Why 4-20 mA Current Signal is Preferred Over Voltage Signal in Instrumentation?

Industrial automation instrumentation mainly employs electrical signals for transmitting process data from sensors and transmitters to control systems. Signals are often classified into two types: voltage signals and current signals. The industrial standard for current signals is 4-20 mA. 

This article explores the reasons behind this selection, the limitations of voltage signals, and the positive aspects of employing current signals, specifically 4-20 mA, in industrial applications.

Voltage Signal Transmission: Limitations and Challenges

Voltage signals are easy to monitor and produce, thus theoretically they could be a useful option for data transmission. Voltage signals do, however, have many practical limits, particularly when used for long-distance transmission.

Voltage Drop and Signal Attenuation

• When voltage signals are transmitted through wires over long distances, they are subject to voltage drop due to the resistance in the wires. 
• This can cause the voltage at the receiving end to be significantly lower than at the sending end, leading to inaccurate measurements of the process variable.
• For instance, consider a transmitter that outputs a 0-10V signal to a control system located several hundred meters away. 
• The resistance in the wire causes part of that voltage to drop, and by the time it reaches the control system, the signal may have degraded, leading to errors in process control or monitoring.

Susceptibility to Electromagnetic Interference (EMI)

• Additionally, voltage signals are more vulnerable to electrostatic and electromagnetic interference (EMI and ESI). 
• Large motors, transformers, and other powerful machinery produce a lot of electrical noise in industrial settings, which makes it easy for interference to pick up and further distort voltage signals, decreasing their accuracy.

Measurement Errors Due to Impedance

• The impedance of the cable and the devices that are connected to it might have an effect on voltage signals, which is another problem with these signals. 
• Alterations in impedance over time, which can be brought on by aging equipment, corrosion, or fluctuations in the environment, can have an impact on the voltage, which can result in drift in the accuracy of measurements.

Click here for How to Safely Check the mA Current of an Instrument Loop Using a Multimeter

Current Signal Transmission: Superior Accuracy and Reliability

• The accuracy, reliability, and noise immunity of current signals are significantly higher than those of voltage signals. 
• In particular, the 4-20 mA standard that is used for current signals offers substantial advantages.
• Let’s explore the reasons behind the widespread adoption of the 4-20 mA current loop in the industry in general.

Click here for 4 to 20 mA Transmitter Output Process Value Calculator

No Signal Loss Over Long Distances

• One of the primary reasons for choosing current signals over voltage is that current does not drop over long distances. 
• Current signal transmission is reliable over long distances, as current does not drop due to wire resistance. 
• In industrial settings, field instruments may be located hundreds or even up to 1,000 meters away from the control room. 
• Despite this distance, the 4-20 mA current signal remains constant, ensuring accurate process data transmission without degradation. 
• This capability makes it ideal for large facilities and plants, where signal integrity is crucial to maintain consistent and reliable process control.
• This consistent current flow is critical in industrial environments where control rooms are often located far from the field instruments, allowing for more flexible plant design without sacrificing signal integrity.

Noise Immunity

• Current signals have inherently higher immunity to electromagnetic interference (EMI) and electrostatic interference (ESI) compared to voltage signals. 
• This is because current loops operate with low impedance, making it difficult for external electrical noise to affect the signal. 
• In contrast, voltage signals, which have higher impedance, are more prone to noise pickup.
• This enhanced noise immunity is especially important in industrial settings where high-voltage equipment and electromagnetic fields are common. 
• A noisy signal can lead to poor process control, erratic behavior, and even system failures, all of which can be minimized with a 4-20 mA current loop.

Linear Response

• The linearity of the 4-20 mA signal also makes it ideal for industrial applications. 
• The current signal maintains a linear relationship between the process variable and the signal being transmitted. 
• This makes it easier to calibrate and more predictable when configuring control systems such as PLC (Programmable Logic Controllers) and DCS (Distributed Control Systems).
• For example, in a temperature measurement system using a 4-20 mA transmitter, 4 mA could correspond to 0°C, while 20 mA corresponds to 100°C. 
• The range in between can be easily divided and calculated linearly, providing an accurate representation of the process variable.

Click here for How to simulate 4-20ma signal with Loop Calibrator ?

Why 4-20 mA Instead of 0-20 mA?

While it is possible to use a 0-20 mA signal for transmission, the 4-20 mA standard has become the norm for several reasons:

Detection of Wire Breaks

• One of the most significant advantages of using a 4-20 mA signal is its ability to detect open circuits or wire breaks. 
• In a 4-20 mA loop, a signal of 0 mA indicates a problem, such as a disconnected wire or a power failure, which immediately alerts operators or the control system. 
• On the other hand, in a 0-20 mA system, a 0 mA signal could either indicate the lower range of the process variable or a failure, leading to potential confusion and delayed response times.

Low-Power Operation

• Many transmitters in industrial settings are loop-powered, meaning they receive power and transmit signals using the same pair of wires. This is typically done with a 24V power supply. 
• The 4-20 mA signal allows for low-power operation while maintaining the accuracy and reliability of the measurement.

Compatibility with PLCs and DCS Systems

• In most control systems, especially those using PLCs or DCS, the 4-20 mA signal is easily converted to a voltage signal for analog-to-digital conversion. 
• This is usually done using a 250-ohm resistor in the circuit, which converts the current into a 1-5V signal (Ohm’s Law: V = IR). 
• The resulting voltage signal is fed into the control system’s analog input module for processing.

Refer the below link for the How a PLC reads the data from field transmitters?

Enhanced Safety in Hazardous Areas

• Another critical advantage of the 4-20 mA signal is its safety in hazardous areas. 
• Because the current in the loop remains below 30 mA, it poses a very low risk of creating sparks or igniting hazardous substances such as gases, vapors, or dust. 
• This low-power operation ensures that personnel are safe from electrical shocks, and equipment operates without the risk of dangerous overheating or sparking. 
• This makes the 4-20 mA loop compliant with safety standards in high-risk environments such as oil refineries, chemical plants, and mining operations.

Why 4-20 mA is the Industry Standard?

The 4-20 mA current signal is preferred in industrial automation and process control for several reasons:

• Minimal signal loss over long distances.
• Noise immunity in electromagnetically noisy environments.
• Easy detection of failures like open circuits or wire breaks.
• Linear response, simplifying calibration and control.
• Enhanced safety in hazardous areas due to low power levels.
• Compatibility with modern control systems, making integration straightforward.

The 4-20 mA current loop has shown to be a reliable and effective way for transmitting process data in industrial settings, even though voltage signals may still have some niche uses. It makes sure that the signals sent by field instruments reach control systems accurately and without distortion or noise, which is essential for maintaining industrial processes’ productivity, safety, and efficiency.

Why is the current signal preferred over the voltage signal?

• Current signals are preferred over voltage signals in industrial applications primarily because current does not drop over long distances, while voltage does. 
• Current remains constant as it flows through the loop, ensuring accurate signal transmission over long cable runs. 
• Additionally, current signals offer better noise immunity, making them less susceptible to electromagnetic interference (EMI) and electrostatic interference (ESI) commonly found in industrial environments. 
• This ensures more reliable and accurate measurements, essential for process control.

FAQ on 4-20 mA current signal

Why is the 4-20 mA current signal mostly used?

• The 4-20 mA current signal is widely used because it offers several benefits over other signal types. 
• It ensures that any current below 4 mA (e.g., 0 mA) indicates a problem such as a disconnected wire or system failure, which helps with troubleshooting. 
• It also allows for simple and accurate analog-to-digital conversion using a resistor to create a 1-5V signal, making it compatible with most control systems. 
• Additionally, the 4-20 mA range provides a linear output, which simplifies calibration and measurement of process variables.

Why is 4-20 mA preferred over 0-10 V signal?

The 4-20 mA signal is preferred over the 0-10 V signal for several reasons:

• No signal loss: Voltage signals can drop over long cable runs due to wire resistance, leading to inaccurate readings. Current signals, on the other hand, do not suffer from this issue.
• Noise immunity: Current loops are less affected by electrical noise, which is a significant concern in industrial environments with high levels of EMI from motors and other heavy machinery.
• Fault detection: A 0 mA signal in a 4-20 mA loop immediately signals a failure, such as a broken wire, while 0 V in a 0-10 V system could be either a valid measurement or a failure, leading to ambiguity.

Why is 4-20 mA preferred over 0-20 mA current in industrial automation?

• The 4-20 mA signal is preferred over 0-20 mA because it allows for easy detection of open-circuit conditions. 
• In a 0-20 mA system, a 0 mA reading could represent either a valid process variable or a failure, making troubleshooting difficult. 
• With 4-20 mA, a reading of 4 mA indicates the lowest possible process measurement, while 0 mA clearly signals an issue such as a wire break or power failure. This improves system reliability and makes fault detection straightforward.

Click here for How to do troubleshooting of a 4-20mA loop?

Why is only the 4-20 mA range considered in industry measurement?

• The 4-20 mA range is chosen in industry because it provides a built-in diagnostic feature. 
• The 4 mA baseline ensures that a signal below 4 mA (including 0 mA) can be used to detect issues like broken wires or instrument failure. 
• Additionally, 4-20 mA loops offer a linear relationship between the process variable and the signal, which simplifies both calibration and interpretation. 
• The 20 mA upper limit is a practical range for transmitting signals while keeping power consumption low in loop-powered devices.

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Why a 0 mA signal is not practical?

• Because it cannot distinguish between an actual zero-point measurement and a system failure like a broken wire or power outage a 0 mA signal is not useful. 
• On the other hand, in a 4-20 mA system, a measurement of 0 mA clearly signals an issue, and 4 mA is the lowest process value. 
• This distinction ensures that problems are promptly identified and appropriately addressed, which is crucial for reliable fault detection and maintenance.