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| Why do we ground the metalic sheath of the cable??
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| TimPerry 2007-02-21, 8:25 pm |
| Ali wrote:
> Why do we ground the metalic sheath of the cable??
so we do not get zapped.
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| Long Ranger 2007-02-22, 3:25 am |
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"Ali" <mohdalibaig@gmail.com> wrote in message
news:1172076745.798239.239670@t69g2000cwt.googlegroups.com...
> Why do we ground the metalic sheath of the cable??
>
Why is a pig's XXX pork, all the way 'round the tail?
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| On Feb 22, 5:34 am, "TimPerry" <timpe...@noaspamadelphia.net> wrote:
> Ali wrote:
>
> so we do not get zapped.
you mean that insulation is not enough... isn't there any connection
with induced voltage phenomena??
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| contrex 2007-02-22, 5:25 pm |
| On 22 Feb, 14:20, "Ali" <mohdalib...@gmail.com> wrote:
> you mean that insulation is not enough...
No. Insulation can break down.
> isn't there any connection
> with induced voltage phenomena??
No. It's earthed, remember?
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| TimPerry 2007-02-23, 3:25 am |
|
"Ali" <mohdalibaig@gmail.com> wrote in message
news:1172153999.985590.253230@p10g2000cwp.googlegroups.com...
> On Feb 22, 5:34 am, "TimPerry" <timpe...@noaspamadelphia.net> wrote:
>
> you mean that insulation is not enough...
if a piece of metal you are touching happen to get hit by lightning wouldn't
you wish it to be grounded?
isn't there any connection
> with induced voltage phenomena??
>
is your question about AC or CATV or other? could you elaborate on what you
would like to know?
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| MassiveProng 2007-02-23, 3:25 am |
| On 22 Feb 2007 06:20:00 -0800, "Ali" <mohdalibaig@gmail.com> Gave us:
>On Feb 22, 5:34 am, "TimPerry" <timpe...@noaspamadelphia.net> wrote:
>
>you mean that insulation is not enough... isn't there any connection
>with induced voltage phenomena??
The connection between it and the EMP pulse of a nuclear
explosion...
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| MassiveProng 2007-02-23, 3:25 am |
| On Fri, 23 Feb 2007 00:17:21 -0500, "TimPerry"
<timperry@noaspamadelphia.net> Gave us:
>
>"Ali" <mohdalibaig@gmail.com> wrote in message
>news:1172153999.985590.253230@p10g2000cwp.googlegroups.com...
>
>if a piece of metal you are touching happen to get hit by lightning wouldn't
>you wish it to be grounded?
>
>
> isn't there any connection
>
>is your question about AC or CATV or other? could you elaborate on what you
>would like to know?
>
All bucket trucks have drag chains!
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| On Feb 23, 10:17 am, "TimPerry" <timpe...@noaspamadelphia.net> wrote:
> "Ali" <mohdalib...@gmail.com> wrote in message
>
> news:1172153999.985590.253230@p10g2000cwp.googlegroups.com...
>
>
>
>
> if a piece of metal you are touching happen to get hit by lightning wouldn't
> you wish it to be grounded?
>
> isn't there any connection
>
>
> is your question about AC or CATV or other? could you elaborate on what you
> would like to know?
Basically I am interested in the grounding of metallic sheath of high
voltage underground cables. Alternating flux will create some voltage
on the metallic sheath. If lightening is the only thing to be
considered then it is better to ground the armoring part first.
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| Beachcomber 2007-02-23, 1:25 pm |
| On Thu, 22 Feb 2007 05:37:40 GMT, "Long Ranger"
<worpylorp@mindspring.com> wrote:
[color=darkred]
>
>"Ali" <mohdalibaig@gmail.com> wrote in message
>news:1172076745.798239.239670@t69g2000cwt.googlegroups.com...
A long power cable with a metalic sheath may be considered to have the
electrical characteristics of a capacitor. Capacitors pass
alternating currents. If left ungrounded, the voltage of a
conductive metal shealth will be at some level above ground, thus
posing a hazard to workers or pedestrians that may come in contact
with it.
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| Salmon Egg 2007-02-23, 1:25 pm |
| On 2/23/07 9:47 AM, in article 45df2643.8209843@news.verizon.net,
"Beachcomber" <invalid@notreal.none> wrote:
> On Thu, 22 Feb 2007 05:37:40 GMT, "Long Ranger"
> <worpylorp@mindspring.com> wrote:
>
>
> A long power cable with a metalic sheath may be considered to have the
> electrical characteristics of a capacitor. Capacitors pass
> alternating currents. If left ungrounded, the voltage of a
> conductive metal shealth will be at some level above ground, thus
> posing a hazard to workers or pedestrians that may come in contact
> with it.
While there is capacitance between the conductors and the sheath, that will
not cause a change in the ac potential on the sheath. If properly installed,
the average potential of the hot conductors and the neutrals or returns will
be zero. A single hot conductor will induce voltage onto the sheath via
capacitance. But that will be bad practice anyway, because the sheath will
act like a shorted secondary turn in a transformer.
Bill
-- Fermez le Bush--about two years to go.
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| So the fuse or breaker trips when the cable is damaged.
Fred
"TimPerry" <timperry@noaspamadelphia.net> wrote in message
news:pv-dnSiViuaxeEHYnZ2dnUVZ_veinZ2d@adelphia.com...
> Ali wrote:
>
>
> so we do not get zapped.
>
>
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| Paul Hovnanian P.E. 2007-02-23, 8:25 pm |
| Ali wrote:
>
> On Feb 22, 5:34 am, "TimPerry" <timpe...@noaspamadelphia.net> wrote:
>
> you mean that insulation is not enough... isn't there any connection
> with induced voltage phenomena??
Think of it as a big capacitor. If the shield isn't grounded, the phase
voltage will charge it to a lethal voltage.
--
Paul Hovnanian mailto:Paul@Hovnanian.com
------------------------------------------------------------------
"Yee-Ha!" is not an adequate foreign policy.
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| Paul Hovnanian P.E. 2007-02-23, 8:25 pm |
| Ali wrote:
>
> On Feb 22, 5:34 am, "TimPerry" <timpe...@noaspamadelphia.net> wrote:
>
> you mean that insulation is not enough... isn't there any connection
> with induced voltage phenomena??
Other than capacitice coupling, the shield helps provide a uniform
potential gradient across the cable's insulation. If the shield is
allowed to float w.r.t. ground potential, then the phase to shield
voltage may rise* beyond the system phase to neutral voltage, placing
additional stress on the insulation and reducing its life.
*By 'rise', I mean that the shield will obtain an a.c. voltage with a
phasor of opposite direction to that of the center conductor, the sum of
which will have a greater magnitude than the system phase to neutral
phasor.
--
Paul Hovnanian mailto:Paul@Hovnanian.com
------------------------------------------------------------------
Quidquid latine dictum sit, altum viditur.
(Whatever is said in Latin sounds profound.)
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| MassiveProng 2007-02-23, 9:26 pm |
| On Fri, 23 Feb 2007 18:14:12 GMT, Salmon Egg <salmonegg@sbcglobal.net>
Gave us:
> While there is capacitance between the conductors and the sheath, that will
>not cause a change in the ac potential on the sheath.
Unless it is mounted on a phone pole in proximity to power lines.
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| Matthew Beasley 2007-02-26, 1:25 pm |
|
"Paul Hovnanian P.E." <paul@hovnanian.com> wrote in message
news:45DF8FD4.F7AA0CC4@hovnanian.com...
> Ali wrote:
>
> Think of it as a big capacitor. If the shield isn't grounded, the phase
> voltage will charge it to a lethal voltage.
The capacitive current flow can also cause ionization of the air on or near
the outer jacket (called a partial discharge). This discharge will generate
ozone and UV, both detrimental to insulation life.
In North American practice, grounded sheilds are optional between 1kV and
2.5kV, and always applied above 2.5kV. (As soon as I say this, I'm sure
someone will come up with a counter example ;-)
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