Which statement correctly describes the relationship between loop diagrams and P&IDs?

• A. P&IDs are more detailed than loop diagrams
• B. Documentation uses only one type of diagram at a time
• C. Loop diagrams are the most detailed diagrams for a control system and contain all details omitted by P&IDs
• D. Loop diagrams show only wiring

Answer: (C)

Loop diagrams carry the most detail for a control system because they lay out the wiring and signal paths for each control loop. They show exactly how sensors, controllers, and actuators are wired, including terminal strips, junctions, power supplies, cable routes, and loop-specific identifiers and data such as tag numbers and setpoints. This level of wiring and instrumentation detail is what you need for installation, testing, and troubleshooting.

P&IDs, by contrast, map the process side: piping, valves, instrument locations within the process, and the relationships between equipment and the overall control strategy. They focus on how the process flows and how instruments interact within the process, but they deliberately omit the granular electrical connections and wiring details that loop diagrams provide.

Thus, loop diagrams are the most detailed diagrams for a control system and include details that P&IDs do not show.