watthour meter and 2-wire circuit

Would a watthour meter (of the typical design used by utilities to meter service) function correctly if it is connected with just a 2-wire circuit?
Assume the load is a pure 2-wire load only (e.g. 240 volts), but the meter was designed for the North American Edison style split 120/240 volt system.
An example of this might be a watthour meter between the utility (source) transformer and the customer (load) transformer, where the load transformer has a 240 volt winding. Could the neutral wire from the source transformer (which would not be connected to the load transformer at all) be omitted and the wattmeter still function? If not, would grounding its neutral terminals get functionality?
I'm not asking about safety issues here, just watthour meter functionality. Assume 180 degree phase relationships from true 120/240 service, as opposed to 120 degree phase relationships from 2 phases of 3 phase 208Y/120 service. But then the 208 volt scenario would be the next question.
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| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
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The standard meter used to measure energy usage on a 3 wire, 120/240V supply in the US has no connection to the neutral. It only connects to the two phase wires, so yes it will work on a two wire system also.
Charles Perry P.E.
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wrote:
| The standard meter used to measure energy usage on a 3 wire, 120/240V supply | in the US has no connection to the neutral. It only connects to the two | phase wires, so yes it will work on a two wire system also.
Thanks. I was hoping no one would say "why not go out and disconnect the neutral yourself and see".
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| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
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Charles Perry wrote:

And if you plug the meter into the meter base backwards it runs backwards. Howinthehell do they do that?
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It works just fine. It is really measuring watts in an analog way so it can compensate for voltage or unbalance between phases.
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| It works just fine. It is really measuring watts in an analog way so it can | compensate for voltage or unbalance between phases.
I just wanted to make sure the circuitry in it wasn't doing things based on voltages relative to the neutral, etc. As was pointed out, it does not even connect to the neutral in the 120/240 volt variety. I was not aware that there are different versions, and different sockets, for the 120/240 versions and the 120/208 versions. I had assumed it was just one and figured a neutral would probably be needed to make it measure right if the phases were varied. And apparently that is true for the 120/208 version (and presumably it could measure correctly on 120/240 as well). But a basic 120/240 version is presumably cheaper to make (no neutral connection, and just one set of circuitry) and should be accurate as long as the voltage is balanced since it is compare the current to opposite phase voltage rather than neutral.
So, if I were to have a transformer with a 240 volt primary and a 120/240 volt secondary, I could, in theory, have the service drop come in with no neutral at all from the utility transformer, and it would measure correctly. Safety would be a different issue.
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| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
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250.24(B) says you will bring the grounded conductor to the service disconnect enclosure. The only ungrounded services are going to be 3p delta and they are very rare, usually needing special permission. We used to keep the neutral out of computer room panels, using only line to line loads, most 3 phase but the neutral was still in the service disconnnect at the "star" grounding point.
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|>I could, in theory, have the service drop come in with no |>neutral at all from the utility transformer, and it would measure correctly. |>Safety would be a different issue. |> | | 250.24(B) says you will bring the grounded conductor to the service disconnect | enclosure.
However, 250.24(B) is not a statement of theory. And I believe it is quite possible that the NEC and most engineers have it wrong. I am only saying possible. I have made no conclusions, yet. But what I do know is that a hazard I am studying is one where there is controversy and vested interests in the status quo.
What I will be doing is exploring all possible solutions and then trying to rule each out as to whether it is practical and/or poses new hazards that would make a worse tradeoff. The question I posted was to know if the watthour meter could impose an impracticality to what might (or might not) be a simple solution. That a watthour meter won't be an issue lets me explore this further to see if it is a practical solution. But it may not be for other reasons. At least I can focus on them.
| The only ungrounded services are going to be 3p delta and they are very rare, | usually needing special permission. | We used to keep the neutral out of computer room panels, using only line to | line loads, most 3 phase but the neutral was still in the service disconnnect | at the "star" grounding point.
The fact that many loads can be connected 2-wire (or 3-wire three phase) shows that there is no fundamental need for one of the current carrying conductors to be a groundED conductor at zero potential relative to the ground bond point. Some loads do need the groundED conductor for legacy reasons (for example the design of typical light bulb sockets makes it especially easy to touch one of the conductors, so we make that be the groundED conductor for safety reasons). But aside from legacy reasons, I think we do not need an actual groundED conductor; what we need are all conductors having system voltages relative to the ground plane, as a delta utilized wye transformer would be (the star point being bonded to an earth ground electrode or the whole grounding system, but not fed as a current carrying conductor).
BTW, the reason I don't just toss the broad issue here (yet) is that I do not believe it will be thoroughly studied that way. It is way too common for people here (and in other areas as well ... it being human nature) to just post the conclusions they have found or learned. In many cases people don't know why the "best" answer they have is the best, but that it is simply what they lerned or what everyone does. But that is not what I want for this issue; I want a thorough exploration of alternatives that might even "fly in the face" of what we (think we) know.
If I conclude that current practice is in fact the best, then there won't be any need for me to post it anyway. If I conclude something else, or just can't reach a conclusion, then I probably will post it, eventually.
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| Phil Howard KA9WGN | http://linuxhomepage.com/ http://ham.org/ |
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wrote:

Indeed. Many marine and naval installations use completely ungrounded systems. The generators are wye-wound, but the neutral is not brought out or connected to any ground or frame. Step-down voltage transformers are delta-delta and the low-voltage 120V circuits are also ungrounded.
Such a system can be more reliable from the standpoint of a single ground fault causes no interruption in service.
daestrom