In ladder diagrams, the rails represent what in a relay logic circuit?

• A. Ground connections
• B. Signal cables
• C. Input terminals
• D. The active and zero volt connections of the power supply

Answer: (D)

The rails in a ladder diagram show the power path for the logic. They are the two conductors that provide voltage to the ladder—one is the active (hot) supply and the other is the zero-volt return. Each rung sits between these rails, and current can flow from the active rail, through the devices on the rung (like contacts and coils), and back to the zero-volt rail. When a complete path exists, a coil energizes.

So the rails aren’t ground or signal cables, and they aren’t input terminals. They represent the power supply and return that power the relay logic.