metalic sheath

Why do we ground the metalic sheath of the cable??

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Ali wrote:

so we do not get zapped.
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you mean that insulation is not enough... isn't there any connection with induced voltage phenomena??
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No. Insulation can break down.

No. It's earthed, remember?
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wrote:

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?
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On Fri, 23 Feb 2007 00:17:21 -0500, "TimPerry"

All bucket trucks have drag chains!
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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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The connection between it and the EMP pulse of a nuclear explosion...
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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.
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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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Ali wrote:

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.
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So the fuse or breaker trips when the cable is damaged.
Fred

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Why is a pig's ass pork, all the way 'round the tail?
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On Thu, 22 Feb 2007 05:37:40 GMT, "Long Ranger"

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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On 2/23/07 9:47 AM, in article snipped-for-privacy@news.verizon.net,

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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Gave us:

Unless it is mounted on a phone pole in proximity to power lines.
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