Voltage across a set of closed N.O. contacts should be

• A. Zero
• B. The source voltage
• C. Infinite resistance
• D. None of the above

Answer: (D)

When a normally open contact closes, it provides a conducting path with only the contact’s own small resistance. The voltage across that path isn’t forced to be zero in reality; it’s determined by Ohm’s law: V = I × R_contact. Since R_contact is very small, the voltage drop across the closed contact is typically a small, nonzero value, not the full source voltage and not infinite. Therefore none of the given simple statements exactly describe what the voltage across closed contacts should be. In practice you expect a small, nonzero drop that depends on the current and the contact resistance. If you observe a large drop, something in the circuit is causing an unusual condition such as excessive current or dirty/misbehaving contacts.