuracy and makes a broken wire obvious.

Why current and not voltage

In a series circuit, the same current flows through every part of the loop. The transmitter regulates that current to match its measurement, so the resistance of a long cable, a terminal block or a connector does not change the value the controller reads, as long as there is enough voltage to drive the current around the loop. A voltage signal, by contrast, drops along the wire and picks up noise more easily.

The range starts at 4 mA rather than zero for two reasons. A reading of 0 mA can only mean the loop is broken or unpowered, so a fault cannot be mistaken for a real low value. And the 4 mA "live zero" leaves enough current to power the transmitter's electronics from the loop itself, which is what makes two-wire instruments possible.

How a two-wire loop works

In the most common arrangement, a DC power supply in the control panel, the transmitter in the field and the controller's input are all wired in series. The transmitter draws its own power from the loop and acts as a current regulator. The controller input usually reads the current as a voltage across a known resistor; 250 ohms, for example, turns 4–20 mA into 1–5 V.

Other arrangements exist. A four-wire transmitter has its own power supply and drives the loop actively, so it must not be connected to an input that also supplies power. Three-wire devices share a common conductor between power and signal. Controller inputs can be "active" (they supply loop power) or "passive" (they expect the loop to be powered elsewhere). Mixing these up, so that a loop has two power sources or none, is one of the most common commissioning faults.

Checking the power budget

The supply voltage has to cover every voltage drop in the loop at full current, with the transmitter still getting its minimum operating voltage. A worked example: a 24 V supply feeds a transmitter whose datasheet asks for 12 V at its terminals. At 20 mA, a 250 ohm input resistor drops 5 V. That leaves 24 − 12 − 5 = 7 V for the cable and anything else in the loop, which at 20 mA allows up to 350 ohms. Long runs, intrinsic safety barriers, isolators and loop-powered displays all use part of that margin, so add them up before assuming the loop will work at the top of its range.

Scaling and the live zero

The transmitter is configured with a lower range value (the measurement at 4 mA) and an upper range value (the measurement at 20 mA). The controller must be configured with the same two values. Between them the relationship is a straight line:

value = lower range + (current − 4) ÷ 16 × (upper range − lower range)

For a pressure transmitter ranged 0 to 10 bar, 12 mA means 5 bar and 8 mA means 2.5 bar. If the transmitter is re-ranged to 0 to 16 bar and the controller is not, the controller will still show a believable number, but a wrong one. Every re-range should be recorded and checked at both ends.

Many transmitters can be set to drive their output low or high on an internal fault, and many follow the NAMUR NE 43 recommendation for those levels. The controller should be configured to recognise them and raise an alarm rather than treat them as a process value. Check which direction each transmitter fails and make sure the control logic does the safe thing in that case: a level transmitter that fails low could cause a pump to keep filling a tank.

HART and digital signals

HART superimposes a digital signal on the 4–20 mA current. The analog value still does the control work, while the digital layer carries configuration, diagnostics and sometimes extra measured values. HART needs a minimum resistance in the loop to work, which is another reason 250 ohm input resistors are common. Fully digital fieldbus and Ethernet-based instruments replace the analog signal altogether; they carry more data on fewer wires, at the cost of more configuration and a network to maintain.

Wiring good practice

• Use twisted pair cable, shielded where there is electrical noise, and ground the shield at one end only, usually in the panel.
• Keep signal cables apart from power and variable frequency drive cables, and cross them at right angles where they must cross.
• Use signal isolators where two grounded devices share a loop, to avoid ground loops that shift the reading.
• In hazardous locations, use intrinsically safe barriers or isolators and certified field devices, installed to the Canadian Electrical Code. See electrical equipment in hazardous locations.
• Label both ends of each loop with the instrument tag and keep the loop diagram current.
• Fuse or otherwise protect each loop individually, so one fault does not take out a whole card.

Troubleshooting a loop

When a loop reads wrong, work through it in order and record what you find. Lock out or follow the site's permit process before opening any panel, and leave work in classified areas or energised equipment to qualified people.

1. Measure the current. A clamp-on milliamp meter reads it without breaking the loop. Compare the current with what the transmitter's display shows and what the controller shows.
2. If the current is right but the controller is wrong, check the controller's scaling, input type and any filtering or clamping in the program.
3. If the current is wrong, check the transmitter's range, its diagnostics and its process connection, then compare against a reference. Our guide to instrument calibration covers this.
4. If the current is zero or stuck, check the supply voltage, fuses, terminals and, with the loop powered, the voltage at the transmitter's terminals.
5. If the reading is noisy, look for shield grounding at both ends, shared conductors and cables run beside drive output wiring.
6. Simulate. A loop calibrator can stand in for the transmitter and drive known currents, which separates field problems from panel problems.

How ehZee can help

We design instrument loops, prepare loop diagrams and I/O schedules, check power budgets and specify isolators and barriers, and we carry out loop checks during commissioning. Design drawings are sealed by a licensed professional engineer where required, and installation is done by licensed electrical contractors. See our instrumentation and controls service, and for the other end of the loop, our guide to PLCs and control panels.

Common questions