Dear All, In a 400V power system, ratio of three current transformers installe

Dear All,
In a 400V power system, ratio of three current transformers
installed incoming switchgear is 4000/1. Maximum three
phases short circuit current of this system is 48kA. One CT
is installed on neutral of upstream transformer (6/0.42kV,
solidly earthed) for 51G protection. Would you please inform
me about the correct CT ratio of this current transformer
(on neutral)??

Thanks a million

Question Posted / majd-s

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Dear All, In a 400V power system, ratio of three current transformers installed incoming switchgea..

Answer / k.prakashchandra

The maximum earth fault current occurs when the phase comes
in contact with the earthed metal part.That will be about
230/0.5 i.e. about 500 A ,ignoring impedance of tr and
taking ER as 0.5 ohms.It is the usual practice to set EFR at
20% of OCR setting.For REFR the CT ratio should be the same
as that of phases.You can have a 2 core 4000:1 NCT and set
the 51G at 0.1 to 0.2 In ,using one of the two secondaries.

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Dear All, In a 400V power system, ratio of three current transformers installed incoming switchgea..

Answer / k.prakashchandra

Further you can not expect much earth fault current in
neutral earthing as the voltage is only 230.Unless the phase
comes in contact with earthed metal part or welding is done
on the ground, this current will be too low to be picked up
by ct.As the power supply points are provided with fuses
,the relay may not get a chance to act at all.

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