Switch voltage

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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) and 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

— Bryan


  1. Avatar Jose De La Portilla says:

    This is why an alligator clip on the. Black lead of your meter is one of the best investments you can make.

  2. Avatar Alex Buchanan says:

    “Across an open switch = applied voltage”

    my interpretation of this situation is that it should read as follows

    “Across an open switch = potential voltage” as the unit is in the off state and thus not applying (using) any of the electrical energy.

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