Electrical path and separately derived sources

J

jmaughan

Member
Location
Tuscaloosa, Alabama
Occupation
Industrial Electrician
How’s it going, just a quick introduction, I’ve been in the trade for 4 years, and I’m very interested in the theory of it all.

Recently i’ve been having conversations about the path of electricity and separately derived systems with my coworkers and supervisors.

From what I can figure through my research and trying to understand how different electrical equipment/concepts work, such as isolation transformers or corner grounded delta systems, Electricity wants to take a path to the source with an inverse relationship to resistance. The source in this instance being the other end of the coil of wires in the secondary side of the transformer. Along with the concept that two separately derived sources, a transformers primary and secondary sides, do not care about returning to each other.

My coworkers, supervisors, and even some engineers disagree with me, but none of them can explain to me how the systems i mentioned work in their idea.

Any help to make me understand that i’m wrong, or reassure me that im not crazy would be appreciated.
 
You are correct. The current returns to its source by all paths. That could also be via a bonded neutral bar of another SDS. All would be tied to a common GES so all could be part of the return path. Normal circumstances, not enough to worry about but YMMV. There is another thread going now that has about 23 amps flowing on the GEC. It’s hard to tell from here if that is high, or normal for that building.
 
ptonsparky said:
You are correct. The current returns to its source by all paths. That could also be via a bonded neutral bar of another SDS. All would be tied to a common GES so all could be part of the return path. Normal circumstances, not enough to worry about but YMMV. There is another thread going now that has about 23 amps flowing on the GEC. It’s hard to tell from here if that is high, or normal for that building.
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Right and following along those lines, if there is a fault on the secondary side of an ungrounded transformer, an isolation transformer, there would be potential on the ground path of the primary side, but no current would flow as there isn’t a complete circuit?
 
jmaughan said:
Right and following along those lines, if there is a fault on the secondary side of an ungrounded transformer, an isolation transformer, there would be potential on the ground path of the primary side, but no current would flow as there isn’t a complete circuit?
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What exactly do you mean by 'potential on the ground path of the primary side'? I think this point is something that is confusing you.

1) 'Potential' is another word for voltage, and is always defined between two points. You can't talk about the potential of a single point, with one important exception: if you define a reference zero point for your measurements, you can assign a potential value to every other point. But even there the potential is taken between two points; its just that one of those points is your zero reference.

2) If you have a conductor, and a potential/voltage between two points on that conductor (say the two ends), then current will flow. If you somehow create this voltage without a closed circuit, then the current flow will act to eliminate the potential, so you can't have a sustained potential on a conductor without some sort of closed circuit with a source for that potential.

3) If you have a fault on the secondary side of an ungrounded isolation transformer, then you won't get any change in the potentials of the primary side, except as referenced to points on the secondary side.

-Jonathan
 
winnie said:
What exactly do you mean by 'potential on the ground path of the primary side'? I think this point is something that is confusing you.

1) 'Potential' is another word for voltage, and is always defined between two points. You can't talk about the potential of a single point, with one important exception: if you define a reference zero point for your measurements, you can assign a potential value to every other point. But even there the potential is taken between two points; its just that one of those points is your zero reference.

2) If you have a conductor, and a potential/voltage between two points on that conductor (say the two ends), then current will flow. If you somehow create this voltage without a closed circuit, then the current flow will act to eliminate the potential, so you can't have a sustained potential on a conductor without some sort of closed circuit with a source for that potential.

3) If you have a fault on the secondary side of an ungrounded isolation transformer, then you won't get any change in the potentials of the primary side, except as referenced to points on the secondary side.

-Jonathan
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By “potential on the ground path of the primary side” I mean a difference of potential compared to the other end of the secondary’s coil. I apologize for not being clear on that.

And when I said current wouldn’t flow, I meant it wouldn’t flow towards the primary’s source, because it only flows if you close the circuit.

I believe we agree on this, but correct me if i’m wrong. My problem now is there are electricians and engineers around me who say this is wrong and for instance, a line from one SDS can have voltage compared to a neutral on a separate SDS because they are both “the source.” Do you know of a good resource i could use to explain this to them?
 
ptonsparky said:
The secondaries, center point, corner ground, etc of each SDS is bonded to a common GES. Consequently you will have a voltage difference from L1, L2, L3 of each SDS to any of the grounded conductors no matter where you decide to choose a reference point.
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Yes, this is 100% true. However we are talking about an unbounded system, such as hospitals isolated systems.
 
jmaughan said:
By “potential on the ground path of the primary side” I mean a difference of potential compared to the other end of the secondary’s coil. I apologize for not being clear on that.

And when I said current wouldn’t flow, I meant it wouldn’t flow towards the primary’s source, because it only flows if you close the circuit.

I believe we agree on this, but correct me if i’m wrong. My problem now is there are electricians and engineers around me who say this is wrong and for instance, a line from one SDS can have voltage compared to a neutral on a separate SDS because they are both “the source.” Do you know of a good resource i could use to explain this to them?
Click to expand...

I think we are both on the same page.

A line on one system _will_ have a measurable voltage to the neutral on the other system, but only because of extremely high impedance parasitic coupling. The think about this parasitic circuit is that any low impedance connection will make it go away.

A useful reference are the various discussions on this forum about the voltage measured in an ungrounded system; for example:

Line to ground readings on ungrounded delta secondary

Ungrounded delta secondary. 480V three phase. Electrician took readings on factory equipment..... and on line side of main breaker. Utility company also got the same exact readings on secondary of transformer. 480V to ground on phase A 480V to ground on phase C 0 V to ground on phase B. We...
forums.mikeholt.com

Secondary of transformer wye system center tap.

In a 480 volt wye system if your center tap was floating and not connected to ground would it carry voltage or potential if so how much and why?
forums.mikeholt.com

With a typical 480V ungrounded system, all of the phase conductors will measure about 277V to ground with a high impedance meter, and 0V to ground on all phases with a low impedance meter. But these numbers are poorly defined, in the sense that any sort of imbalance or wiring fault can change them quite a bit without any change in the function of the system. The same 480V ungrounded system will measure 480V, 480V, 0V phase to ground if one of the phases is faulted, and could actually measure > 480V to ground in some fault conditions.

-Jonathan
 

Understanding the Neutral Conductor

Dennis, here's a really basic lesson, and we'll start with direct current, because the fact that we use alternating current is not relevant to the basics: Picture a battery, like you'd find two of in a flashlight. You have a terminal at each end, with a difference in potential of 1.5 volts...
forums.mikeholt.com
 
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