Which statement correctly contrasts P&ID diagrams and loop diagrams?

• A. Loop diagrams are the most detailed form of diagrams for a control system, and thus they contain all details omitted by P&IDs; P&IDs show the setup of the control devices
• B. P&IDs are more detailed than loop diagrams
• C. Loop diagrams show only wiring
• D. P&IDs show only mechanical layout

Answer: (A)

Understanding the difference between these diagrams helps you see how control systems are documented at different levels. P&IDs map the process piping and instrumentation, showing where devices are installed and how the process streams flow, including the general setup of control devices and their connections to the process, but they don’t detail the electrical wiring or the complete signal paths inside a control loop. Loop diagrams zoom in on a single control loop and lay out every electrical connection and signal route — from transmitter to controller to final element — including power supplies and terminations. Because of this, loop diagrams are the most detailed for a control system, containing details that P&IDs omit, while P&IDs illustrate the placement and relationships of the control devices within the process. The other statements don’t fit because P&IDs are not just mechanical layouts, and loop diagrams encompass more than wiring, with emphasis on complete control loop interconnections.