Breaker Selection

http://ecmweb.com/mag/electric_shortcircuit_calculation_methods/

http://electricalengineeringtour.blogspot.com/2007/09/circuit-breaker-sizing-on-fault.html

what if the transformer data is not given??

Search the net and any transformer manufacturer would have published guideline values of the transformer's no-load loss, full-load loss & typical percentage impedance. This can be taken as a guideline for your short circuit calculations. The MCCB's breaking capacity MUST be more than the worst expected fault current for a three phase short circuit at the point of installation of the MCCB in the network.

Do not forget to include any cable or line impedance that may be present between the transformer and the MCCB location.

but it does depend on the transformer data my question is if i know the load only and system voltage can i calculate the fault current or the breaking capacity which has to be considered for the selection of the breaker??

No! Not possible! You MUST know the capacity of the source, which is going to pump the short circuit current.

That is correct but confusing for the average person. The capacity of the source refers to the peak current in a very small portion of time. Frequently it is more than 1000+ times as great as the RMS value of the current rating.

This is the point at which the potential to evaporate metallic parts of the breaker becomes very important. It seems that voltages in the 400 to 480 VAC are especially good at sustaining an arc that can become quite deadly if it is not suppressed by appropriate design of the breaker. This is why the source is important so that you don't underestimate the potential for a deadly flash.

this scenario is applied to those conditions if the short circuit is occured on transformer site what if the short circuit is occured on load site moreover i have given you the scenario that i have the load of 100Amps and sytem voltage of 440VAC i dont know the main power source rating weather it is of 500KVA or 1000KVA because it is the distribution transformer

actually i am working on the project in which i need to select the proper breaker for ther main incoming supply the client told me the total load is 600Amps and i am confused weather to use the MCCB of 600Amps rating but of which breaking capacity??

Doesn't it depend on the wiring?

http://cr4.globalspec.com/search/sitesearch?do=show&sort=textmatchrank&srch=British%20Standard%207671&order=asc

truely it is but the question remain as it is the breaker of which breaking capacity is the most appropiate for the given scenario???

so the bottom line is you have to know the transformer data for the calculation of fault current and if we have such a scenario of the electrical distribution in houses if short circuit occur in any of the house it has the direct influence on its respective distribution transformer??

In a house, the incoming fuse, earth fault device, circuit fuse or plug-top fuse will act first, leaving the transformer still energised. If it didn't, then the house owner would not be very popular with fire insurance companies.

but if the short circuit is occured inside the house what is the consequence then and if that short circuit current also depends upon the connected transformer???

Well said ee65. I didn't explain very well so I'm giving you a GA!

I was trying to think of a mechanical equivalent to this question and I find it very hard to think of one.

I suppose one could say that the purpose of a breaker is to protect the wire downstream of the electrical distribution point. However, if that wire were to vaporise and the resulting plasma was infinitely conductive then the burning question (all puns intended) would become one of killing the power feed to the plasma. Perhaps that will explain why it is important to know how much energy the supply could deliver.

By the way, for those who have not experienced a large interruption of power such as is the topic of discussion, it may require you to change your underware afterwards.