Answer / syed rafiudeen

18. The GI conductor sizes shall be as listed below.

(1). HT switchgear, structures, cable trays & fence, rails,
gate and steel column
50 X 6 mm

(2). PCC & MCC EARTH PITS
50 X 6 mm

(3). DB
25 X 6 mm

(4). Motor rated above 55 KW & above
50 X 6 mm

(5). Motor rated above 22 KW & up to 55 KW
40 X 6 mm

(6). Motor rated above 5.5KW & up to 55KW
25 X 6 mm






(7). Motor rated up to 5.5 KW
8 SWG. GI wire

(8). Lighting panels & local panel
25 X 6 mm

(9). Control & relay panel
25 X 6 mm

(10). DG frame, excites
50 X 6 mm

(11). DG & TX’mer neutral
50 X 6 mm cu flat

(12). TX’mer body
50 X 6 mm

(13). Push button station
8 SWG. GI wire

(14). Receptacles & welding outlets
8 SWG. GI wire

(15). Lightning Arrestor
25 X 3 mm copper

(16). PLC & WF panels
25 X 3 mm copper

Answer / shreenidhi

There is no general rule to arrive at the exact no of earth
pits.But discharging of leakage current is certainly
dependent on the area/cross section of the material.In
other terms for any equipment the earth strip size is
calculated on the current to be carried by that strip.First
the leakage current/current to be carried is calculated and
the size of the strip is determined/sized.For most of the
electrical equipments like Transformer,DG set etc.,the
general concept is to have 4 nos earth pits.2 nos for body
with 2 seperate strips and the pits shorted and 2 nos for
neutral with 2 seperate strips and the pits shorted.The
general rule is that for neutral ,the strip size should be
capable to carry neutral current of that equipment & for
body the strip should be capable to carry half of neutral
current.For example for 100kVA transformer,the full load
current is around 140A.The strip connected should be
capable to carry atleast 70A(neutral current)which means a
strip of GI 25x3mm should be enough to carry the current
and for body a strip of 25x3 will do the needful.Here I am
taking the strip size that is generally used as
standards.However a strip with lesser size which can carry
a current of 35A can be used for body earthing.The reason
for using 2 earth pits for each body and neutral and then
shorting them is to serve as back up.If one strip gets
corroded and cuts the continuity is broken and the other
leakage current flows throgh the other run thery by
completing the circuit.Similarly for panels the no of pits
should be 2 nos.The size can be decided on the main incomer
breaker.For example if main incomer to breaker is 400A,then
body earthing for panel can have a strip size of 25x6 mm
which can easily carry 100A.

Answer / ramkumar

it depends on the resisitivity of soiland time for fault
current to pass through(1 sec or 3 sec).there is a formula
which gives the area for earthing required.this is is
divided by area of one earth plate.this gives the no of
earthpits required.

Answer / g. janakiraman

no off earth pits are decided by considering the total
fault current to be dissipated to the ground in case of
fault and the current that can be dissipated by each earth
pit.
normally the density of current for GI strip can be roughly
200 amps per squre cam. based on the lenght and dia of the
pipe used the no of pts can be finalised. this has to be
worked out.

Answer / babloo

the no.of earth pits is usually calculated considering the
fault current, for suppose if the short ckt occurs b/w the
2 phases the fault current will be 3times to the normal
current. For example A 55KW motor carrying the current of
80A during normal running , when the short ckt occurs the
fault current has to dissipate through the earth, so the
earth must be around 25 X 3mm strip.

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