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

2 Answers
3866 Views
I also Faced
E-Mail Answers

Answers were Sorted based on User's Feedback

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.

Is This Answer Correct ?

0 Yes

0 No

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.

Is This Answer Correct ?

0 Yes

0 No

Post New Answer

More Electrical Engineering Interview Questions

what is phase to phase fault? what is line to line fault?

6 Answers

What is the basis for selecting Vk/2 or Vk/4 for REF and Differential protection of Transformer and What is the significance of magnetising current

0 Answers ABB, Jindal,

Explain the advantages and disadvantages of thermal power plant?

0 Answers

what happens if dc supply is given and the remedies for it.

0 Answers

what is grounding & what is the value of groumding rod 2.5m?

3 Answers

Why different values of B’s (magnetic flux density) are chosen for DC machines and Induction Motors?

1 Answers

advantages & disadvantages of Lagging and leading power factor .

0 Answers KMSC,

What is the make of breakers, transformers, generators? What are their capacities and ratings?

1 Answers

Why in USA supply voltage parameters(230V,60Hz)are different,is there any advantage please brief.

1 Answers APC, DRJ ARDE,

How to find the maximum load that can supply by a particular(say 315kva) KVA rated transformer?

2 Answers ME,

what is inverter and converter

7 Answers Shrishti Power Technologies,

What is effect of over voltage and under voltage on ACB

0 Answers EFS,

For more Electrical Engineering Interview Questions Click Here