Answer / bikash chandra pal

WAHT IS EARTHED SYSTEM??

when the source(it may be feeding generator neutral point
or feeding transforme neutral or a bus through a zig-zag
transformer etc)neutral point of a perticular voltage level
of a system is earthed(may be solidly or through a
resistance/inductance), then that perticular voltage level
of the system is called as earthed system.

WAHT IS UNEARTHED SYSTEM??

When the above said neutral point is not earthed then that
perticular voltage level of the system is called as
unearthed system.

ADVANTAGES OF EARTHED SYSTEM

The system eathing(solidly or through a resistance or
impedance) is done to provide a returne path of earth fault
current, which in turne helps

1)To isolate the faulty part of the system due to earth
fault by sensing earth fault current.

2)To Reduce the insulation requirement of phase elements of
equipments connected in that voltage level & cost of the
equipment/s in turn. VIZ: Earthed grade cables are less
costlier than unearthed grade cables.

3)Fenomennon like ARCHING GROUND can be avoided.

So,
System with lesser Equipment cost(refer SL.NO.(2)above),
health of the concern part of the system(Refer sl no.(1) &
(3)above) remain protected.


DISADVANTAGES OF EARTHED SYSTEM

Possibility of interruption of power supply due to earth
fault,is more in EARTHED SYSTEM than UNEARTHED SYSTEM.

ADVANTAGES OF UNEARTHED SYSTEM

1)Possibility of continuation of power supply is more in
UNEARTHED SYSTEM, as tripping of system due to first earth
fault is not happens.

2)sensing of presence of earth fault through sensing of
concern system's neutral displacement is possible & planned
maintenance/earth fault clearence of the system is
possible,which supports better availity of the system &
lesser interruption of process.

DISADVANTAGES OF UNEARTHED SYSTEM

1)Isolation of the faulty part of the system due to earth
fault by sensing earth fault current is not possibele due
to first earth fault.

2)Insulation requirement of phase elements of equipments
connected is higher Hence cost of the equipment/s is
higher. VIZ: unearthed grade cables are costlier than
earthed grade cables.

3)Fenomennon like ARCHING GROUND is there.

So,
Capital investment is higher for unearthed System, But
availability of system is better.

Answer / bikash chandra pal

The following is in reference to Mr.S Bhattacharrya's
answer. The following I wish to point out for readers who
are learning.
1) It is correctly indicated that VOLTAGES 415VOLT SYSTEM
GENERALLY NEUTRAL IS DIRECTLY GROUNDED AND THIS IS CALLED
EARTHED SYSTEM. But, it is to note "33KV & above system" is
normally grounded solidly (not that "above 33kv system" is
grounded solidly as indicated in mr. Bhattacharrya's reply)

2) Midium voltage level 3.3kv to 11kv(not upto33 kv) are
normally grounded through Impedance or resistance to limit
earth fault current. there are so many terms to describe
such system e.g. "high resistance(or impedance)
earthed" / "low resistance (or impedance)
earthed / "effectively earthed" /"non effectively earthed"
etc. but not termed as " UNEARTHED SYSTEM" (as indicated in
Mr. Bhattachrrya's reply.)

3)For information,"high resistance(or impedance)
earthed" / "low resistance (or impedance)
earthed / "effectively earthed" /"non effectively earthed"
terms are defined based on "co-efficient of earthing". What
is co-efficient of earthing??---- go through some text book
like "sunil s Rao" it will be clear to you.

4) For information "unearthed system " is that system,
which is not earthed at all, ie.nither solidly or through
resistance.

5) Please note, Earthing through Resistance or impedance ,
limits the "Earth fault current" to desired value as per
system requirement, But it is having no effect or
connection with system's Short circuit current(STC) or
Fault level, as indicated in Mr. Bhattacharyya's reply.

6)"----CALLED UNEARTH SYSTEMS AND THIS SYSTEM IS VERY
STABLE COMPARED TO DIRECTLY GROUND SYSTEM "---- is not a
true statement again given by Mr. Bhattachryya in his
reply. Both the system is quite stable. System stability is
not depended on system grounding. To know about system
stability you may refer any TEXT book for example "NAGRATH
KOTHARI".
systems "earthed" , "resitance or impedance earthed"
or "unearthed", having their own advantage / disadvantage
already explained to some extent in my earlier reply no.1
above.

Answer / s bhattacharya

BASICALLY AS PER LATEST IEC ALL VOLTAGE SYSTEMS WHETHER
400KV OR 415VOLT THE NEUTRAL HAS TO BE GROUNDED AT EACH
LEVEL IN ONE POINT WHETHER SOLID GROUND OR HIGH IMPEDANCE
GROUND.
THE METHOD OF NEUTRAL GROUNDING DEPENDS ON SYSTEM VOLTAGES
1.FOR HV SYSTEMS GREATER THAN 33KV THE NEUTRAL IS GROUNDED
DIRECTLY TO EARTH.
2.FOR VOLTAGES 33KV TO 4kv THE NEUTRAL IS GROUNDED EITHER
THROUGH NGR OR NGT (AS PER THE SYSTEM REQUIREMENT TO LIMIT
THE SYSTEM SHORT CIRCUIT CURRENT IE LIMIT FAULT LEVEL)AND
THIS IS CALLED UNEARTHED SYSTEM.
3.VOLTAGES 415VOLT SYSTEM GENERALLY NEUTRAL IS DIRECTLY
GROUNDED AND THIS IS CALLED EARTHED SYSTEM.
HIGH IMPEDANCE GROUNDING IE FROM 4kv TO 33kV SYSTEMS ARWE
CALLED UNEARTH SYSTEMS AND THIS SYSTEM IS VERY STABLE
COMPARED TO DIRECTLY GROUND SYSTEM.

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