Switch voltage

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

—Bryan

Related Tech Tips

Compressors and Pumps
Let's go with dictionary definitions of each one first. pump noun 1. A mechanical device using suction or pressure to raise or move liquids, compress gases, or force air into inflatable objects such as tires. compressor noun 1. an instrument or device for compressing something. Both compressors and pumps move fluids, and a fluid: Yields or […]
Read more
EPR valves 101
I don't do much in the way of “rack” refrigeration. However, I recently had a conversation with experienced rack refrigeration tech Jeremy Smith, and he got me thinking about EPR valves. I've heard EPR (evaporator pressure regulator) valves called suction regulators or hold back valves. In essence, they hold back against the suction line to […]
Read more
Start Capacitor & Inrush, Facts & Myths - Part #4
Now is the part where we get specific about start capacitors and inrush. If you haven't read the first three parts, please do so before reading this one, or it may not make sense. (You can find Part #1, Part #2, and Part #3 at these links.) I'm going to come out and say it […]
Read more

2 responses to “Switch voltage”

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

  2. “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.

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.

loading

To continue you need to agree to our terms.

The HVAC School site, podcast and daily tech tips
Made possible by Generous support from