The 4-20 mA current loop is often considered to be more effective at signal transmission than the 0-20 mA current loop.
Here are the main reasons:
Detection of Wire Breaks & Faults
4-20 mA Loop: The 4 mA “live zero” signal distinguishes between a zero signal and a fault state. If the current falls below 4 mA, it signifies a problem with the loop, helping in fault detection.
0-20 mA Loop: A zero signal (0 mA) can indicate either that the process variable is zero or that there is a defect in the loop, which makes fault detection more difficult.
Signal Integrity & Noise Immunity
4-20 mA Loop: The 4-20 mA loop provides improved noise immunity as the signal is constantly greater than 4 mA. Any noise (or) interference is far less probable to make the current to drop to zero, hence preserving signal integrity.
0-20 mA Loop : The signal may be closer to 0 mA, but it may be more vulnerable to noise and interference, resulting in less reliable signal transmission.
Power Efficiency
4-20 mA Loop: With a 4 mA baseline, the loop can supply two-wire transmitters continuously. This is especially beneficial in distant or hazardous areas where many power sources are impracticable.
0-20 mA Loop: The loop must be configured to handle circumstances where the current is zero, which could necessitate additional power management measures for connected devices.
Standards and Compatibility
4-20 mA Loop: The 4-20 mA current loop is a widely used industrial standard that ensures interoperability with a variety of devices and systems. This uniformity facilitates integration and maintenance.
0-20 mA Loop: Less frequent, and may cause compatibility concerns with normal equipment designed for 4-20 mA operation.
Signal Resolution
4-20 mA Loop: Creates a higher signal resolution across the same current range because it does not transmit signals at 4 mA or below. This improves measurement precision and dependability.
0-20 mA Loop: Signal transmission occurs across the whole range, but the existence of the zero-current condition can impair effectiveness.