All circuits exibit various levels of Capacitance, which is external from the circuit conductors. In order for larger currents to flow, the external Capacitive Fields must be charged to a [somewhat] constant level. This "Line Charging" is between any and all points of different Potential.
Line charging "Couples" insulated and isolated circuit conductors to each other, via the Capacitive Effects between conductors and points of different Potentials.
Simply, between any and all circuit conductors, there's a Capacitive Charge that must be created and held during current transfers, or prior to current transfers - like a circuit opened with a toggle switch, or a closed circuit with no connected loads [Receptacle branch circuits, for example].
For conductors inside of a conduit, the Capacitive circuits are between all conductors, and all conductors and the conduit. Also there's a Capacitive Charge between the conduit and the Earth ground.
For Utility power systems, there's a Capacitive Charge between the circuit conductors, and between each conductor and the Earth Ground.
Between Utility sides and "User Sides" [Primary / Secondary], the Capacitive Coupling occurs primarily across the Transformer's windings, but also occurs between the Coupled effects on the Primary to Earth Ground, and the Secondary to Earth Ground circuits.
These are when Ungrounded AC systems' voltage to ground become really excessive.
Roughly, the Capacitive Coupling effect is present whenever there's current flowing, or available to flow on circuits, and there's any difference in Potential - either externally or internally to the conductors.
Kind of trick stuff, isn't it???
A working example to use is: Line Charging [Capacitive Coupling] is the reason GFCIs aren't set to trip when the slightest inbalance of current is sensed. If the GFCI was set to trip at as low as 1 milliamp, circuits with lengths of like 100 feet [for example] would trip the GFCI, and the circuit being protected would be in proper working condition [AKA no one is being shocked].
Sorry to be so technical in the explanations.
It's just great that you are here asking these questions!!! It would be nice if I can figure an easy way to explain this stuff, so it's not quite as overwhelming.
Good luck with your studies!
Scott " 35 " Thompson Just Say NO To Green Eggs And Ham!
Geeez Scott, great answer (Great Scott, geeez what an answer, with deference to Jimmy Olson/Perry White for us geezers).
Cindy it means you can have a voltage present in an uncharged conductor because of it's near proximity to an charged conductor. For all practical purposes, there's nearly no difference between capacitive and inductive coupling. Also because of another thread I've been in lately, it does not happen in DC conductors, only ones with a frequency.
There have been real problems with control circuitry run with load conductors in conduits because when you push the stop button, it won't because of coupling. that's the normal danger. It also becomes apparent when you run low voltage conductors in the same manhole, etc., with medium (high)voltage conductors, it has killed more than one.
OK, coulda been "Great Caesars Ghost" on the TV show.
Great answer Scott. I would also like to jump in with Mr. Corron and add that it is very important in medium and high voltage applications that the discharge the conductors when working on equipment. The conductors will hold a charge.
Not that I remember of course, not having been born yet.....ahem. They were trying to make Perry think he was crazy, his statue of Caesar also talked. Sorry to change subjects on ya, Cindy, but you know were subject to 'wing' off into a tangent at any moment.
before i go too far here, when you use a megger to test a large service [~1200A]that has recently been deenergized, you use an aligator clip and probe tool, or something similar, to discharge the bus bar so you dont get zapped. the charge that builds up on the bus, is it correct to say that was from a capacitive coupling with the other conductors?
from pauls reactance formula, i can see how higher frequency and capacitance numbers will make lower capacitive reactance values. since its in ohms like resistance, then lower capacitive reactance is better, right? but it takes higher capacitance to get the lower reactance, or resistance if i can say that, which is a good thing. so higher capacitance is not all bad?
capacitive coupling across primary and secondary? how much of an affect does this have in farads or ohms for normal installations sized per code, any percentages? also how much coupling is normal between primary and secondary grounds?