In electrical circuits, "Common" (often abbreviated as COM) refers to a single point or conductor that serves as a shared return path for current from multiple circuits or components.
Using a common connection simplifies wiring layouts by reducing the total number of terminals and cables required. This optimizes space and results in a cleaner, more organized system design.
However, a potential drawback is that the common line becomes a single point of failure. A disconnection or loose contact in the common line can affect all connected circuits. For this reason, critical applications may prefer isolated circuits--where each has an independent return path--to enhance system reliability.
In DC control circuits, particularly for PLCs and sensors, two configurations are standard: "positive common" and "negative common." In a negative common system (typically used with NPN logic), the shared line connects to the negative terminal (0V) of the power supply. Conversely, in a positive common system (typically used with PNP logic), the shared line connects to the positive terminal. selection depends on the application, regional industrial standards, and safety requirements.
Common connections are frequently consolidated using terminal blocks. It is crucial to distinguish "Common" from "Ground" (Earth). While a circuit's common line may be connected to ground in some configurations, they represent different concepts. "Ground" typically refers to a safety connection to the earth or chassis for protection, whereas "Common" functions as the circuit's reference point and current return path. Additionally, the term appears in "common-mode noise," which describes noise currents flowing in the same direction on both power lines relative to ground.
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