Questions & Answers > 45S on a Single Phase 3 Wire 120/240V installation

Brad, could you explain the potential coil that looks like it is connected to the 2 bottom middle terminals of the meter.It shows that one potential coil connects to the top second left terminal to the bottom 3rd terminal from left and potential coil connected to top 3rd terminal from left to 3rd bottom terminal from left showing a total of 3 potential coils with the 3rd bottom terminal being common to all three. We use this meter a lot on single phase 120/240 volt service with what i will refer to it as A phase connected to the 2nd top terminal and B phase connected to the top 3rd terminal from left and we ground the 2 middle terminals so they would be common. We consider this to operate as a two stator meter measuring A to neutral and B to neutral and these two phases 180 degrees apart with their own currents working with them. When we test this meter with a powermaster test kit we actually have to test it as a form 4 meter with the description of one potential coil and two currents as powermetrix does not have a protocal written for the way we use it. It does test fine like this but i guess i wonder why powermetrix does not have a test script to test it the way it is working in the field and this is common for us to use this form 45 to measure 3 wire single phase load.But i would like to know what that 3rd potential coil that is connected to the two bottom middle currents is for.


February 16, 2015 | Unregistered CommenterBuck

Hey Buck,
It is interesting that PowerMetrix does not have a test program for the 45S meter. It has been around for several years and it (along with the 5S and 35S) is one of metering’s most versatile meters. The 35S (ABB) and 45S (GE) came into being when auto-ranging meters were first introduced and they are and were designed to be direct replacements for the 5S electromechanical meter.

You are correct to see the 45S as a two element meter and, like I say above, is a direct replacement for a Form 5S meter. From Blondel’s Theorem it requires n-1 meter elements to measure any metered load correctly. When applying the 45S into a single phase, three wire, 120/240 volt service this meter is being correctly applied as a Blondel’s solution. Many power companies trick Blondel’s Theorem by installing a Form 4S meter (a single element meter) for the single phase three wire, 120/240 volt service and for the 4S in this installation and it is seen as acceptable metering (by industry standards).

Back in the days of electromechanical meters it would have been a Form 5S meter that should have been used to meet Blondel’s Theorem in its application into a single phase, three wire, 120/240 volt installation. Again, not many power companies did it this way but this would have been the correct way to do this. Blondel’s says n-1 meter elements, but it also says that you “must also make a common connection to all of the potential coils in the meter to the conductor where there is no current coil” (which in your single phase installation is the neutral conductor). With the 45S meter installed instead of a Form 5S meter you are still doing exactly what Blondel’s requires by placing the neutral in both of the lower center meter terminals. It is fine to do it this way but you really only need to place it at the bottom right center position (looking at the meter from the front) to make the Blondel’s connection to two of the three potential coils in the meter.

The first element, first meter potential coil, that starts at the top center left ends at the bottom center right and it is matched with the current coil in that element (top far left to bottom far left). There is a second potential coil in this element that connects the bottom two center meter terminals and in this single phase application it is not used. In fact, it is not used when the 45S is used to meter three phase three wire DELTA service either. It is only used when the 45S is used to meter a three phase, four wire DELTA installation with two CTs. The second element in the 45S meter has one potential coil (connected from top center right to bottom center right) and this element’s current coil is connected from top far right to bottom far right.

So as you are metering your single phase, three wire, 120/240 volt service with the 45S meter you are applying it correctly. The first phase (phase A) from the service will be metered in the first element with ‘A’ current in the first element current coil and A-N voltage impressed across the first element first potential coil. The second phase (phase B) from the service will be metered in the second element with ‘B’ current in the second element current coil and B-N voltage impressed across the second element potential coil. Additionally (and by reminder), the second potential coil in the first element (connected from bottom left and bottom right center) is not used in this single phase installation.

If you want to hear and see when this first element, second potential coil is used then please check out our YouTube Channel where we have posted the Metergod Monday video from 2/9 where we discussed the 5S, 35S and 45S meters and the differences in them. I discuss them in a three phase, three wire and a three phase four wire applications (this is where the second potential coil is used). I didn’t talk about the 45S in a single phase, three wire, 120/240 volt installation, but the above explanation for the single phase should help you understand that it is not used then.
You can copy and paste this link into your browser to view it:

As a suggestion, if the PowerMetrix has a test program for a 35S meter then you can probably test the 45S meter with that test program for this single phase, three wire, 120/240 volt installation. You will place 120 volts to neutral on the two meter potential coils and test amp current in both current coils. If the PowerMetrix does have a test program for the 35S and you need additional assistance in this process let me know and I will go through this on another Metergod Monday and from there you should be able to do the 45S meter test this way.

Be Safe All Ways,

February 16, 2015 | Unregistered CommenterBrad