Calculating Short Circuit Current: Tips & Advice

[Ivan001]

Good evening to everyone!

I'm pretty new in this kind of job, so every help or advice will be helpfull and I will appreciate it.
I know that short circuit current from picture in attachment depends on impedance and reactance of Source MV, transformer example (10kV/0.4kV) and cables. So if I get, calculate all of this impedances I will be able to calculate short circuit current and here begins my problems.

1.) If I have all impedances (sources, transformer, cables) I will be able to calculate short circuit current of each circuit(1,2,3,4). Isc of each circuit is different while the cross section of cables are different, too. But, how to get total Isc on busbar, in attachment it's 9.1kA?

2.) Sometimes when you make a project for some house in village it's difficut, impossible to get all this information about reactance and impedance of transformer and source (MV network), how to calculate Isc in such situation?

3.) How do you calculate Isc for different type of earthing system (TN-S, IT, TT)?

THANK YOU!
Regards,
Ivan

 

[anorlunda]

Are your circuits actually 3 phase?

There are multiple types of shorts. Line to line. Line to ground.

Your schematic looks very simple, but most people use matrix methods to calculate short circuits.

Do you have a textbook that describes short circuit calculations?

 

[Ivan001]

Are your circuits actually 3 phase?

There are multiple types of shorts. Line to line. Line to ground.

Your schematic looks very simple, but most people use matrix methods to calculate short circuits.

Do you have a textbook that describes short circuit calculations?

@anorlunda my questions are for short circuits claculation of house installation (for example), so there is always information (Isc) on busbar like on picture in attachment. It's probalby Imax sc. So, my questions remains same.
I found some examples of calculating short circuits line to line, line to ground etc, but this principles are useless in calculation of short circuit in house installation (like on picture in attachment).

 

[anorlunda]

You asked a very general question. A very general answer is yes, the impedances and the detailed wire layout (including grounding) are necessary for short circuit calculations. If you don't have the data, or the experience to estimate, then you need the assistance of a qualified power engineer.

The risk is underestimating short circuit current. If a low estimate results in a too small circuit breaker, then there may be a short circuit that the breaker can not clear. If that happens, fires in the houses may result.

We can not give you a better answer without more detail. As a consulting engineer myself, I would not give an answer without an on site inspection. Internet posts are not enough.

Because this involves public safety, I do not think we should discuss it more on PF. Please, find a local power engineer who can help you.
Thread closed.