Short Circuit Calculations

It is to ensure that the cable can withstand without damage the momentary pulse of heat that is dissipated prior to the circuit protective device disconnecting the short circuit current.

FOR MINIMUM SIZE CABLE SELECTION, WHTHER SHORT CIRCUIT LEVEL OF BUSBAR IS CONSIDERED OR THE TOTAL LOAD OF THE SYSTEM?

IS THERE ANYBODY TO CLEAR MY CONFUSION?

THANX

Hello naleghari271@hotmail.com

PWSlack actually answered your Question, in the above Posted reply.

Please remember that the use of CAPITALS in an Internet Forum, is the same as shouting at the reader, and is considered to be not good manners.

Now, as to your Question....

Your choice of cable is related to the total load expected, with the requirement that the cable must be able to withstand the (PSC = Prospective Short Circuit) current at the point of electrical supply.

The Electrical Engineer of the Electrical Distribution Authority (The Electrical Supplier) is normally able to advise the PSC at your cable connecting point, where you connect to the electrical supply.

Example: There would be no point in having an installation with a rated load of 2MVA, a Switchboard/Busbar rated PSC of 30MVA, and a supply cable with rated PSC of 5MVA, if the Supply point rated PSC was 10MVA, because if there was a major fault at the switchboard busbars, the supply cable would explode, if the supply point OCB/ACB or fuses failed to disconnect in time.

Does that assist you?

Please advise further

Kind Regards....

Thanks for your detailed reply.

Is the Prospective Short Circuit Current, total load of the system conected to the busbar/switchgear or its the short circuit level of the busbar/switchgear.

Example. Short circuit Level of busbar is 70 KA and total load connected to the busbar, is 5 MW. For minimum cable size slection, which figure should i consider, whether 70 KA, or Current related to 5 MW in the following formula for minimum cable size selection.

I=224A(ln(.........).

Which value of I should I consider to calculate A.

Thanx

Hello again, naleghari271@hotmail.com

You have given some figures in your above reply.

You have omitted to state the Supply Voltage.

Is this a class or homework Question?

Please reply, with

Kind Regards

Its MV switchgear, u can say 6 to 11 kv is ur supply

Hello again, naleghari271@hotmail.com

You need to give a fixed Voltage, not a <".....6 to 11 kv is ur supply">

You omitted to answer my question: Is this a Class or Homework Question?

Please reply, with correct answers.

You could also advise your location (Do that in your Profile), because your location may have a bearing on your local Regulations, Insurance Requirements, etc.

Kind Regards....

for MV 6 kv

This is a project of cable reengineering at which i m working

Consult an appropriate Mentor at the facility.

Hello jack of all trades

I have given you a GA point for your good answer.

It appears the question asker is out of his depth.

He has not bothered to answer my questions properly, and still we do not know location of site, which is quite important from local requirements and Regulations also Insurance reasons.

Kind Regards....

Sizing busbar for current and cables for fault rating seem to be very common questions here.

People can find CR4 but they don't seem to be able to find any of the numerous cable sizing guides on the internet.

??????

Hello V.I.Abraham

From me for your well-explained Post.

Kind Regards....

Short circuit calculation are required to define the size of the cable in order that the maximum temperature which can damage the insulation is not exceeded this temperature is different for XPLE or PVC.

Method of calculation is always the same adding geometrically the impedance from the source up to the S/C point.

better consider motor contribution for sc study

WHAT IS FORMULA OF FIND OUT OF K.A IN THE BUSBAR,CIRCUIT BREKAR ALSO FORMULA FOR CT VA CALCULATION

Simply stated the three phase sc current is source voltage (ph-n)/source impedance.

ct va is a bit complex depending on a number of factors.