How do normally open and normally closed contacts contribute to safety interlocks?

Prepare for the NCCER Electrical Level 2 Control Systems and Fundamental Concepts exam. Utilize flashcards and multiple choice questions with hints and explanations. Ensure success on your exam!

The main idea here is how NO and NC contacts work together in a safety interlock to ensure a guarded, safe state before operation. A normally closed (NC) contact is closed when the interlock coil is not energized, so if power is lost or the guard opens, that contact opens and the safety circuit is interrupted, forcing a stop. A normally open (NO) contact is open until the interlock is energized, at which point it closes and completes the path needed to allow operation. Using both types lets the system clearly indicate a safe state: the NC contact provides a fail-safe break if the guard is not in place or power is lost, and the NO contact only completes the control circuit when the guard is properly closed and the interlock is energized. Together they enforce safe sequencing, so the machine can start or continue only when all safety conditions are met, and will stop if any condition is not satisfied.