What is the purpose of an interlock in ladder logic?

• A. To prevent an output from energizing until another has energized
• B. To latch an output
• C. To reset a counter
• D. To invert a signal

Answer: (A)

Interlocks in ladder logic enforce a safe, controlled sequence between outputs. The idea is that one output can only energize after another one has already energized, preventing conflicting or unsafe states. In practice, you condition the path to energize the first output with a contact that represents the second output’s status—usually a normally open contact of the second output. When the second output isn’t on yet, the contact is open and the first coil cannot energize. Once the second output is energized, the contact closes and the first output can energize (assuming all other conditions are met). This sequencing is a common way to ensure machines operate in a safe order and to prevent two functions from stepping on each other. It’s not about latching an output, resetting a counter, or inverting a signal—the interlock’s role is to gate one output behind the state of another.