Answer / nirav shah

Unearthed cable have more insulation strength as compared
to earthed cable.When fault occure phase to ground volage
is root 3 time the normal phase to ground voltage. so If we
used earthed cable It may be chances of insulation puncture.
SO unearthed cable are used.Such type of cable are used in
6.6 KV system where resistance type earthing are used.

Answer / samsung

no above is wrong
earth cable and unearth cable get differantiated with size
of insulation( thickness) , unearth cable it is more and if
l-g fault takes place on unearth cable voltage rise will be
sqrt 3 times phase voltage on healthy phase

Answer / prasad

Earthed cable requires less insulation as compared to un
earthed cable because in case of any fault the circuit will
complete through earth and less damage to cable will be
occurred . For unearthed cable,in case of fault the
protective system will function or the cable will burst.
Hence the insulation level of the unearthed cable will be
more than earthed cable and the voltage range will be
accordingly changed. However for transmission of
HT ,earthed cable will be more economical due to low cost
where as unearthed cables are not economical but insulation
will be good.

Answer / vipul dhruv

what nirav said is right but the thing is he didn mention
on which phases the voltage will be root 3 times the phase
to ground voltage. actually it is on the other two phases
other than the faulted phase that the voltage will shoot to
1.73 times the normal phase to ground.
regards.

Answer / e.k rajan nair

In 3phase earthed system, phase to earth voltage is
1.732times less than phase to phase voltage. therefore
voltage stress on cable to armor is 1.732times less than
votage stress between conductor to conductor. whre as in
unearthed system, (if system neutral is not grounded)
pahse to ground voltage can be equal to phase to phase
voltage. in such case the insulation level of conductor to
armor should be equal to insulation level of conductor to
conductor. as a thump rul we can 6.6KV UE cable is equal to
11k earthed cable ie 6.6/6.6kv UN cable can be used for
6.6/11kv earthed system. because each core of cable have
the insulation level to withstand 6.6kv so between core to
core insulation level will be 6.6kv+6.6kv = 11kv

Answer / ravi kaslikar

Power Cables can be classified into Earthed and Unearthed
cables. The terms "earthed" and "unearthed" refer to the
application for the which the cables are to be used.

By Earthed system, we mean a three phase system whose star
point is grounded directly. In this case, in the event of
the earth fault, the voltage in the faulty phase will
become zero. The voltage between the healthy phases and the
ground will be the same, 11kV/1.732 or 6.6/1.732.

However, in case of an earth fault in an ungrounded system,
the voltage between the healthy phases and the ground will
be equal to the phase to phase voltage. i.e. 11kV or 6.6 kV.

Hence, the insulation level of these cables needs to be
higher.

Thus a cable suitable for a 11kV earthed system will be
suitable for a 6.6kV unearthd system.

Answer / 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.

Answer / sai seddy

well said jack. we are all electrical engg.what we
know,post the answer.nirav shah post the answer was correct
to my knowledge. Mr. Mittal Boye knows only.

Answer / rakhi kawatra

Earthed System : the neutal point or mid point connection
is earthed in such manner that even under fault conditions
he max. voltage that can occur between any conductor and
earth does not exceed80% of the nominal system voltage

Unearthed System : an electrical system which does not
fulfil the requirement of the earthed system.

Answer / jack

All the above answers are true partially & its not upto
that extreme engineering to consider the current rating.
extint, but one Mr.Mittal boye who doesnt have
sense give the answer. u mit...........

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