Answer / siddharth tayade

there is no need of star point for earth fault detection we
can connect the c.t. as per the earth fault requirement

Answer / budhaa lal

because we use the CBCT both side(feeder side and in motor
winding R,Y,B.)and for earth fault protection we give the
both ct's output to the motor protectio relay. and when
earthfault ocurres in motor there is unbalance in core
respectively and sensing by motpro relay and it gives
tripping as per setting.

Answer / engr. l. o. osuji

Hi Pratikjain,
The answer in #4 is partly correct. Your question is on the
motor and not on the feeder. Provide appropriate cts and
wire correctly using an over-current earth fault relay.
Note, this relay has three operating elements known as red,
yellow and blue-phase elements. The yellow-phase element is
for earth fault. This relay works on the principle that Ir
+ Iy + Ib = 0 for a balanced load. This summation is
algebraic. In times of an earth fault the above equation is
no longer equal to zero hence the residual current flows
through the yellow-phase element as an earth fault provided
your cts and relay are properly connected. In this way the
cb for the motor is tripped by the yellow-phase element as
an earth fault if the relay is properly set.

Answer / venkatesh

the fundamental condition for connecting neutral to earth is that the neutral voltage should not be higher than supply potential .but in star connected squirrel cage induction motor the neutral potential was reached uplimit potential during starting.so, neutral was not connected to earth in that case while in transformers there is no problems of increased neutral potentials. though they were similar in some aspects

Answer / velusamy

THE PURPOSE OF PROVIDING STAR CONNECTION IS 1.DEPENDING ON
THE DESIGN REQUIREMENT OF EQUIPMENT 2.TO PROVIDE PROTECTION
AGAINST DIFFERENTIAL

Answer / rajesh

As the neutral has high potential when the motor starts.
And else decreases while running. This may cause high flow
of current to earth which is earth fault.

Answer / nanoty

Because it is a high voltage rating motor and Star
connected.There is no need of earth fault protection in
this case.

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