Which logic element describes two or more normally closed switches in parallel?

• A. NAND
• B. NOR
• C. AND
• D. OR

Answer: (A)

Normally closed contacts are closed when their coil is not energized and open when energized. Placing two NC contacts in parallel means the circuit will conduct as long as at least one contact is closed, which happens whenever at least one corresponding coil is not energized. The only time the circuit stops conducting is when both coils are energized and both NC contacts open. If you map coil energized (true) and not energized (false) to logic inputs, the output is true except when all inputs are true, which is the NAND function. So this arrangement describes a NAND gate. The other options don’t fit because they would require the path to conduct under different input conditions (any input true for OR, all inputs true for AND, or the inverse of OR for NOR), which isn’t the behavior of two NC contacts in parallel.