Question { IBM, 63462 }

Why capacitor bank does not require in DG Set?

Answer

DG set supplies necessary current to load. Its (internal)
Power Factor is governed by its design. DG sets like
transformers are rated in KVA i.e. KVxI.
Whatever be the load power factor, a DG set's internal power
factor will be governed by its design.
If a reactive load such as inductor or capacitor of
equivalent capacity (LOAD KVA = DG set's KVA)) is connected
across a DG set's terminals, the power factor measured at
these terminals will be nearly 0. However in this situation
also DG set's internal PF wikk be its designed PF.
Power factor is a parameter with respect to its measurement
location.
If load supplied by a DG set consists of large number of low
power factor loads which eats up only very small portion of
its total capacity and the balance load is taken by a high
power factor load, then the power factor at DG set's
terminal will still be high. But the DG set's internal power
factor will still be governed by its design.

Question { Bhel, 9558 }

what is the earth resistance ?

Answer

Earth is a large mass and its potential is zero (neither
positive nor negative). When a matter of positive or
negative potential comes in touch with earth it gets
neutralised by supply or absorption of electrons by the
earth. Thus earth is a neutraliser.

An earth conductor connected to a buried earth pipe/plate
acts as carrier of the neutralising electrons. All of this
happens through the soil in contact with the earth pipe/plate.

Similar to current flow through a conductor there is gradual
dissipation of voltage from earthing terminal point(source)
to large mass of earth(Sink) through the soil in contact
with the earthing pipe/plate. There is gradual potential
drop acroos this surrounding soil.

Resistance of this surrounding soil to flow of current is
called as soil resistivity. Lower the resistivity of soil
lower will be the earth resistance and providing better
earthing i.e better drain point of risky high voltage.

Question { 5308 }

length of 4 core cable 95mm from substation LV VFD panel to
Motor at field approximately 150 meter. Motor stop and power
has been isolated from LV MCC ( Substation )to disconnect
Motor for services. After Motor completed service propose to
reinstall and reconnect Motor Cable, found earth cable
sparkling during touching at metal / motor body.
Inspect by clamp meter found earth cable have current flow
AC 1-2amps and all earth cable at MCC LV also have current
flow as same value. why it is happen?

Answer

Cable insulation together with cable conductors form
capacitance. On nergising a cable this capacitance gets
charged and cable charging current flows even if no load is
connected at cable end. Similarly when cable is disconnected
from power source it retains the original charging energy
until it finds a conducting path to earth for discharge.
Hence before touching a disconnected cable all its
conductors including metal sheath and armours should be
connected to a earth path so that the stored energy in the
cable gets discharged.