Equipment ground for 18KV syetem

[Mila2015]

I'm installing 18KV underground cable from a new tap-box, to an existing one (see oneline attached). The iso-phases and tap-boxes are isolated (frames are not bonded) in all locations. Each Tap-box has a Power Transformer for metering. Through this set-up, both GSUs will be feeding separate AUX transformers. Is there a reason why I shouldn't install equipment ground with underground cable? I'm thinking a ground fault in new cable may return through existing tap-box ground bar and inject itself into both GSUs somehow. Any thoughts?

P.S. Red items in oneline is new equipment.

 

[RRaghunath]

Quote "The iso-phases and tap-boxes are isolated (frames are not bonded) in all locations." Unquote.
I can't believe this. There cannot be any metallic frame near live part that is not bonded to earth. Check.
Isolated Phase Busducts are bonded at both ends but connected to earth at only one end. If tap-boxes are bonded to the isolated phase busducts, then there is no need for separate earthing of tap boxes.

 

[Mila2015]

Sorry. Both enclosures are bonded to earth, but they're isolated from each other (iso-phase and tap-box) using "insulating bellows". The Iso-phase is grounded only at one location to avoid current circulation. Also, tab-boxes are bonded to earth (through grounding ring). I don't think bonding the 2 tap-boxes is required, or is beneficial but wanted to see other thoughts.

 

[RRaghunath]

Quote "I don't think bonding the 2 tap-boxes is required,.." Unquote
Why do you say that - are these tap-boxes not metallic??
As I said, any conductive part near live parts has to be connected to earth for the sake of safety.

 

[Mila2015]

I agree each tap box needs to be bonded to earth at each box location, but do we have to pull equipment ground cable from one box to the other? As I explained, we're installing underground 18KV shielded conductors from one box to the other.

 

[RRaghunath]

Yes. It is the practice - Lay earth conductor to which you will take taps to earth the tap-boxes etc.
Shield in the cables is not recommended to be used to bond the earths at different locations.