Fault current calculations 2

[immsk]

Hi I am attempting to do the short circuit current calculations to determine an appropriate fuse size and I was wondering if some one could give me some pointers. The calculations are for a run between a 3ph 600/347V padmounted transformer and a switch gear. We are putting in a new 3ph service 600/347V, 1000AMP service and I would like to do some calculations in order to ensure that we have appropriate fusing selected to comply with ESA and utility requirements. If some one could walk me through the calculations or inform me of what specs and numbers I need to have before I can commence the analysis I would really appreciate it.

 

[sspeare]

You should do this work under the direction of an experienced Engineer. Nobody can simply give the "know-how" to you. You have to study. I suggest reading the Bussmann publications on fault calculations. See the link below.

 

[shamsdebout]

The first step is to get the available fault current at the source. Contact the utility go get this information or you can use tables of typical values.

Here is a good reference:

http://www.cooperindustries.com/con...ch_Lib_Short_Circuit_Current_Calculations.pdf

 

[shamsdebout]

I second sspeare's comment and he beat me to the link. I guess I was onto something.

 

[Gragas]

The structure and information in your question shows you should probably not be doing this unsupervised

 

[DaveMeharg]

I would recommend the GE guide to short circuit calculations. If you google this you should find the link below which has a very helpful document on calculating short circuit.

http://www.geindustrial.com/publibrary/checkout/GET-3550F?TNR=White Papers|GET-3550F|generic

The basic things you need to do is create a simple one line diagram. You then will need to know the transformer size and impedance data. You also need to know the cable conductor size and length between transformer and your new switchgear. It would also be good to have the utility short circuit information. However, you could assume an infinite utility source to simplfy the calculations. The GE guide can walk you through taking the information above and converting it into short circuit. The GE guide also has short circuit calculations worked out for given transformer sizes (although they tend to be based on the USA sized transformers).

You will find that your transformer has the biggest impact on your short circuit levels downstream of the transformer. It will provice the biggest impedance that will limit the short circuit levels. Just knowing your transformer kVA size and impedance will allow you to do a quick calcuation to get you in the ball park in terms of short circuit levels. There are other considerations such as the X/R ratio when working with circuit breakers and other devices which the GE document also mentions.