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#126902 - 03/05/01 03:09 PM Skin effect
sparky Offline
Member

Registered: 10/18/00
Posts: 5545
"skin effect" as i understand it, is related to HZ. a while ago, i came across a link from a link from a Telsa link, where a rather bold individual actually had a pix of himself discharging a bolt of lightning from his pointed finger like Zeus. He claimed that he was unaffected , due to this phenomenon. He also had a do-not-try-this-at-home disclaimer, not that i was at all inspired to.


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#126903 - 03/09/01 05:14 PM Re: Skin effect
Tom Offline
Member

Registered: 01/01/01
Posts: 1069
Loc: Shinnston, WV USA
In out lawsuit happy society, it is always best to have a disclaimer.

Skin effect is also related to current density. To prove this, doubling the wire size (circular mills) does not double the ampacity (check the tables in the NEC). When really high current density conditions are anticipated, the busbars may be specified as hollow copper tubes (sounds suspiciously like water pipe, wonder if there is a dual use listing?).

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

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#126904 - 03/09/01 07:52 PM Re: Skin effect
Bill Addiss Offline
Member

Registered: 10/07/00
Posts: 4196
Loc: NY, USA
Tom,

Whoa! That would be a Great Patent to have! Self Cooling Busbars/Hot Water Heaters!

To add to your comments, for this reason stranded conductors can carry a higher current per AWG because of more surface area than solid conductors.

Sparky,

If you can find that Link I'd like to see it. Here, I'll trade you one:

(remember that disclaimer)

http://ducttapeguys.com/

Bill

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#126905 - 03/09/01 11:32 PM Re: Skin effect
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
Skin Effect kind of throws a Monkey Wrench into the commonly known way that current flows through a conductor, along with the way the charges get pumped in.

Real, or Ohmic Resistance is the resistance offered by the conductor to the passage of electricity [the basis of Voltage Drops]. Although the Specific resistance is the same for AC or DC, the Total resistance of a wire is greater for AC than DC.
This is due to the fact that there are induced EMFs [Voltages] in a conductor in which there is an alternating flux.

These EMFs are greater at the center than at the circumference [edges], so that the Potential Difference tends to establish currents that oppose the current at the center and assist it at the circumference.

The current is thus "Forced" to the outside of the conductor, thus reducing the effective area of the conductor.

Normal charge "Pumping" of the conductor is to begin the input charge flows from the center of the conductor, then pump the conductor full of charge [electrons] from there until it's at X capacity. The majority charge [the actual current that will be carrying the energy to the load] flows in the outermost areas of the conductor's surfaces, while the power supply continues to pump charges into the center of the conductor.
AC tends to create a Reluctance to the way the charge gets pumped into the conductor by introducing the mentioned EMFs in the center of the conductor.

Kind of interesting, isn't it??

You think I wrote it wierd, take a look at the way it's written in my Electrical Engineering books!!
Looks like outer space stuff

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

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#126906 - 03/10/01 04:17 AM Re: Skin effect
sparky Offline
Member

Registered: 10/18/00
Posts: 5545
Scott & Tom;
yes interesting, how do harmonics and or hertz play a part? if they indeed do play a part.

Bill, that's a fun link....
my favorite is that duct-taped duck!



[This message has been edited by sparky (edited 03-10-2001).]

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#126907 - 03/10/01 04:51 AM Re: Skin effect
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
Steve,

Well, since the Skin Effect will change with frequency, Harmonics will do their part to contribute.
Harmonics are basically reflected AC at frequencies above the fundamental Hz.
Example: Hz fundamental = 60 Hz, 3rd harmonic = 180 Hz.
This is kind of why the transmission frequency and generated frequency for interstate power grids stay at 60 Hz, rather than increasing the frequency. 25 Hz is another option, but this would make trouble with electric motors and discharge lamps [plus you would really be able to see an Incandescent lamp flicker at 25 Hz!!].

Aircraft and others that don't generate high power [actually use large conductors] can benefit from increasing the fundamental frequency to a higher level, such as 400 Hz.

Smaller conductors at power frequencies generally aren't affected by the skin effect, however, large conductors are affected. The size affected usually begins around 500 Kcmil [500 MCM].

The above instances kind of explain why you won't see very large conductors carrying Radio Frequencies!! The Feeders from a High Power TV Transmitting Amplifier to the Broadcast Antenna would most likely be an array of paralleled circular conductors.

Tubes have less skin effect resistance than flat conductors of the same cross section.

Aluminum conductors are affected less by skin effect than Copper conductors of similar cross sectional size, because of the greater Resistance of Aluminum.

All this talk about skin effect is making me want to grab the books and re-read those areas!! I'll add some comments later this weekend as they comeup.

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

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#126908 - 04/19/01 02:58 PM Re: Skin effect
Anonymous
Unregistered


Quote:
Originally posted by Tom:
doubling the wire size (circular mills) does not double the ampacity (check the tables in the NEC).


This is true. But skin effect is hardly the reason or even a reason. Doubling a solid conductor's circular mils (cross-sectional area) increases its circumference by only 41%. Circumference has a lot to do with dissipating heat. Heat dissipation ability has a whole lot to do with ampacity.

As Scott35 said, skin-effect doesn't come into play at 60 Hz until the conductor is really thick.

For copper the skin depth at 60 Hz is about 0.854 cm (centimeter) which is a wire diameter of 1.71 cm - which is about two-thirds of an inch.

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#126909 - 05/05/01 09:11 PM Re: Skin effect
Steve T Offline
Member

Registered: 02/14/01
Posts: 312
Loc: Oak Park, IL, USA
I have seen a diagram showing a wire going thru a conduit with the conduit bonded on both sides of the wire(in essence a parallel conductor) A 100a load was on the circuit and 97a was traveling on the conduit and only 3 on the wire.

With this is mind--a simple residential service, rigid nipple thru the wall between the panel and meter socket, since both the service panel and meter socket are bonded to the neutral conductor, will a majority of the current be traveling on the conduit nipple?

Would a bond bushing on each side of the nipple with a conductor connected to the neutral from the bushing increase the skin effect by creating a lower resistance path to the neutral in the panel and metersocket?

I make contractors remove one bond bushing when I see this arrangement, quoting objectionable current (from 250-2? don't have my book in front of me). Am I off base?

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#126910 - 05/06/01 08:50 AM Re: Skin effect
golf junkie Offline
Member

Registered: 04/22/01
Posts: 511
Loc: York, NE
since both the service panel and meter socket are bonded to the neutral conductor, will a majority of the current be traveling on the conduit nipple?

Probably yes, depending on locknuts being tight with a good connection at both enclosures. The power will obey ohm's law, path of least resistance and all that.

Would a bond bushing on each side of the nipple with a conductor connected to the neutral from the bushing increase the skin effect by creating a lower resistance path to the neutral in the panel and metersocket?

I don't think so. As stated in the thread above, skin effect is non-existent at 60hz with the possible exception of large conductors.

I make contractors remove one bond bushing when I see this arrangement, quoting objectionable current (from 250-2? don't have my book in front of me). Am I off base?

Removing the bonding bushing doesn't change the electrical circuit. The enclosures are still bonded through the locknuts and sharing current with the neutral and grounding conductor. The purpose of the bushing is to ensure continuity in the case a concentric K.O. should pull out or a locknut comes loose.

If you really want to remove the nipple from the circuit the only way to do it is use PVC. So yes, I think that you are off base.

later,
GJ

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#126911 - 05/06/01 10:56 AM Re: Skin effect
gpowellpec Offline
Member

Registered: 12/06/00
Posts: 128
Loc: Irving, Tx USA

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