Upstream And Downstream Short Circuit Current

Who designed this?

Do you mean you have 3 Ring [Main Units] of 11[12] kV 25 kA/1-3 sec.? Usually the rated breaker current could be 630 A [See Schneider Electric Medium Voltage Distribution Catalogue RMU 24 kV].

If the 3 RMU are connected in series and the ends are connected to the same source, then the maximum load current would be 630 A.

According to Nexans Cables Catalogue [for BS 6622 6.35/11 kV cable Cu/XLPE/AWA/PVC], maximum current for single-way duct [underground] could be 570 A.

[120 RHO, 15oC Earth temperature, 0.8 m depth].

See: Nexans 6-36kV Medium Voltage Underground Power Cables XLPE insulated cables.

The short-circuit current [one cable per phase] may withstand is 40 kA/3 sec. [from rated 85 oC up to 250 oC]

Thanks for your responce.

What i mean is i have three switchgears.One will be considered as upstream substations having two feeders going to two downstream substations. Then this two downstream substations are connected together.This will be a ring circuit. However downstream interconnection will be open in both places and will be keeping as a redundancy source.

Is this run as a ring with one point of supply or an open ring with two points of supply?

Open ring with two points of supply.

In Electrical Engineering power distribution it is always better to draw sketch when you request for any clarification, so that we can understand better to guide you better.

There is mistake in your question. Short circuit rating is same for 3 substation but the withstanding time is different. Nobody design time based protection nowadays. It shall be energy based protection . Calculate the fault level for 3 substations and the Equipment/switchgear manufacturer to confirm the withstand time. There is nothing wrong to consider 3 seconds. But it will increase the cost of switchgear.Normally, short circuits shall be protected within milli seconds. No need to withstand for 3 seconds.
Better to re-check and discuss it with consultant engineer for your project.