Transformers - Calculating the HT Fault Level

Hi there,

Prospective symmetrical (Short Circuit) current = Short Circuit Level / (Sqrt(3) * voltage)

Cable Short Circuit Current = (Cross Section * K)/Sqrt(t)

Where K = 115 for copper conductorss, paper insulated.

t = time to withstande short circuit current

goto www.aberdare.co.za and download their cable facts and figures handbook. It gives the answers to all these and more. Beware - these standards/codes are applicable to South Africa and may vary in your country!

Regards,

Craig

You do not have the information to make that calculation. Check with the utility. They should be able to give the the fault current at the point you require.

Please reference the National Electrical Code article 450 for guidance.The requirements vary, based upon primary voltage and the transformer impedance.It varies from 300% to 600% of the primary current rating for Circuit Breakers, but for fuses, it is 300% of the primary current rating, for tranformers over 600 volts.

If less than 600 volts primary, reference table 450.3(b) it varies from 125% to 250% depending on the rated primary current, whether it is internally protected, and the transformer impedance.

If the NEC is not available in your area, please provide data plate info from transformer and I can help you more.

He is asking for the primary fault current. I don't see where you suggestions apply.

I stand corrected.

I misread the OP question.

Hi all,

I also misread the question - I gave a formula to calculate the fault current of the cable on the HT side.

My apologies.

As stated earlier - you need to get the impedance.

There are four methods you can use (depending on the accuracy you require)

1. Ohmic method

2. Per-unit method

3. Computer software

4. Point-to-point method

A simple way is as follows;

R=V/I = source impedance

where V = primary voltage and I = primary current

Conductor resistance / number of conductors = conductor impedance

source impedance + conductor impedance = total impedance

Primary voltage / total impedance = Short circuit current (RMS)

The conductor impedance you would have to look up in the relevant tables peratining to the codes governing your country.

Regards,

Craig

hi for short ckt calculations get the ETAP software .this will help 2 u

This depends on your system.

If the HT supply is directly from utility, the answer given by wareagle is correct. You need to check with utility company; otherwise the fault current has to be calculated using any of the methods such as Per Unit, MVA Method or using Software like ETAP (www.etap.com), Easy Power (www.easypower.com), SKM Tools: DAPPER (www.skm.com) etc. However, for calculating fault, you have to have sufficient data of your system.

- MS