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

Skills in the Toolbag
The old adage goes: “When all you have is a hammer, everything looks like a nail.”  For some techs I know, even having a hammer can be challenging. I was pretty new in business, and my first real “employee” hire in the HVAC part of Kalos was my brother, Nathan. Many of you know him, […]
Read more
20° ΔT (Delta T), A Lazy Rule of Thumb
  What should the delta T (ΔT) be? What do you mean!? 20 degrees, of course! We are referring to the air temperature drop across an air handling unit or evaporator coil in cooling mode. Actually, depending on who you ask, the answer will range between 18-22 degrees Fahrenheit. Heck, I’ve heard some insist that […]
Read more
Introduction to SORIT Valves
Photo Courtesy of Parker/Sporlan There are many brands and styles of evaporator pressure regulating valves (EPR), but none as common as the Parker/Sporlan SORIT and ORIT valves. The diagram above clearly shows some of the common applications. An EPR or “hold back” valve maintains a set suction line pressure and, therefore, coil temperature. That is […]
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Your email address will not be published.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