SDS Grounding/Bonding.. Ground Loop?

[texie]

Out of curiosity, what is the purpose of the transformer?

 

[LarryFine]

Please see post #11 and let me know what you think...

Does the orange path I've highlighted not effectively connect the two neutrals together?

Yes, it indirectly does. And, while I agree with you and have asked about it here, somehow, grounding the SDS to the same Earth does not disqualify it from being an SDS. Actually extending the primary neutral and connecting the secondary to it does, I think.

However, and this is, I think, what makes the difference, any accidental fault or over-current on the secondary side would not cause additional current in the primary neutral, because the secondary is as far as that current would attempt to flow to operate OCPDs.

 

[Jerramundi]

Out of curiosity, what is the purpose of the transformer?

Thinking about running 240V for the long run for VD purposes. The graphic is misleading. It's actually 300+ ft between the Main Panel and the Transformer and have some sensitive electronic equipment on the other end. I've read the isolation helps with harmonic issues and filtering out voltage spikes... but I still have to more learn about harmonics. But I don't wanna get into that debate again.

 

[winnie]

I suppose. I'm still tempted to argue that said multiple points to Earth create a connection between the two neutrals... but perhaps I'm thinking about it wrong. It sounds like my plan is good. I'm just having hard time NOT overthinking it.

Hey. I do that all the time.

There _is_ a connection between the two neutrals. No problem with that.

But only a _single_ point on each neutral is connected to external stuff.

Follow any current path from source through 'hot' then the load then 'neutral' back to the source. If in following that loop current can leave the neutral, flow through an egc or soil, and return to the neutral at a different location, you have a problem.

But if there is only one connection from neutral to earth you are fine, even if that connection also goes to another neutral.

 

[Fred B]

If my understanding is correct, to be a sds, no connection other than hot legs go back to panel, to isolate that neutral. New trans on load side needs bonding ground and neutral to clear faults on new panel. I could be wrong.

 

[Jerramundi]

If in following that loop current can leave the neutral, flow through an egc or soil, and return to the neutral at a different location, you have a problem.

Your comment SEEMED reassuring but then I read the above... isn't "the problem" you've identified here almost exactly what I've outlined in my diagram? (Albiet my concerned connection is between the metal components/bonding and not the soil).

Per your concern, which seems applicable to my diagram, the current could leave the secondary circuit via fault, flow through the secondary ground rod, through the soil, to the primary side ground rod and thus to the primary neutral?

I'm wondering if I've misread the section on SDS's and the secondary side should just be bonded to the primary side EGC, with no GEC or GE on the secondary side other than the link to the primary side grounding system via the EGC.... but that seems nonsensical because per that logic, the only grounding system in an entire city's grid would be that of the POCO.

 

[jim dungar]

Per your concern, which seems applicable to my diagram, the current could leave the secondary circuit via fault, flow through the secondary ground rod, through the soil, to the primary side ground rod and thus to the primary neutral?

No it cannot. Current flows back to its source, it is trying to flow in a loop.

Yes, a secondary fault current could flow through dirt, up the ground rod and to the primary neutral. But at that point it would flow through the metal paths eventually returning to its source, the secondary neutral, through the bonding jumper. You show this very clearly in your orange path where the current never flows on the primary neutral.

You are trying to solve problems that don't really exist.

 

[winnie]

Per your concern, which seems applicable to my diagram, the current could leave the secondary circuit via fault, flow through the secondary ground rod, through the soil, to the primary side ground rod and thus to the primary neutral?

But it doesn't. Current is always trying to get back to its source. This is one of those key concepts that you should repeat until you get it:

Current is trying to get back to its source. It follows all paths back to its source.

If you have a secondary 'hot' fault, current will flow through the bonded metal trying to get back to the transformer neutral. Other then very small amounts of capacitive coupling, it isn't trying to get anywhere else.

-Jon

 

[LarryFine]

Per your concern, which seems applicable to my diagram, the current could leave the secondary circuit via fault, flow through the secondary ground rod, through the soil, to the primary side ground rod and thus to the primary neutral?

Why would any secondary current want to flow to the primary? Its source(s) would be the secondary windings.

The soil is considered not conductive enough to provide a current-carrying pathway, which is also why we cannot rely on it for fault-clearing or as a neutral conductor.

 

[don_resqcapt19]

If my understanding is correct, to be a sds, no connection other than hot legs go back to panel, to isolate that neutral. New trans on load side needs bonding ground and neutral to clear faults on new panel. I could be wrong.

The grounding conductors tie the two systems together and that is provided for in the definition of a separately derived system.

Separately Derived System. An electrical source, other than a service, having no direct connection(s) to circuit conductors of
any other electrical source other than those established by grounding and bonding connections.

 

[Jerramundi]

But it doesn't. Current is always trying to get back to its source. This is one of those key concepts that you should repeat until you get it:

Current is trying to get back to its source. It follows all paths back to its source.

If you have a secondary 'hot' fault, current will flow through the bonded metal trying to get back to the transformer neutral. Other then very small amounts of capacitive coupling, it isn't trying to get anywhere else.

-Jon

For a second there I thought you were busting out some Welcome Back Kotter on me, lol.

 

[Jerramundi]

No it cannot. Current flows back to its source, it is trying to flow in a loop.

I am aware. I guess I just struggle in seeing the secondary side of this particular type of SDS as 100% separately derived even though conventional wisdom dictates that it is.

Yes, a secondary fault current could flow through dirt, up the ground rod and to the primary neutral. But at that point it would flow through the metal paths eventually returning to its source, the secondary neutral, through the bonding jumper...

Just for the sake of intellectual argument, and I'm not trying to be difficult, just curious... I can't see the current going all the back through the primary EGC to the secondary neutral. Seems to me like that path would be of much greater resistance than just going up the primary ground rod to the primary neutral.

You are trying to solve problems that don't really exist.

Just trying to be thorough brother.

 

[Jerramundi]

Why would any secondary current want to flow to the primary? Its source(s) would be the secondary windings.

The soil is considered not conductive enough to provide a current-carrying pathway, which is also why we cannot rely on it for fault-clearing or as a neutral conductor.

I suppose that's true. That the fault on the secondary would much sooner go through the SBJ to the secondary neutral than it would the secondary GEC, GE, the soil, the primary GE, the primary GEC, and finally the primary neutral.

I'm just trying to be thorough because there's so much concern raised about load side connections of neutral to case, etc.

I don't typically work with SDS's aside from standard single family residential installations like landscape lighting... so yea, just being careful.

 

[Jerramundi]

The grounding conductors tie the two systems together and that is provided for in the definition of a separately derived system.

Excellent quote. That helps ease my anxiety a bit.

 

[LarryFine]

Your wanting to understand and asking questions are admirable.

 

[Jerramundi]

Your wanting to understand and asking questions are admirable.

I've worked with too many a-holes that put on this cloak of perfectionism and refuse to ever admit that they might not know something. I understand where it comes from because as a society we've birthed and fostered the expectation of perfection (which is ultimately an illusion), but it just prevents you from developing any sense of humility and learning from your peers. I used to be afraid to ask questions, not anymore.

You've been warned Forum. This cat is curious,... just forget the later half part of that proverb, lol.

 

[Fred B]

The grounding conductors tie the two systems together and that is provided for in the definition of a separately derived system.

Thanks for the clarification, I think my confusion was with the original drawing showing the neutral carrying past the transformer not ending at secondary winding. I got stuck on the neutral, But i guess it would just change purpose at that transformer bonding with the grounding system and tieing back to the original panel. Is that right?

 

[LarryFine]

I was interested in wiring and electronics by the age of six. In the first grade, I built a foxhole radio after reading about it in a book from the school library.

Dick and Jane? Oh, please! I was capable of reading anything by then, as well as doing math, so my teacher let me play with radios as long as I was quiet.

So, a lot of my understanding electrical application comes from my understanding electrical theory. The rest is the hardware, which I learned as a helper.

 

[winnie]

I am aware. I guess I just struggle in seeing the secondary side of this particular type of SDS as 100% separately derived even though conventional wisdom dictates that it is.

What would happen to your concern if there were no primary? In other words if you had a generator that you needed to ground rather than the secondary of a transformer? A generator can also be an SDS.

Now consider what happens if the prime mover of this generator were an electric motor? Still an SDS.

The transformer simply dispenses with the rotating bits. The primary is not electrically connected to the secondary, power is carried by a magnetic field. Electrons get moved by the magnetic field and the current must return to the secondary.

Jon

P.S. for pedantic purposes we are saying 100%, and for code purposes it is 100%, but the reality is that it is 99.99999%; there is some capacitive coupling and insulation leakage so some very small amount current actually will flow via the primary ground rod.

 

[Fred B]

What would happen to your concern if there were no primary? In other words if you had a generator that you needed to ground rather than the secondary of a transformer? A generator can also be an SDS.

Now consider what happens if the prime mover of this generator were an electric motor? Still an SDS.

The transformer simply dispenses with the rotating bits. The primary is not electrically connected to the secondary, power is carried by a magnetic field. Electrons get moved by the magnetic field and the current must return to the secondary.

Wow never thought of it that way, thanks. But would you carry 4 wire back to first panel as implied in the original image posted by jerramundi or just the bonding ground?