No ground going to the panel

[bjenks]

Need some sanity checks. I am looking at a 15 story condo that has the MDP on the first floor. There is a 600A breaker that feeds (2) sets of 350kcmil conductors up to the top floor's Rooftop panel. The conduits leave the MDP in a two PCV conduits and only have three conductors and one neutral, NO Ground wire. At the Rooftop panel, I see a metal conduit coming in to the panel with the phase conductors and the neutral, no ground conductor.

Neutral and ground are isolated and the neutral goes to the neutral bus. Then all the loads ground go to their load tapped off the ground lug.

 

[bjenks]

Part of the message is missing, so I guess I should ask the question again:

They are complaining of the single large RTU's phase/VD protector sensing a problem and after 5 resets disable the load. The cost to fix run a ground conductor up the the panel in the two conduits is enormous. I measures the N-G voltage at the rooftop panel and got 0 volts. I am wondering if a lightning storm could be causing the neutral-ground to float and thus tripping the relay. I would hate to say to pull a ground conductor up and it will solve the problem. Am I going down the right path or barking up the wrong tree?

 

[resqcapt19]

If you have access to building steel, the easiest fix would be a one to one ratio delta/wye transformer and bond XO to the building steel.

 

[bjenks]

So I would take the existing neutral conductor and convert it to an equipment gounding conductor. Then take the X0 and bond it to building steel. That is a great answer!

 

[wareagle]

Bjenks quote"So I would take the existing neutral conductor and convert it to an equipment grounding conductor. Then take the X0 and bond it to building steel. That is a great answer!"
I do not think what resqcapt19 is saying. Use the transformer to power the RTU. If you convert the neutral to ground you have created another problem.

 

[bjenks]

I don't need the neutral going in to the transformer, only going out. However, I do need a equipment bonding conductor sized to 250.122 running back to the ground of MDP where my 600A feeder breaker is located right? Then per 250.30 the transformer is a seperately derived system I have to bond the neutral to ground in the transformer and then run a grounding electrode conductor to building steel sized at 250.66.

Right now there isn't any equipment grounding (bonding) conductor going up to the rooftop distribution panel, but there is a neutral. I was going to bond that to the transformer ground and then go back to the MDP and take the neutral connection of the feeder and bond it to the ground bus, which is where the main bonding jumper is.

Am I doing this wrong or did I not originally explain the problem correct or maybe I missed something still.

 

[resqcapt19]

bjenks,
Yes that is exactly what I was saying. You would use the existing neutral as the EGC for primary of the transformer and refeed everything from the secondary of the transformer. This, when correctly installed, will provide the required equipment grounding of the roof equipment and may be a less expensive fix than pulling an EGC from the main. This would all hinge on the availability of a code suitable grounding electrode at the roof. If this is a steel frame building, that should not be an issue.