Question 44 3AE02 - Third Assistant Engineer (Alt)

The Correct Answer is B **Why option B ("4 to 20 mA") is correct:** The **4 to 20 mA current loop** is the dominant and universally accepted industry standard for analog signal transmission in instrumentation and control systems (such as PLCs, DCS, and SCADA). This range offers several key advantages for industrial applications: 1. **Noise Immunity:** Current loops are inherently more resistant to electrical noise and voltage drops over long distances compared to voltage signals. 2. **Standard Range:** The 4 to 20 mA range represents the full scale of the measured process variable (e.g., 4 mA often represents 0% of the variable, and 20 mA represents 100%). 3. **Live Zero (Zero Suppression):** The use of 4 mA instead of 0 mA provides a "live zero." This means that if the current drops below 4 mA (typically to 0 mA), the system can immediately detect a fault, such as a broken wire, sensor failure, or loss of power to the transmitter. If the standard were 0 to 20 mA, a 0 mA reading could represent either 0% measurement or a system fault, leading to ambiguity. **Why each of the other options is incorrect:** * **A) 0 to 10 mA:** While this is a current range, it is not the primary industry standard for control systems. Crucially, it lacks the "live zero" (4 mA minimum) feature necessary for reliable fault detection in critical industrial environments. * **C) 1 to 10 A:** This current range is far too high for standard signal transmission. Industrial signal loops are low-power (milliampere range) to minimize power consumption and reduce heat. Ampere-range currents are used for motor control or heavy-duty power distribution, not for instrumentation signals. * **D) 4 to 20 A:** Similar to option C, this current range is extremely high. Using amperes instead of milliamperes would require substantially larger components, cables, and power supplies, and would be highly inefficient and dangerous for low-power signal transmission purposes.