What would happen in two completely isolated systems, say generators for simplicity, if one hot leg from one system makes direct contact with the hot leg from the other system. With no direct path back to source, would there be any current flow?
Direct contact between conductors of differing systems
- Thread starter Pinnie
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wwhitney
Senior Member
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- Berkeley, CA
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No. If one system is floating relative to another, the first connection simply establishes the voltage relationships between the two systems. No current would flow through that connection, since as you point out, there would be no circuit. It takes a connection between two points with a well-defined voltage relationship to make current flow. Which a second different connection between those two systems would now be, curtesy of that first connection.
Cheers, Wayne
Cheers, Wayne
Can you elaborate on this?
In a single system, when you bond two things, their potential attempts to equalize. Whether that be for the purpose of equipotential bonding (pools) or when you have a normal circuit and load etc.
With two separate isolated systems, I can’t see the motivation for two points from each system to seek equipotential.
With two separate isolated systems, I can’t see the motivation for two points from each system to seek equipotential.
Two wires connected to the same point will be equal in potential because they are electrically continuous. What that potential will be depends on where you call "ground" for the sake of measurement.
wwhitney
Senior Member
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- Berkeley, CA
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A voltage difference between two points occurs when there is something pushing charge between the two points (from one to the other in the case of DC, or back and forth in the case of AC). If you have point A connected to system 1 which is isolated from system 2 which contains point B, there is nothing pushing charge between A and B. u
If you now connect A and B, we no longer have two points, but if we take two other points, point C in system 1 and point D in system 2, there is still nothing pushing charge between C and D. System 1 is pushing charge around system 1, system 2 is pushing charge around system 2, but nothing is pushing charge between them.
Now if we make a second connection between C and D, there are two possibilities. One is that C is at the same voltage as A, and D is at the same voltage as B. Then still there is nothing pushing charge between C and D. But if either C's voltage relative to A is non-zero (which voltage is entirely within System 1), or D's voltage relative to B is non-zero (which voltage is entirely within System 2), then we have something to push charge between C and D, and we see a current in the connection between C and D. As you pointed out, it takes two connections to make a circuit to allow current to flow.
Here "system" is anything with a source, like a battery, a utility transformer, a PV cell with sun shining on it, etc.
Cheers, Wayne
- Location
- Illinois
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- retired electrician
I saw a couple of low voltage systems where there were only 3 wires available and both AC and DC was needed. They tied one leg of both systems to a common conductor so they could get both AC and DC at the other end using only 3 conductors.
A little thing I like to call "creative wiring".
winnie
Senior Member
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If you think about it, what is described in the first post is exactly what we do when we 'ground' a system. The 'neutral' is just one of the system terminals, and its voltage is not defined until we connect it to 'everything else' by grounding that terminal. You could equally ground any of the terminals (though some won't be permitted by code; the physics would allow it).
kwired
Electron manager
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That first connection is often via a grounded conductor from each system. So unless both systems are identical voltage systems that are "in phase" with one another and it is the "same phase conductor" making contact, which is what they do when they intentionally parallel sources with each other, there is likely to be a voltage between the two conductors mentioned in OP. Amount of current that flows will depend on what that voltage is and how much impedance is in the circuit path. The closer to being in phase they are the lesser the voltage will be between them.
LarryFine
Master Electrician Electric Contractor Richmond VA
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It's also why multiple utility transformers can have their neutrals tied together by a single system neutral.
So voltage is completely limited to what you’re measuring it to then. If I take one meter lead to a 120v main lug and the other to a cookie, I’ll read 0 volts because they have no continuity at all. If I go X1 on one system, then to X0 on another isolated system I would get 0 volts again yeah?
wwhitney
Senior Member
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- Berkeley, CA
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- Retired
It would have to be completely isolated, so at most one system could be a grounded system (X0 connected to earth). If both systems are grounded, then that common connection to earth is already the first connection between the systems, so you will be able to measure voltage.
Cheers, Wayne
Right. Good clarification.
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- Wisconsin
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In the real world you will often have some amount of coupling capacitance so a 'phantom' voltage might be noticed. This is the principle behind some non-contact voltmeters.
winnie
Senior Member
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- Springfield, MA, USA
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- Electric motor research
As @jim dungar notes, no system is perfectly isolated; there is always leakage and capacitive coupling. So lets add an additional caveat: the impedance of your meter is low compared to any such leakage.
What will happen is that if you put one lead of your low impedance meter on one terminal a system, and the other lead on a terminal of a different system, you will get very close to 0V. A very small current will flow through the meter, caused by the leakage, but a small current through a low impedance means a very low reading.