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How would we calculate the no of earthing pits required
for motor /panel
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Answer Posted / 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.
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