what is the difference between earthed and unearthed cable?
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Answer Posted / n. ramesh
In unearthed cables (usually ht cables), the core
insulation will be rated for Uphase-phase as the healthy
phase conductors will be at √3 times Uphase-neutral with
respect to earth in case of a single phase to earth fault
(when laid in ground). Whereas in the case of earthed
cables, the healthy phase conductors will be at Uphase-
neutral with respect to earth even when there is single
phase to earth fault. Hence, in the case of earthed
cables, the core insulation will be rated for Uphase-
neutral.
Further, in earthed cables, a current equal to the current
through the phase conductors will flow through the shield,
which will heat up the shield. Hence, current rating of
cables meant for earthed application will be slightly lower
than the current rating of cables meant for unearthed
application for the same conductor cross sectional area.
Generally 6.6 kV and 11kV systems are earthed through a
neutral grounding resistor and the shield and armour are
also earthed, especially in industrial power distribution
applications. Such a case is similar to an unearthed
application but with earthed shield (some times called
solid bonding). In such cases, unearthed cables may be
used so that the core insulation will have enough strength
but current rating is de-rated to the value of earthed
cables.
But it is always better to mention the type of system
earthing in the cable specification when ordering the
cables so that the cable manufacturer will take care of
insulation strength and de-rating. Also it is better to
use the cables for the type of system earthing they are
meant for.
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