Direct contact between conductors of differing systems

Pinnie said:
What is leakage current?
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You know people say that some stuff is a conductor and other stuff is an insulator? That's not really true. Everything is a conductor, but some conductors really suck. We use the worst conductors as insulation.

Then there is also capacitive leakage current which is the flow of current due to charging and discharging the natural capacitor formed by conductors being in close proximity.
 
Leakage current is pretty much any current that flows in an undesired path (leakage) in a normally functioning system. This would include the tiny current through a completely intact insulation system.
 
winnie said:
Leakage current is pretty much any current that flows in an undesired path (leakage) in a normally functioning system. This would include the tiny current through a completely intact insulation system.
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That kind of leakage is generally due to capacitance, assuming the voltage is below the insulation rating. Is still real current though and enough of it will trip GFCI devices.
 
So leak
kwired said:
That kind of leakage is generally due to capacitance, assuming the voltage is below the insulation rating. Is still real current though and enough of it will trip GFCI devices.
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so it sounds like leakage current is a commonly misunderstood concept. I’m assuming it’s intended to mean “current that physically penetrates the insulation of conductors” ( likely unintentionally). Or as someone else put it, insulation conducts electricity, just very poorly.
 
Insulation does actually conduct, and not just capacitively. It may just be nano- or picoamps, but over a long enough distance it can be substantial.
 
CoolWill said:
Insulation does actually conduct, and not just capacitively. It may just be nano- or picoamps, but over a long enough distance it can be substantial.
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And that leakage current would typically be traveling from conductors to metal parts / egc yes?
 
Pinnie said:
And that leakage current would typically be traveling from conductors to metal parts / egc yes?
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Yes and to other conductors. The insulation is like a resistor between the conductors. A really really high value resistor.
 
kwired said:
Everything is a conductor if a high enough voltage is applied ;)
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True, but IMHO slightly different than leakage current.

Every insulator has a tiny amount of _DC_ conductivity. I mean really really tiny. Things like charged ions slowly diffusing through plastic insulator tiny. For some industries this sort of DC leakage is pretty significant; high temperature insulators for process heating systems are often very leaky.

On top of this, real world insulators act as the dielectric in a capacitor, so if you have an AC voltage you will get conductivity caused by the capacitance of the various conductors separated by the insulators.

If you apply a high enough voltage, you will ionize and break down the insulating material. Once this happens you no longer have an insulator, you have a plasma.

For a physics demo the teacher applied DC to a glass rod. No current measured. Then the teacher used a torch to heat up the glass rod until current could flow by sodium ions moving in the near molten glass. Once it got to that minimum temperature the current flow caused the glass to keep heating up until it was glowing dull and soon melted apart.
 
winnie said:
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).
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Which brings up the point regarding the first response, in that if ANY parts of either systems are referenced to ground, that ground connection completes a circuit.
 
What potential would you read line to neutral point in an ungrounded wye (you can have an ungrounded 277/480Y I believe). You would still get near nominal voltages yeah? In my mind the grounding of your transformer would not substantially not effect the available fault current value.
 
I believe you can use a wye transformer to supply an ungrounded system, but that code does not permit you to use the neutral to supply any loads. US electrical systems are often designed with the expectation that the neutral is a grounded conductor near ground potential, and in an ungrounded wye system the neutral could end up at an elevated voltage for an extended period of time.

If you had an ungrounded wye, and measured L-N on the transformer, you would expect normal L-N voltages.

If you have an ungrounded transformer, the fault current between terminals is completely unchanged. However since most faults are between the system and ground, the current that actually flows in most faults is substantially reduced. (The fact that most faults are from the system and ground is the rational behind 'slash rated' breakers.)
 
Pinnie said:
What potential would you read line to neutral point in an ungrounded wye (you can have an ungrounded 277/480Y I believe). You would still get near nominal voltages yeah? In my mind the grounding of your transformer would not substantially not effect the available fault current value.
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You can read any value between two non-referenced points. In your case 277V L-G is most likely in a balanced system, like one with no load, however the real world is not always balanced. It is even possible to read up to 2X L-L during some fault condition.
For fun, hold one 1 voltage probe in your hand and touch the other probe to random surfaces, remember being told how old timers would touch live conductors?

Available fault current has nothing to do with grounding, except for the probability of a L-G fault occurring.
 
Does an ungrounded transformer lack EGC directly to X0? I know you need a ground fault monitoring system in an ungrounded or impedance grounded system. It’s always a little mental gymnastics when talking about grounding, remembering the egc/mbj purpose vs GECs purpose. I have them in two different categories, truthfully, in my head. When I hear ungrounded system, I assume we’re saying system without GEC, not a system without egc/mbj.

FYI I plan on getting Mike holts grounding and bonding book soon.
 
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