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.
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 ;-)
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
Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.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
On 2/23/07 9:47 AM, in article email@example.com,
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.
-- Fermez le Bush--about two years to go.
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