Equipment ground cable installation

[bxny]

We are designing the grounding for roof mounted HVAC units for an industrial building. The roof will consist of precast concrete planks covered by rigid insulation and topped with a membrane. The equipment grounding cable will be bare copper. In the interest of minimizing the number of roof penetrations, we are considering laying the equipment grounding cable on top of the precast concrete planks and below the rigid roof insulation, and connecting to each of the HVAC unit enclosures that would be grouped together, which would form a loop requiring only one penetration down to the next floor. (After this long introduction, my apologies) my question is, should we be concerned about the creation of a fire hazard by having the bare ground wire in contact with the rigid insulation?
Thanks, in advance for any and all responses.
Bxny

 

[EEJaime]

I am not sure what about the HVAC units would make you want to establish a seperate ground grid, but it seems to me that you are on the wrong road. Do you not have an equipment grounding conductor in each power feed to the units? Why would you not bond the equipment enclosures to the equipment grounding conductor that is part of the electrical distribution system? Establishing an isolated ground grid for equipment seems dangerous and inneffective. What path to earth is taken by any ground currents present? Potential differences between your equipment ground of the electrical system and this seperate "ground grid" will very possible seek alternate paths which would potentially lead to hazards to personnel. If the single point of penetration to the next floor is to bond this to the equipment ground system then I don't know what you have accomplished by running this ground conductor on the roof. I am afraid I just do not understand exactly what it is you are attempting. I think more information is necessary before any recommendations can be made.

 

[dpc]

One of the purposes of the equipment grounding conductor is to provide a low impedance path for ground fault currents. To accomplish this, the best place for the grounding conductor is inside the conduit with the feeder. This is basically required by the NEC. If the grounding conductor is separated from the phase conductors, especially if you have steel raceways, the impedance seen by a ground fault goes up dramatically and makes the fault current much more difficult to detect and clear.

What you are talking about is a supplemental grounding system that cannot replace the equipment grounding conductors. As mentioned, if you elect to install this grounding system, it must be bonded to the building ground system, preferably at multiple locations and also bonded to the equipment grounding conductor at each HVAC unit.

But if you run a proper equipment grounding conductor (green wire) you don't really need anything else per NEC.

As for the fire hazard issue, I don't really know - sorry.

 

[bxny]

Sorry. I meant supplemental ground, which is bonded to the building grounding system.

 

[davidbeach]

What do you anticipate achieving with your supplemental ground? It really wouldn't be serving any purpose except adding cost to the project. Spending the same money to up size the ground conductor with the supply circuits would provide much better grounding at the same cost.

 

[bxny]

My intention here is not to debate whether using a supplemental ground is appropriate. My real interest here is whether a fire hazard would exist.
Thanks for all your input.
bxny

 

[alehman]

If there is a bad connection that happens to be exposed to fault current (or lightning current), then I would think there is some risk. I wouldn't want it there, if for no other reason than simply because it is impossible to inspect or maintain. What does the roofing installer think? Why not put it on top? This sounds a bit like a lightning protection system.

 

[dpc]

Normally, when lightning protection systems are installed, the bare copper cross connections just set on top of the roof. I agree with alehman that it would be best to keep them where you can see them, or protected in a conduit.

 

[jghrist]

I agree with alehman also. Put it on top. Another reason would be to reduce the number of roofing penetrations needed, and keeping the chance of leaks to a minimum.

 

[bxny]

We will have a lightning protection system on this roof, as well as the supplemental grounding for the roof mounted HVAC equipment.

 

[davidbeach]

Why not just use a solid copper roof?

 

[alehman]

NFPA780 requires that the rooftop HVAC equipment be bonded to the lightning protection system, and will probably require air terminals mounted on the equipment. I don't see the point of the supplementary grounding.

 

[stevenal]

bnxy,

Why not share your reasons for the extra grounding? The forum works best when information travels both ways. Maybe those confused by your application can learn something.