What Makes a 0 mA Signal Impractical?
Why a 0 mA Signal is Impractical in Process Instrumentation
Long the industry standard for providing analog data in process control, the 4-20 mA signal The reliability, accuracy over great distances, and simplicity of fault identification of this range appeal. Still, why not use a 0–20 mA signal instead? Three main factors define a 0 mA signal as impractical:
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Circuit Design Complexity and Cost A 0 mA signal indicates essentially an open circuit that is, no current is flowing. Electronics designed to precisely detect and operate with 0 mA add complexity and expense. Even at 0% measuring, a 4-20 mA signal always shows measurable current, therefore simplifying design and enhancing signal integrity.
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Fault Detection is Impossible It can be challenging in a 0-20 mA loop to determine if a 0 mA reading denotes a true 0% measurement or a system failure (such as damaged wire, power outage). By contrast, a 4 mA live-zero baseline enables rapid fault identification; any dip below 4 mA indicates a malfunction.
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No Power for Loop Devices Transmitters and displays running two-wire (loop-powered) draw power from the signal loop. A 0 mA signal generates no power, hence devices turn off. The 4 mA baseline guarantees that gadgets get the lowest current required to run constantly.
Clearly the 4-20 mA signal is the better option over 0-20 mA in industrial applications since it not only provides greater fault detection and device dependability but also safer and more practical for long-distance transmission.