Fault Level Calculation of Electrical System

The calculation of greatest fault current of system is required for calculation of the capacity of the switchgear or bus bar or cable. The source of the supply will be the consideration.The MVA/kVA capacity of the transformer or generator which is the source. The simple equation to find the short circuit current is

1.1 x V _L

I = --------------- Amps

√ 3 x Z

I - short circuit current

Line voltage is multiplied by 1.1 to take care of 10% rise in voltage

Z Impedance in ohms

% Z x10 x kV ²

Z = ---------------------

kVA

Detailed calculation method is available in www. ecmweb.com

You must read it if you are really interested to learn the subject.

There are so much reading material on the subject in the net itself

Similarly read this link

http://books.google.co.in/books?id=yiT9Doq8TRwC&pg=PA184&lpg=PA184&dq=short+circuit+calculation+of+bus+bar+&source=web&ots=L1ww-bULxl&sig=Ker1LfRB8E9pWC5-fKnyNqPi_K0&hl=en#PPA184,M1

Hi

To evaluate the fault level of a system follow the steps noted below

Base kVA as 1000 kVA or 1MVA

% System imepdance at 250mVA as 100% ( I have considered 11kV system and assumed 250mVA (Max)

Convert %System Impedance at 1MVA = 100/250 = 0.4%

%tx Impednace = 4%

% total impednace = 4.4%

Fault level @ 415V side = 100/4.4*1 = 22.7MVA

Fault Current @ 415V = 22.7*10^6/ 1.732*415 = 31KA

Max fault current 31*1.1 = 35KA (due to Variation in system voltage)

Krishnan Subramoniam

AECOM

Sydney - Australia

What will be the fault level of the supply with

1. 415V, 144KVA? transformer impedence= 0.08, Vsec=100V, VA= 4000

2. 11kV, 25MVA? trans imped= 0.069, Vsec=800V, VA= 3.2MVA

Thanks a lot

check here http://ecmweb.com/mag/electric_shortcircuit_calculation_methods/?smte=wr

Thank you very much..

max. impedense value for any rating for the transformer is cosidered as 5% of the rating. therefore for example:-

for 1000KVA, THE FAULT LEVEL CURRENT WILL BE

1000x5%=1000*.05=50KA

ASIF HUSSAIN

Wrong....absolutely wrong