When testing a set of N.O contacts with a DMM, what should be the reading when the switch is activated?

In the context of testing normally open (N.O.) contacts with a digital multimeter (DMM), when the switch is activated, the expected reading is 0 voltage. This is because N.O. contacts are designed to remain open until they are activated, at which point they close the circuit.

When you test with the DMM, if the contacts are functioning correctly, you will measure continuity or a closed circuit, resulting in no voltage drop across the contacts. This means that effectively the voltage present could be considered as 0 volts across the DMM leads when the switch is closed, confirming that the circuit is complete and current can flow.

If the switch is operating as intended, any applied voltage upstream of the contacts would be able to pass through when the contacts close, but measuring directly across the contacts in the closed state should ideally show a resistance close to 0 ohms or a voltage of 0. Thus, a reading of 0 voltage indicates that the contacts are indeed closed and functioning correctly.