Short Circuit Analysis

[Golestan]

Hi,
Can someone please briefly explain and show a simple example of a short circuit analysis. We are required to show this on our MEP plan sheets for obtaining building permit. Thanks

 

[davidbeach]

That's for your electrical engineer to do, it isn't a mechanical item.

 

[apowerengr]

Davidbeach I Agree - even though the building plan reviewer probably won't know what they'd be looking at anyhow.......

 

[waross]

The intent of such requirements is often to assure that the equipment that has been selected is adequate to withstand the fault currents that may be imposed on it.
The panels and breakers are rated in available fault current.
(There is a safety factor to allow for the greater asynchronous currents that will occur in the real world. The available fault current rating is based on a conservative X/R ratio and is safe for normal distribution transformers.)
Dividing the transformer rated full load current by the percent impedance voltage will give the available fault current at the transformer terminals. This should be less than the rating of the installed equipment.
In the event that the available fault current exceeds the rating of the equipment the next step is to find some tables and look up the impedance of the service conductors. Recalculate the current with the addition of the cable impedance.
If the available fault current is below the rating of the installed equipment the installation is suitable.
If the available fault current is above the rating of the installed equipment the installation is not acceptable.
Either the equipment must be replaced with higher rated equipment or the available fault current must be reduced.
The calculations are much simpler than a rigourous sulution of actual asymetrical fault currents for a reason.
The calculations are simple to understand and compute. The results are conservative and are easy to apply.
The probability of an unsafe installation as a result of a mistake in calculations or a lack of understanding of asymetrical fault currents is very low.
One cheap and simple way to reduce available fault current is to increase the length of the service conductors until the impedance is sufficient to limit the available fault current to the rating of the equipment.

even though the building plan reviewer probably won't know what they'd be looking at anyhow

Rest assured that the AHJ plan checker will understand that the available fault current must be less than the rating of the equipment. You are responsible for the accuracy of the calculations, not the plan checker.
respectfully