In industrial automation, many technologies come and go, but the 4 to 20 mA signal has remained unchanged for decades. From oil & gas plants to water treatment facilities, from power stations to chemical industries, this signal continues to be the backbone of reliable measurement and control.
Many technicians use 4–20 mA signals daily, yet only a few truly understand why this standard is so powerful and how small mistakes can cause wrong readings or plant issues.
This article explains 10 essential things every technician should know about 4 to 20 mA signal.
1. Live Zero Advantage
One of the biggest strengths of the 4 to 20 mA signal is the concept of live zero. In this system, 4 mA represents zero process value, not zero signal. For example, 0 bar pressure or 0 m³/h flow is shown as 4 mA, not 0 mA.
This design allows technicians to easily detect problems. If the signal suddenly drops to 0 mA, it clearly indicates a cable break, power loss, or transmitter failure. This simple feature alone makes troubleshooting faster and safer compared to voltage signals.
You can also read our article: Difference between Live Zero and Dead Zero in 4-20mA Signal
2. Noise Immunity
Industrial environments are full of electrical noise caused by motors, VFDs, contactors, welding machines, and high-voltage cables. Voltage signals such as 0–10 V are highly sensitive to this interference.
The 4 to 20 mA signal works on current, not voltage, which makes it naturally resistant to electrical noise. Even if noise is induced in the cable, the current remains almost unchanged. This ensures stable and reliable measurements, even in harsh industrial conditions.
3. Long Distance Transmission
In large plants, transmitters are often installed hundreds of meters away from control rooms or PLC panels. Voltage signals suffer significant losses over long distances due to cable resistance.
With a 4 to 20 mA loop, the signal can travel hundreds or even thousands of meters without accuracy loss. As long as the loop has sufficient power supply voltage, the measurement remains correct. This is why 4–20 mA is preferred for remote field instruments.
4. Loop Power Concept
In a 4 to 20 mA system, the same loop carries both power and signal. This is called a loop-powered transmitter. The transmitter draws power from the loop and modulates the current according to the process value.
This design reduces wiring complexity, improves safety in hazardous areas, and allows easy integration with PLCs, DCS systems, and indicators. It also enables the use of intrinsic safety barriers in explosive environments.
5. Open Circuit Detection
Another major advantage of the 4 to 20 mA system is open circuit detection. Under normal operation, the signal always stays between 4 and 20 mA.
If a wire breaks or a terminal becomes loose, the current drops below 4 mA. Modern PLCs and control systems immediately recognize this condition as a fault, triggering alarms and diagnostics. This early warning helps prevent wrong decisions based on false data.
6. Ground Loop Risk
Although the 4 to 20 mA signal is robust, improper grounding can still create problems. When a loop is grounded at multiple points, ground loops may form.
Ground loops introduce unwanted currents into the measurement circuit, leading to fluctuating readings, drift, or noise. To avoid this, technicians must ensure proper grounding practices, especially when connecting field instruments to control panels located in different electrical zones.
7. Shielding Rule
Signal cables used for 4 to 20 mA loops are often shielded to protect against electromagnetic interference (EMI). However, shielding must be applied correctly.
The standard rule is simple: ground the shield at one end only, usually at the control panel side. Grounding both ends can create a ground loop through the shield itself, defeating its purpose and introducing noise into the signal.
8. Scaling Accuracy
A 4 to 20 mA signal is only meaningful when correctly scaled. The transmitter may be configured for a range such as 0–100 °C or 0–10 bar, but the PLC or DCS must be scaled to match exactly.
Incorrect scaling results in wrong engineering units, misleading operators and affecting control logic. Always verify transmitter range, PLC input configuration, and displayed values during commissioning and maintenance.
9. Cable Resistance Effect
In voltage systems, cable resistance directly affects signal accuracy. In contrast, one of the biggest advantages of current loops is that cable resistance does not affect the measured current, within the loop’s compliance voltage.
This means technicians do not need extremely thick or expensive cables for accurate measurement. However, total loop resistance must still remain within the power supply capability to avoid saturation issues.
10. Industry Standard Signal
Despite the rise of digital communication protocols like HART, Modbus, and Ethernet-based fieldbuses, the 4–20 mA signal remains the global industry standard.
It is simple, universally supported, easy to troubleshoot, and highly reliable. Most smart transmitters still use 4–20 mA as their primary output, often combined with digital overlays such as HART for diagnostics.
What we learn today?
The popularity of the 4–20 mA signal is not accidental. It is the result of decades of practical field experience, safety considerations, and engineering reliability. For technicians, understanding these 10 key principles is essential for correct installation, commissioning, troubleshooting, and maintenance.
Mastering the basics of 4–20 mA signals not only improves measurement accuracy but also builds confidence in handling real-world industrial challenges.
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