Low Voltage Cables Sized Based on Short Circuits
Low Voltage Cables Sized Based on Short Circuits
(OP)
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?
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?






RE: Low Voltage Cables Sized Based on Short Circuits
RE: Low Voltage Cables Sized Based on Short Circuits
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?
RE: Low Voltage Cables Sized Based on 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.
RE: Low Voltage Cables Sized Based on Short Circuits
RE: Low Voltage Cables Sized Based on Short Circuits
RE: Low Voltage Cables Sized Based on Short Circuits
RE: Low Voltage Cables Sized Based on Short Circuits
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.
RE: Low Voltage Cables Sized Based on Short Circuits
RE: Low Voltage Cables Sized Based on Short Circuits
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.
RE: Low Voltage Cables Sized Based on Short Circuits
David Castor
www.cvoes.com