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 not only makes wiring easier but also results in a cleaner and more organized circuit design.
However, a drawback of this approach is that a failure, such as a disconnection or poor connection in the common line, can affect all circuits sharing it, creating a single point of failure. For this reason, in critical applications, isolated circuits (each with its own independent return path) are often preferred to enhance system reliability.
In DC circuits, particularly in PLC and sensor wiring, two configurations are common: "positive common" and "negative common." In a negative common system (often used with NPN transistors), the shared common line is connected to the negative terminal (0V) of the power supply. Conversely, in a positive common system (often used with PNP transistors), the common line is connected to the positive terminal. The choice between them depends on the application, regional standards, and safety considerations for how the circuit should behave in a fault condition.
In common wiring, terminal blocks are mainly used to connect.
In some cases, PLCs and other devices have internal, commonly wired circuits and common terminals. It is crucial to distinguish "common" from "ground" (or "earth"). While a circuit's common line is sometimes connected to ground, they are fundamentally different concepts. "Ground" typically refers to a safety connection to the earth or the chassis for protection, whereas "common" serves as the circuit's current return path and reference point. Relatedly, the term is also found in "common-mode noise," which describes noise currents that flow in the same direction on both the positive and negative power lines.
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