Substation Secondary Source Grounding

[pippey]

We have a substation control building with a secondary source fed off of a distribution line outside the substation. There is a pole mounted, 12470/240V single phase transformer that has the neutral grounded at the pole. The service runs from the pole, underground to the substation across the street.
I think we should bond the neutral to the ground grid of the substation again at the service disconect to the control building, but have been questioned about this. No one has a good answer about why we shouldn't. Is there something I am missing here?

 

[waross]

Most North American codes require a ground connection at the panel. If the panel is in the substation it will be grounded to the main grid or if grounded independently such ground must be inter connected with the main ground grid. Running equipment inside the sub that is grounded outside the sub may result in serious touch potentials in the event of a ground fault in the main supply or distribution circuits.

No one has a good answer about why we shouldn't. Is there something I am missing here?

Yes.
The possibility of serious and possibly fatal touch potentials between grounded surfaces in the event of a serious ground fault.
Put another way, a serious ground fault may develop significant voltage gradients in the surrounding earth. There may be a large voltage difference between the station ground potential and the ground potential of a point outside the station ground grid.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

What codes apply in your case?

If NEC, (judging from your voltages and use of word 'ground'), neutral to ground bond is required at each service entrance equipment, regardless of how may times the source neutral is grounded on the line side.



Rafiq Bulsara

http://www.srengineersct.com

 

[pippey]

rbulsara,
We are a US utility and exempt from NEC requirements.

warss,
Am I correct that from your answer that you are in agreement with grounding again at the service entrance?

I think some concern now is that if there is a fault in the substation, that by bonding the outside service transformer neutral to the substation ground grid (and at the pole the transformer hangs from), a rise in substation ground grid potential could cause a rise in ground potential around the pole too, and because the pole is outside the equi-potential plane provided by the substation ground grid, there could be a step/touch potential hazard.

 

[rbulsara]

pippey:
Even then good engineering judgment should take precedence. You are looking for a guidance for a building power and there is no better guide than NEC, whether or not it legally applies.

Strive to provide a low impedance path for ground fault currents, especially if you want the OCPD to open under fault.






Rafiq Bulsara

http://www.srengineersct.com

 

[jghrist]

If the substation is the source of the distribution line, then the line neutral will be bonded to the substation ground grid anyway. There will be a possibility of transferred voltage at the distribution pole whether or not you ground the service neutral at the station.

Even if the distribution line comes from a different source and the neutral is not connected to the grid, you may still have a transferred voltage problem. If you do not ground the service neutral at the station, then during a high side ground fault, you may have a high voltage between grounded station equipment (which will be at the station GPR voltage) and the secondary neutral (which will be at some remote ground voltage). This situation would be similar to bringing a phone line into the station without isolation.