Fault Rating of Parallel Cables

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DerekR

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Hello anyone

I am looking at a power system, running at 6.6kV, where the designer is proposing to use parallel cables to feed from the HV switchboards. I have calculated a minimum core size for the cable based on the prospective fault currents that will occur of about 70sqmm.

The designer is proposing to use 2 off of approximately 50sqmm in parallel to achieve this figure. My understanding is that, regardless of the loading the minimum cable size must be 70sqmm. Therefore he needs to use 2 off 70sqmm otherwise the cable will not survive the potential fault current if a single cable fails.

Has anyone any thoughts on this?
 
In short, if you use parallel runs of smaller cables, the ground in each of those runs must be sized as if there were only one full-sized run. If there is a phase to ground fault, at which time the full load will only be transmitted through one ground.
 
Thanks for that. It does somewhat confirm my understanding. What I was also interested in is, if you get a phase to phase fault (3 phase system), I think the samething applies as it does with the ground fault
 
SC Rating of Parallel Cables?

SC Rating of Parallel Cables?

Depends on where the fault is. The purpose of checking the cable size against short circuit is to prevent cable damage during a down stream fault. The parallel wires will share fault current to the downstream fault with the currrent split depending on the relative impedance of the two cables. The current split may be enough to allow use of the smaller cable.

If the fault is in the cable run itself, the cable is already damaged and has to be replaced anyway so why add costs to prevent possible damage to something that has already failed?

There are installations where the larger size would make sense. For example, if the cable runs through underground ducts and vaults in a manner that allows replacement of the faulted section without removing the complete run. Or if the high temperatures during the short circuit could damage other cables in the same cable tray.
 
Can you even parallel 50sqmm? In circ. mils thats is smaller than 1/0 although only slightly. I believe 310.4 mentions that conductors must be 1/0 or larger to parallel.
 
Fault Rating of Parallel Cables

Thanks for the comments. They have proved very useful in getting my thoughts together on this.:smile:
 
rcwilson said:
The purpose of checking the cable size against short circuit is to prevent cable damage during a down stream fault.
Click to expand...

I have to admit I have never made this check. Is it common for a PHASE cable to become damaged due to too much fault current passing through it for too long of a time? I would think that the OCPD would open before this became an issue unless there was a failure of the OCPD.

Parallel runs will help with dividing the fault current due to relative impedances, but you will also have smaller cable runs than if you just had a single run of larger wire.

Does everyone check the PHASE cable against fault current magnitude/clearing time? And how does one check this?
 
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