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#6202 - 12/26/01 03:22 PM capacitive coupling  
Cindy  Offline
Member
Joined: Jun 2001
Posts: 196
PDX, OR, US
Is this something that can be described in 24 words or less?
Don't want to burden anyone with having to write a book like the last question I asked, much appreciated though.
C


Work Gear for Electricians and the Trades

#6203 - 12/26/01 04:50 PM Re: capacitive coupling  
sparky  Offline
Member
Joined: Oct 2000
Posts: 5,311
My 2 ct's;

Capactive Coupling is the reactance of EMF to proximal items.

PS......
( while back when i thought i was a nuetral, Scott explained that i was indeed more of a capactitor..)


#6204 - 12/26/01 06:42 PM Re: capacitive coupling  
Tom  Offline
Member
Joined: Jan 2001
Posts: 1,044
Shinnston, WV USA
2 conductors separated by an insulator make a capacitor. Capacitors will pass AC. Therefore, one live wire can cause a voltage in a de-energized conductor.

23 words, one number & one acronym. Close enough?

Tom

[This message has been edited by Tom (edited 12-26-2001).]


Few things are harder to put up with than the annoyance of a good example.

#6205 - 12/26/01 07:03 PM Re: capacitive coupling  
pauluk  Offline
Member
Joined: Aug 2001
Posts: 7,520
Norfolk, England
I'm sure the folks here don't mind giving longer explanations, just so long as you don't mind reading them. [Linked Image]

The actual capacitance between two conductors (measured in farads, or more commonly a smaller division such as the microfarad) will depend upon:

(a) the area of those conductors,
(b) the distance between them, &
(c) the type of insulation material between them (known as a dielectric).

Once you have calculated or measured capacitance value, you can determine the reactance (& thus the current that will flow) from:

X = 1 / (2 x pi x f x C)

where f=frequency in use & C=capacitance in farads). X will be in ohms.


#6206 - 12/26/01 07:06 PM Re: capacitive coupling  
Scott35  Offline

Broom Pusher and
Member
Joined: Oct 2000
Posts: 2,707
Anaheim, CA. USA
A really quick and dirty description would be:

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 S.E.T.


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

#6207 - 12/26/01 07:59 PM Re: capacitive coupling  
George Corron  Offline
Member
Joined: May 2001
Posts: 717
Lorton, Va USA
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. [Linked Image]


#6208 - 12/26/01 08:14 PM Re: capacitive coupling  
The Watt Doctor  Offline
Member
Joined: Dec 2001
Posts: 435
Mont Belvieu, TX
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.


The Watt Doctor
Altura Cogen
Channelview, TX

#6209 - 12/26/01 09:49 PM Re: capacitive coupling  
Bill Addiss  Offline
Member
Joined: Oct 2000
Posts: 3,878
NY, USA
George,

"Great Caesars Ghost" sounds like the right one. They even had an episode where someone dressed up like Caesar to come to Perry in a dream for some reason.

Bill


#6210 - 12/26/01 11:13 PM Re: capacitive coupling  
George Corron  Offline
Member
Joined: May 2001
Posts: 717
Lorton, Va USA
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.


#6211 - 12/26/01 11:55 PM Re: capacitive coupling  
Cindy  Offline
Member
Joined: Jun 2001
Posts: 196
PDX, OR, US
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?

my brain hurts, gotta stop for now -C-


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