MV Cable Shield Currents

[steve66]

I see that MV cables are supposed to be derated for shield currents when they are grounded at multiple points.

But I can't find anything that explains how to calculate the derating. Does anyone know where this is explained? I don't have access to the IEEE books, but I assume it would be covered in one of those.

I assume more derating would be required for longer cables, since the source would have to supply the entire shield current, and that current would add to the load current at the source.

So does anyone know if the derating is significant for short runs like less than 300 feet? By significant, I mean more than 10% or so.

 

[steve66]

Found some info. Sounds like its less of a problem for smaller cables (<250 KCM), and less of a problem if all 3 phases are in the same conduit.

And it doesn't depend on the cable length. Not sure I completely understand it, but it looks like he heating effect in W/ft has more of an impact than the additional current needed to generate that wattage.

 

[Julius Right]

In my opinion, the derating due to shield current in case of multiple grounding points it has to be as in case of single section of a cable both ends grounded. The section of maximum length dictates the derating.

If Pc=I^2*R it is the main conductor losses and Ps= I^2*Rs*Xm^2/(Rs^2+Xm^2) it is the shield losses then, if Ia the ampacity of the cable if only one end is grounded or the phases are cross bonded [see IEEE 575] then the factor k=Pc/(Pc+Ps) it is the derating factor. For parallel single core cables, the Ps is different and you may calculate it according to EPRI EL-5036-V4 [see attachment]

 

[Julius Right]

And further explanations