2009-2010
A Comparative Study between Cast Iron Galvanised Iron
Conductor used for Earthing of Electrical System
Compiled By
In Alphabetical Order)
Arjun Meena
Ashish Khandelwal
Ashok Jingar
Suparna Saha
Com
m p ar
arat
atii v e Stu
St u d y b et
etw
w een Cast
Cas t Ir
Iro
on &
A Co
Galvanis
Ga
lvanise
ed Iron Condu
Conductor
ctor used for Earth
Earthing
ing of
of
Electr
Ele
ctrical
ical System
EXECUTIVE SUMMARY
•
•
•
•
•
•
•
Earthing is a very wide topic for which vast literature still feels to be less. Lots of
earthing practices are present today and one has to choose suitable earthing practice
according to the needs & design requirement. One has to consider various factors
like soil resistivity, moisture, and resistivity of surface material, material of earthing
conductor & other technical parameter like magnitude of fault current, shock duration
etc during design procedure. Among various factors, choice & selection of conductor
of material is to be considered both economically & technically.
technically .
There
are various
choices
associated
with
the material
the &
conductor
it could
be copper,
zinc coated
steel,
Cast iron,
Galvanised
Ironofetc.
selectionlike
of each
of
these materials depends
depends upon the type of application for which they are being used.
used.
The qualities
qualities associated
associated with good earthing system
system are that
that it should be of low
electrical resistance, having good corrosion resistance, & it should be able to
dissipate high fault current repeatedly.
As per IS 3043:1987
3043:1987 P.19 Cl.9.1, electrode
electrode resistance
resistance is affected
affected by the shape
shape of
electrode. Pipe is considered to be the better shape than plate as the surface being
circular, the distance from the centre to any point is always equal which
facilitates uniform distribution of fault current from electrode to earth. Also pipe has
multistage current dissipation
dissipation..
There is a general belief among engineers that CI is better earthing conductor than
GI. Economically, GI is considered to be better than CI but maintenance wise & life
expectancy wise, CI is considered to be better. IS 3043 also considers CI as better
Earthing conductor.
conductor. But it is not true that CI is always better for every application
In CI pipe earthing, the presence of
of phosphorus
phosphorus is such that Iron phosphide
phosphide is
is formed
on the pipe surface which forms a matrix & act as barrier layer, preventing any further
corrosion. It has longer life due to large thickness & corrosion resistant properties.
properties.
For power station
station earthing it is considered to
to be the best choice
choice (as per IS
3043:1987). For watering the pit to retain moisture content, CI Pipe earthing doesn’t
require any extra pipe as holes are provided from top to bottom of pipe.
•
•
•
•
Galvanized Iron Conductor being less expensive & easily available is used more
frequently as earthing conductor. G.I. pipe electrode earthing requires 15-20 micron
galvanization Plus Charcoal & Salt etc. It has been found by tests that average loss
in weight of galvanized Iron electrode is not more than 0.5 % per year compared to
pure cast iron/wrought iron whose average loss in weight is 2.2% per year(as per
IEEE Std.80-2000).
Std.80-2000).
In some places,
places, CI can’t be used at
at all as earthing
earthing conductor
conductor where GI rods find its
its
importance. Like in case of deeply driven rods where CI pipes installation is
considered to be costly, there GI rod found its usage. With the use of proper backfill
compound like bentonite & hot dipped galvanizing, GI earthing system finds
substantial reduction of resistance of earthing & also finds increase in its life
expectancy value by 3 times.
As GI pipe corroded
corroded rapidly
rapidly & its condition
condition deteriorates
deteriorates year after year, GI pipe
pipe
earthing needs to be changed every 4 years. Also for watering of pit, additional pipe
is required. It sometimes fails to activate safely devices. Distribution of short circuit
current is less in terms of charge dissipation and, therefore, generates high potential
at the pit resulting in low fault current which is sometimes insufficient to trip fault
protection relays. Also for electronic equipment & appliances, GI pipe earthing is not
a stable platform.
Conclusion: This paper makes no ruling on whether Cast Iron or Galvanized Iron is
better as earthing electrode but it provides the basic information for choosing the
more appropriate earthing conductor for a particular application. We have presented
a basic discussion based on application, electrode type, & most important based on
IS that may allow Engineer to think on its own to evaluate which type of earthing
conductor is better.
A Co
Com
m p arat
ar atii v e St
Stu
u d y b et
etw
w een Cast
Cas t Iro
Ir o n &
Galvanis
Ga
lvanise
ed Iron C
Condu
onductor
ctor used for Ea
Earth
rthing
ing of
of
Electr
Ele
ctrical
ical System
Abstract: Earthing is a very wide topic for which vast literature still feels to be less. Lots of
Earthing practices are present today and one has to choose suitable earthing practice
according to the needs & design requirement. One has to consider various factors like soil
resistivity, moisture,
moisture, resistivity of surface material,
material, material of earthing
earthing conductor
conductor & other
technical parameter like magnitude of fault current, shock duration etc during design
procedure. Among various factors, choice & selection of conductor of material is to be
considered both economically & technically. There are various options present for the
selection of conductor of material & in making this selection; one should understand the
theory behind the each option. It is outside the scope of this paper to discuss every
conductor.
This paper intent to present a comparative study between Cast Iron Conductor &
Galvanised Iron Conductor used as earthing material. It is the intention of this paper to
assist the engineer in making decision on the subject by presenting basic advantages &
disadvantages involved in selection of Cast Iron or Galvanised Iron conductor & also make
one understand the complexities involved in selection of conductor of material. It is the
application
applicatio
n & other parameters which decide the selection of conductor of material.
I Introduction
Earthing of electrical equipment is very important for the safety of operating personnel as
well as for proper system operation. By earthing we mean connecting the electrical
equipmentt to the general mass of the earth which has a very low resistance.
equipmen
Earthing is necessary for
•
Discharging the accumulated charges to ground to prevent hazard to man &
machine.
•
Stabilizing system voltage with respect to ground.
•
Fast & efficient operation
operation of protective
protective gear in case of an earth fault.
fault.
•
•
Efficient return of fault / leakage current to its source without
disturbance to the running system.
causing
any
Human Safety - Equipment
Equipment Safety - Avoid
Avoid ignition
ignition due to static charge
•
Protection
lightening against
- Protection
induced voltage
volagainst
tage - Protection
again
st storageagainst
voltagesurge voltage -Protection against
The qualities associated with good earthing system are that it should be of low electrical
resistance, having good corrosion resistance, & it should be able to dissipate high fault
current repeatedly.
There are various choices associated with the material of the conductor like it could be
copper, zinc coated steel, Cast iron, Galvanised Iron etc. & selection of each of these
materials depends upon the type of application for which they are being used. Somewhere
plate earthing is recommended & somewhere pipe earthing is recommended. As per IS
3043:1987 P.19 Cl.9.1, electrode resistance is affected by the shape of electrode. Pipe is
considered to be the better shape than plate as the surface being circular, the distance from
the centre to any point is always equal which facilitates uniform distribution
distribution of fault
current from electrode to earth. Also pipe has multistage current dissipation.
II Ea
Earth
rth Ele
Electro
ctro des
IS 3043 recommends wide variety of types of earth electrode. The type includes earth rods,
earth plates, & underground structure metal works (as per IS 3043:1987 P.19 Cl.9.2). The
soil
resistivity
of theelectrode
ground is (as
probably
single most
important
factor
thebe
determination
determina
tion
of the
type of earth
per ISthe
3043:1987
Cl 8.0.1).
Rods
can in
only
as effective
as the contact they make with the surrounding material. Thus, they should be driven into
pure ground, not disturbed (backfilled)
(backfilled) ground. In some locations, low soil resistivity is found
to be concentrated in the topsoil layer, beneath which there may be rock or other impervious
strata, or a deep layer of high resistivity. In such circumstances, the installation of copper
earth tape, or pipes or plates, would be most likely to provide a satisfactory earth electrode
resistance value. Whatever forms an earth electrode takes, the possibility of soil drying &
freezing, & of corrosion must be taken into account. Preferably, testing of the earth
electrode should be carried out under the least favourable conditions, i.e. after prolonged
dry weather.
III Earthing Conductors
Earthing conductors which are defined in IS 3043 as a protective conductor connecting the
main earthing terminal of an installation to an earth electrode or other means of earthing
must be adequately sized particularly where buried partly in the ground, and be of suitable
material & adequately protected against corrosion & mechanical damage. There are various
materials available for conductors but their detailed discussion is outside the scope of this
paper. For example, copper is considered best material for conductor because of its
excellent conductivity, cathodic & resistant to corrosion property. Also at some places,
copper clad steel is also used for underground rods. Iron is another material used as
material
for conductor
in thetwo
form
of Cast Iron Pipes & Galvanised Iron plates/rods. Let us
have a discussion
on these
in detail;
A. Cast
Cas t Ir on Condu
Con duct
ct or
Generally, Cast Iron (CI) is used as Pipe earthing material (as per IS 3043:1987 p.20 cl
9.2.2). Pipe Earthing is considered to be durable & reliable as reliability depends on
powerful metallurgy & surface area of the electrode. CI pipe is considered to be having
double surface area. In CI pipe earthing, the presence of phosphorus is such that Iron
phosphide is formed on the pipe surface which forms a matrix & act as barrier layer,
preventing any further corrosion. It has longer life due to large thickness & corrosion
resistant
it is resistant
to residual
chlorine/
sulphate
which sometimes
present inproperties.
soil in largeAlso,
quantities.
In C.I. pipe,
excellent
and uniform
dissipation
occurs and
ideally no maintenance is needed because of no upsurge in ‘Ohmic Value’. Also less
number of pipes is required as surface area contact is more due to bigger diameter. As per
IS 3043:1987, CI pipe earthing comprises of installation in which pipe level is kept 4” X 6 “
above the earth with chamber and can be checked any time. For power station earthing it is
considered to be the best choice (as per IS 3043:1987). For watering the pit to retain
moisture content, CI Pipe earthing doesn’t require any extra pipe as holes are provided from
top to bottom of pipe.
If CI pipe is compared with galvanized iron, the relative thickness is almost 5 times of GI &
hence life expectancy of CI pipe is more than double as compared for GI pipe. Where
electrode of large surface area is necessary to provide requisite current carrying capacity,
earth plates are recommended. These are generally
generally of cast iron, not less than 12.5mm thick
& perfectly ribbed & are usually 1.2m X 1.2 m.(as per IS 3043:1987 P.47 cl 20.5.2).
But above all advantages of CI pipes depends on certain conditions & applications. CI is not
favourable for every application. In pipe electrode earthing, change of diameter has
relatively minor effect & size of pipe is generally governed by resistance to bending or
splitting. It is apparent that the resistance diminishes rapidly
rapidly with the first few feet of driving,
but less so at depths greater than 2 to 3 m in soil of uniform resistivity. In this case, Pipes
may be of cast iron of not less than 100mm dia, 2.5 to 3m long & 13mm thick(as per IS
3043:1987 Cl.9.1). Such pipes cannot be driven satisfactorily & may, therefore, be more
expensive to install than plates for the same effective area.
On the contrary plate earthing using cast iron also proved to be costly & expensive with
certain limitations. The size employed for plate is normally not greater that 1.2 X 1.2m. For
conventional sizes, the resistance is approximately inversely proportional to the linear
dimensions, not the surface area i.e. a 0.9 X 0.9 m plate would have 25% more resistance
than a 1.2 X 1.2 m plate. Also, the depth at which plates are set should be as such to
ensure surrounding soil is always damp(as per IS 3043:1987 P.20 cl.9.2.1). Also CI in its
pure form can’t be used. Addition of phosphorous material is necessary to make it rugged &
better corrosion resistant.
B. Galvanize
Galvanized
d Iron Conductor
Galvanized Iron Conductor being less expensive & easily available is used more frequently
as earthing conductor. G.I. pipe electrode earthing requires 15-20 micron galvanization Plus
Charcoal & Salt etc. It has been found by tests that average loss in weight of galvanized
Iron electrode is not more than 0.5 % per year compared to pure cast iron/wrought iron
whose average loss in weight is 2.2% per year(as per IEEE Std.80-2000). Plate electrode
when made up of GI or steel shall not be less than 6.3mm in thickness (as per IS 3043:1987
Cl.9.2.1).
In some places, CI can’t be used at all as earthing conductor where GI rods find its
importance. Like in case of deeply driven rods where CI pipes installation is considered to
be costly, there GI rod found its usage. With the use of proper backfill compound like
bentonite & hot dipped galvanizing, GI earthing system finds substantial reduction of
resistance of earthing & also finds increase in its life expectancy value by 3 times. The
maintenance rate also decreases on the other side.
maintenance
But use of GI is discouraged because of certain properties which are unavoidable. The
galvanized coating eradicates with time & results in reducing the life of earthing. The life of
GI rod is considered to be around 6 years. In G.I. pipe, up to 40mm dia pipe can be used &
hence, is having lesser area of contact with earth which results in poor dissipation of
current. Also maintenance wise, GI earth pit requires maintenance at regular intervals.
In case of Plate earthing, as GI plate is inside the earth & only GI strip comes out from pit; if
any continuity problem comes, it can’t be checked. As GI pipe corroded rapidly & its
condition deteriorates year after year, GI pipe earthing needs to be changed every 4 years.
Also for watering of pit, additional
additional pipe is required. It sometimes fails to activate safely
devices. Distribution of short circuit current is less in terms of charge dissipation and,
therefore, generates
generates high potential at the pit resulting in low fault current which is sometimes
insufficient to trip fault protection relays. Also for electronic equipment & appliances, GI pipe
earthing is not a stable platform.
IV. Conclusion
This paper makes no ruling on whether Cast Iron or Galvanized Iron is better as earthing
electrode but it provides the basic information for choosing the more appropriate earthing
conductor for a particular application. We have presented a basic discussion based on
application, electrode type, & most important based on IS that may allow Engineer to think
on its own to evaluate which type of earthing conductor is better.
There is a general belief among engineers that CI is better earthing conductor than GI.
Economically, GI is considered to be better than CI but maintenance wise & life expectancy
wise, CI is considered to be better. IS 3043 also considers CI as better earthing conductor.
But it is not true that CI is always better for every application. In our discussion we have
shown, in some cases, GI is better where CI type earthing installation is considered to be
more expensive & technically not right. Also, with the use of GI as hot dipped & proper
backfill compound, good earthing practice can be obtained.
V REFERENCES
[1] IS 3043:1987
[2] IEEE std. 80-2000_Grounding
80-2000_Grounding
[3] IEE Wiring Matters| Autumn 2005| www.iee.org | Earthing: Your Questions Answered-By
Geoff Cronshaw
[4] IEEE Std.142-1991
Std.142-1991 IEEE recommended
recommended practice for Grounding Of Industrial &
Commercial Power Systems
VI Compiled By
Arjun Meena, Ashish Khandelwal, Ashok Jingar, Suparna Saha