minimum short circuit current calculation 1

[eliahud79]

in a case that two parralel cables connect a load
i used to calculate the minimum short circuit current at at the cabled end near the load by taking into cosidaration the two cables impedances.
but i noticed that some electrical software take into considaration for minimum short circuit calculation only imppedance of one cable

what is the correct calculation according to standards?

 

[jghrist]

What standards? I don't know of any US standards that would specify this. If it is possible to have a fault at the location in question where the current is carried by only one of the parallel cables, then using one cable impedance would be appropriate for minimum fault calculations.

 

[cranky108]

I've had people in the past ask me for the minimum fault current. Because I really did not know what they were asking, I would say zero.

Now I just ask what they want the short circuit study for.

If they can spell out why they want the study for, or what it means to them, then they may not get what they expect.

 

[7anoter4]

First of all the "maximum" and "minimum" short-circuit current is stated in IEC60909 by manipulated
the constant C from 1.1 to 0.95 depends on voltage[low,medium,high]. Ik"=C*Un/Zsc initial symmetrical short-circuit current. Now Zsc is calculated for the maximum possible [ for minimum current] and, of course, exists this possibility
that only one cable[from 2 parallel] to remain connected[the European lugs and connectors are not so reliable sometime]
and if this could indeed increase significantly the circuit impedance-I doubt, anyway-one could take it into consideration.

 

[eliahud79]

thank you

the problem is for very long cables, it is almost impossible to
protect the the cable for minimum short circuit of only one cable (of 2 in parrelel)
for such cases can i take in considaration the minimum short circuit of 2 cables in parrelel

 

[burnt2x]

I guess these ideas in the IEC world are put up to specify a window where designs can be roughly assessed. If you have the minimum, you will not design something below that level but you would also like to shave off some costs by not going beyond the maximum.

Back to the OP:
I think there will be other factors relative to whether the fault current level is the minimum or maximum, not just the dropping of one of the two circuits in parallel. The available fault current sources, the size of generating units, their location from the common point of interconnection, etc. , will have a bearing as to the possible level of fault available at certain points of the system as well as at the instant of occurrence of a fault. With the existing open-access, it is not always possible to know which units are connected to the system, how big the reserves are and how far these sources are. Having said those, the minimum could just be the historical data you can have (least power generating units on-line, the farthest generating unit, etc. . plus the condition initially proposed; one circuit dropping and the other circuit left to supply the electrical load, actual or perceived.

 

[eliahud79]

thank you

my case is lv o.4kv system
for long cables the miminimum short circuit current determind mainly by the cables impedance.
the design for two cables in parrerel, there is no option to work with only one cable connected.

 

[7anoter4]

The maximum and minimum fault current must be calculated, and each will be used for different purpose. The maximum fault current is used to determine the breaking and making rating of the circuit breakers. The minimum fault current is
used for protection relay co-ordination.
So, if you will use the maximum short-circuit current in your protection system and if the cable is the main impedance, than if only one cable will remain in circuit the protection will not work as it is too high fitted.
In order to check the cable for short-circuit 60% of maximum short-circuit current with both cables connected will be fair.
For maximum current you may take the System [Grid] Power as infinite, but for minimum you have to ask from the Utility.
Medium voltage system power at transformer high voltage terminal could be from 200 to 500 MVA.

 

[cranky108]

I don't know about different values coordination, and ratings.
I've seen as many coordination problems for higher impedance faults, as for almost bolted faults.

And quite often on the distribution system there are several system configurations, and what is frequently desired for minimum fault (in the US)is a larger system impedance for calculations of motor starting.

Don't get me wrong about what I am interpeting you as saying. If there are more than one understandings of a term meaning, this is the place to find that out.

So from what I am seeing there is a deffinition in IEC world for minimum fault?

 

[7anoter4]

The calculation method of IEC 60909 determines the sc currents at location F using the equivalent voltage source, c*Un/sqrt(3), defined as the voltage of an ideal source applied at the short-circuit location in the positive sequence system, whereas all other sources in the system are ignored (short-circuited).
All network feeders, synchronous and asynchronous machines are replaced by their internal impedances.