Is transformer always 'source' to which current returns?

[Elquery]

TL;DR Summary

In a typical U.S. residential 120/240 split phase system, is the local transformer always the source to which current (charge?) must return? Or can it bypass and go direct to other PoCo elements/generators?

I am not sure if I should use the word 'current' or 'charge' but my question is whether, in a typical U.S. residential 120/240 split phase system, is the local transformer always the source to which current (charge?) must return? Or can it bypass and go direct to other PoCo elements/generators?

I am arguing that the transformer creates an isolated conductive path, and so even though the power distribution system is earthed in multiple locations, the return current will always return to the transformer and will not bypass said transformer through the Earth (going straight to the PoCo generators, etc.).

 

[anorlunda]

There are single-wire-earth-return power systems in a few places on Earth.
https://en.wikipedia.org/wiki/Single-wire_earth_return
However, they are very rare exceptions.

In most power systems, return of current to the source occurs almost 100% trough the wires, not the Earth. Transformers are passive devices that do not change that. Specific schemes such as 120/240 split phase do not change that.

The earthing connections in most power systems are there for safety reasons. They mitigate risks in abnormal situations. In ideal normal cases, no net current flows to/from Earth in any of those connections.

 

[phinds]

A transformer to which the current does not return is called an open circuit.

 

[Elquery]

Thanks anorlunda.

Let me clarify a bit more the principle I am getting at:
I understand that SWER's exists and that typical distribution systems are not relying on Earth return current. My question is really more about concept and this is an example to get at that.

So even with SWER, could the current from a distribution transformer bypass that SPECIFIC transformer and return to a higher voltage transformer ground connection upstream? Or does the circuit need to be entirely completed at that local transformer?
Hopefully that's making sense.

And If I am correct that it needs to happen at the local transformer, how would one best argue that (scientifically/rationally) in a debate with one who thinks current can bypass to upstream components (all the way to the PoCo generators).

 

[anorlunda]

I don't understand your focus on the transformer. You could ask that question about any component in the circuit, including 1 foot of wire.

Rather than words, let's look at a picture. Pay attention to the ground point in this circuit. The source is shown to be DC, but it could be AC.

Now we apply Kirchoff's Current law to every component in the circuit. No current flows out of the top of the source unless an equal current flows into the bottom. No current flows into the top of the load unless an equal current flows out of the bottom.

• Now, if the current from the load to the ground point equals the current from the ground point to the source, how much current flows into the ground?
• What difference would it make to the currents in this circuit if the ground point did not exist, or if it was moved to another point?
• Suppose there was a second ground point at the bottom of the load; what changes?
• Now suppose we had those two ground points but we remove the wire between them. Now what changes?

I also don't understand what you mean by "bypass" Please draw a schematic of what you have in mind. You can sketch it in paper, take a good quality photo with your phone, and copy/paste it into a post.

 

[Dullard]

Ultimately, any current 'leaving' the transformer must return to it - that's the definition of a circuit.

For your 'second transformer' case, current would only flow 'to' it if it shared a reference (like a ground) with the source transformer, and that would only be so that the current could return to the source transformer via that shared reference.

If you can come up with a good way to persuade people who are simultaneously certain and wrong, I'd love to hear the details.

 

[Elquery]

I don't understand your focus on the transformer. You could ask that question about any component in the circuit, including 1 foot of wire.

Rather than words, let's look at a picture. Pay attention to the ground point in this circuit. The source is shown to be DC, but it could be AC.

View attachment 294147

Now we apply Kirchoff's Current law to every component in the circuit. No current flows out of the top of the source unless an equal current flows into the bottom. No current flows into the top of the load unless an equal current flows out of the bottom.

• Now, if the current from the load to the ground point equals the current from the ground point to the source, how much current flows into the ground?
• What difference would it make to the currents in this circuit if the ground point did not exist, or if it was moved to another point?
• Suppose there was a second ground point at the bottom of the load; what changes?
• Now suppose we had those two ground points but we remove the wire between them. Now what changes?

I also don't understand what you mean by "bypass" Please draw a schematic of what you have in mind. You can sketch it in paper, take a good quality photo with your phone, and copy/paste it into a post.

Thank you, this clears it up.
My focus on the transformer, I suppose, is because it's a real world situation (and the point of contention) and I cannot think of an equivalent scenario with a simple conductive circuit. By scenario I mean where there is a closed loop and then another closed loop adjacent, forming a figure-eight. In a purely conductive circuit it would be a short circuit. So the 'source' (as one uses the word when saying "current must return to source") get's 'moved' to a new location (probably not the best way to phrase that but...)

I see from your response that the fundamental situation need not be a transformer and applying Kirchoff's current law to any node will do the trick.

I wouldn't be-able to draw a schematic of the 'bypass' because it can't happen! :)