Mysterious Earth Cable

[czping]

Hi all,

I found a earth cable linking the excitation transformer (230kV/660V) LV terminal box to the generator excitation panel (earth bar). The power cables from the transformer to the panel is single core without screen/shealth.

I was wondering what is the purposes for this earth cable?? If it is for earthing the transformer's terminal box, why can we just link it to the transformer body and earth at site? Instead, a cable was laid all the way from the panel to the transformer (abt 100 meters).

I found out this earth cable because "hot spot" was detected using thermography at the earthing cable termination at the transformer terminal box.

Thanks in advance!

 

[jghrist]

It is for earthing the excitation panel. Even though the excitation panel is probably earthed elsewhere, an equipment earthing conductor should be run with the phase conductors to reduce the return impedance in case of a ground fault.

 

[czping]

Sorry jghrist,

I might not have explained my situation properly.

The excitation panel is already earthed through a earth bar in this panel. On the other hand, the transformer terminal box is not earthed locally but has this earth cable running all the way to be connected to the excitation panel earth bar.

Also, can you elaborate "to reduce the return impedence in case of a ground fault". Thank you!

 

[jghrist]

I assumed that the transformer 660V is the source for the excitation panel. Is this a grounded source? If so, then any fault current to ground on the excitation panel would flow back to the transformer source. The reactance of the fault current path depends mostly on the distance between the phase conductor and the ground return path. Running a grounding conductor along with the phase conductor minimizes this distance and the reactance.

 

[czping]

Yeah jghrist, you are right, the transformer 660V is the source for the excitation panel. However, the transformer is delta-delta and there is no neutral for grouding.

The grounding is done at the panel in this case. However, our whole station makes use of a common earth grid, so I presume that earthing done anywhere is the same.

I got a comment from a colleague that the earth cable might have been pulled along with the power cable, so since it is there, just earth it at a convenient point.

What do you think?

 

[jghrist]

Equipment grounding for an ungrounded system is covered in NEC 250.30(B). Metal enclosures must be connected to the grounding electrode at any point from the source (transformer) to the first system disconnecting means. If the first disconnect is at the excitation panel, you could bond the equipment grounding conductor to the grounding electrode there. At least that's how I would interprete the rule.

 

[czping]

Thanks jghrist, that's very useful!

 

[jbartos]

Suggestion: Check for transformer secondary fuses location since they are also disconnecting means.