Switch Voltage

On an energized, intact circuit, you will read voltage across an OPEN switch when testing with a voltmeter. However, you probably won't read significant voltage across a CLOSED switch.

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 the potential difference between two points. It's not simply a measurement of how much “electricity” can be measured at one point. (Current or amperage represents the actual electrons moving through a circuit. It's so easy to learn that I even taught my sons about basic electrical theory.)

So, just remember:

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

Comments

Jose De La Portilla
Jose De La Portilla
2/7/18 at 07:58 AM

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

Alex Buchanan
Alex Buchanan
2/16/18 at 09:10 AM

“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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