Low Voltage Cables Sized Based on Short Circuits 1

[ThePunisher]

For ANSI installation, I was informed that LV cable sizing should not be based on short circuit withstand.

Is this equally true for IEC installations? The problem with sizing LV cables with short circuit withstand is we are going to grossly oversize the cable. Can this be waived? Does anyone had experience in working around with IEC installations that had the same situation?

 

[davidbeach]

I don't know the answer to the question about IEC practice, but you would only need to size for fault current at the load end of the cable rather than the source end of the cable. If the cable is of any length at all, the fault current at the load end will be considerably less than at the source end.

 

[ThePunisher]

Thanks David, I hope somebody from the IEC world will share their technical opinion.

On your comment, the short circuit at end of the cable would be considerably less by calculation. Does any standerd depict that the termination end (thus considering the cable length) should be the basis comparison to the cable withstand?

Will it be prudent to compare the short circuit withstand to the equivalent short circuit at the point where the cable comes out from the substation....assuming this is where the high probability of short circuit starts?

 

[7anoter4]

As davidbeach explain, in IEC world also, if the short-circuit occurs in the cable it self you cannot protect the remaining undamaged portion of the cable. The protection could protect the cable only for exterior short-circuits.
The cable size has to be at first stated according to load current, checked for the voltage drop and for short-circuit at the opposite to the source end.

 

[thinker]

But you can not always define a source and load sides. Example - regenerative drive system where power flow is bi-directional.

 

[7anoter4]

Good point thinker! Then you have to check twice as the source and the load interchange their positions.

 

[davidbeach]

But in all cases you are only concerned with through faults. 99.99% of the time you can ignore something like a regenerative drive, it will produce far less fault current than the system it is regenerating into.

 

[PHovnanian]

You typically protect cables from through faults because, for a fault in the cable run, the cable has already been damaged.

The protection is to keep a fault downstream (or external to the cable in the case of bi-directional power flows) from damaging an otherwise good cable.

This logic may have to be modified in the event you are dealing with direct buried cable systems where your maintenance practice is to expose the faulted section and splice it out. Then, it would be worthwhile protecting the upstream unfaulted part of the cable run.

 

[cherry2000]

Very basic calculations with fault currents available at bus, with subsequent impedance of the length of cable, reveals that fault currents fall to negligible values at the end of the cable. Here the premise is that if the cable routing has followed the proper codes/specs, it will be a well maintained raceway and no mechanical damage to the cable is envisaged. In such a situation, it is always cable ampacity and voltage drop which determine LV cable sizing. One can always do a simple calc for a sample length of cable (say the shortest from the substation leaving the substation) and prove that Isc values are low.

 

[juleselec]

When sizing LV cables for short circuit temperature rise, I usually take the conservative option and model the fault at the start of the cable (rather than the end as other commenters have suggested). Obviously if you use the max fault current at the bus for sizing a small feeder cable you're going to run into strife. But you need to consider the effect of the protective device in limiting the fault current.

Most fuses, MCB's and MCCBs have some form of current limiting and the vendor literature will typically either provide maximum i2t let-through energies or some sort of curve showing the let-through current for a prospective fault.

When you calculate the min cable size based on short circuit temp rise using these let-through energies, you'll find that they generally aren't a problem unless you have an extremely high short circuit capacity upstream.

 

[dpc]

If the fault is in the cable itself, the temperature rise will not the primary concern. The temperature rise is related to through faults as mentioned previously.



David Castor

http://www.cvoes.com