Get Tech Tips
Subscribe to free tech tips.
On an energized, intact circuit, you will read voltage across an OPEN switch, not across a CLOSED switch, when testing with a voltmeter.
Both sides of a closed switch are electrically identical (or at least very close). Therefore, there should be no movement of electrons between the leads of your voltmeter.
I have seen many new apprentices get confused when they measure across the points of an energized (closed) contactor or between two energized low voltage circuits, and they measure 0 volts.
Voltage measurement is always a measurement of potential difference between two points, not simply a measurement of how much “electricity” can be measured at one point.
Across a closed switch = 0 volts (or if it does display voltage, it is the voltage drop across the switch)
Across an open switch = applied voltage