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Potential Voltage/Applied Voltage and Troubleshooting

I’ve often said that as human beings, we often make the mistake of making things more difficult than they are. One example of this is the simplified idea of troubleshooting an HVAC electrical circuit from the perspective of reading a wiring diagram and applying the concept of what I like to refer to as Potential Voltage and Applied Voltage.

To illustrate, consider the diagram in Figure One below that shows the circuitry of an electric furnace.

Figure One


On this diagram, you can easilily locate test points related to Applied Voltage and Potential Voltage. For example, isolating one of the heating elements, we can see that there is an N.O. (Normally Open) switch shown wired in series with it, and the identifiers on this particular switch are M1 and M2 (M, meaning Main set of contacts on a sequencer …SEQ #1 in this case…..), and in a sitution in which a technician would want to find out if this circuit is working like it should, a smple test for Applied Voltage would be a good place to start.

Checking with a voltmeter directly at the wiring connections for the heating element to find out if the required 240-volts was being applied or not would allow us to begin to evaluate this particular circuit in the event we were troubleshooting a situation in which the customer was complaining that the unit wasn’t heating properly. And, depending on the results of that test, we would be able to find out if the element itself, or the switches wired in series with it, could be a source of a problem.

For the sake of creating a troubleshooting scenario, let’s say that the answer to the question, “Is there voltage applied to the element?” is no. The reading we get with the meter is 0-volts, not the 240-volts that would allow the element to provide heat as long as it OK, meaning that it would have the proper resistance if we checked it with an ohmmeter.

But….I digress. We’re not talking about using an ohmmeter to check resistance, we’re talking about doing a “hot” test with a voltmeter to determine which component in the circuit could be a source of the problem, so, back to the idea of Applied Voltage and Potential Voltage….

Once our first test showed no Applied Voltage, our next step would be to check the switches wired in series with the element ot see if they were doing what they were supposed to do, which introduces the idea of checking for Potential Voltage.

Moving to the left of the element, there is a fuse. Checking directly at the terminals of this fuse would be implmenting a check for Potential Voltage because of a simple rule regarding a switch (which, is technically what a fuse is….just a switch that only opens once). And that rule is, “Voltage can be read across an open switch”.

And, in the event that we read 240-volts at the fuse terminals, it would prove six things, all with one check of the meter:

1. The fuse is open.

2. The heating element is not open.

3. The limit switch to the right of the element is not open.

4. The M1 and M2 terminals of the sequencer are closed.

5. The fuse shown at #1 on the main terminal block on the L1 side is not open.

6. The fuse shown at #1 on the main terminal block on the L2 side is not open.

The above can be proven as true when you consider tracing the Potential Voltage circuit we’ve set up by checking the fuse.

If you trace from the left terminal connection of the fuse, you’ll go all the way back to the L1 side of the line. If you trace from the right terminal connection of the fuse, you go all the way back to the L2 side of the line, proving along the way that the element, the limit switch, and the fuse at #1 on the L2 terminal block must be in the condition I mentioned above, because if any of those  conditions didn’t exist, we wouldn’t be able to get the Potential Voltage reading at our test points.

Like I said….no reason to make things more difficult than they really are.

Learn from yesterday…..Live for today……Look forward to tomorrow