Interior Ground Ring?

[Chris3585]

I am working on designing a grounding system for the building. We would like to install a ground grid to bond ever other steel column to ground (including interior columns). Normally, we route the ring around the exterior of the building. The existing building is pre-cast concrete with no slab so installing a ground ring around the interior to connect the grid to would be easier than trying to get it outside of the building. Is there an issue with not extending the ground ring outside of the building?

 

[Smart $]

... Is there an issue with not extending the ground ring outside of the building?

Depends on whether you have a compliant GES without using an NEC compliant ground ring. If yes, then no problem. If no, you'll either have to use an exterior ground ring, or install other electrodes to make a compliant GES.

 

[Chris3585]

I think so. I will drive three ground rods 10' apart at the electrical service (outside the building). We will then ground to the building steel and to the water main.

Do you know why most people install the ground rings around the exterior?

 

[GoldDigger]

I think so. I will drive three ground rods 10' apart at the electrical service (outside the building). We will then ground to the building steel and to the water main.

Do you know why most people install the ground rings around the exterior?

So that it is in soil contact over its entire length. Otherwise it would just be linking lots of electrodes together.

 

[eric9822]

I think so. I will drive three ground rods 10' apart at the electrical service (outside the building). We will then ground to the building steel and to the water main.

Do you know why most people install the ground rings around the exterior?

250.52(A)(4) Ground Ring. A ground ring encircling the building or structure, in direct contact with the earth, consisting of at least 6.0 m (20 ft) of bare copper conductor not smaller than 2 AWG.

 

[GoldDigger]

250.52(A)(4) Ground Ring. A ground ring encircling the building or structure, in direct contact with the earth, consisting of at least 6.0 m (20 ft) of bare copper conductor not smaller than 2 AWG.

Putting it in direct contact with the earth inside the building could be a problem too, even if the code did allow it.

 

[Chris3585]

Would it be against code to install the ring on the interior of the building? I'm not sure how we would core through the pre-cast to get to the exterior?

 

[GoldDigger]

Would it be against code to install the ring on the interior of the building? I'm not sure how we would core through the pre-cast to get to the exterior?

The ground ring referred to here is a form of ground electrode. You only need to have one connection to it, to whatever point the GECs come together at.
There is no reason not to create an EGC ring inside the building for ease of attachment, although it would not eliminate the need to run EGCs directly to each load along with the current carrying conductors.
There is also no reason for you not to create a set of bonding jumpers between columns if you want to, but unless any of the column connections qualify as a UFER ground, you would not have a required grounding electrode as a result. Just a bond to building steel.

 

[Chris3585]

The ground ring referred to here is a form of ground electrode. You only need to have one connection to it, to whatever point the GECs come together at.
There is no reason not to create an EGC ring inside the building for ease of attachment, although it would not eliminate the need to run EGCs directly to each load along with the current carrying conductors.
There is also no reason for you not to create a set of bonding jumpers between columns if you want to, but unless any of the column connections qualify as a UFER ground, you would not have a required grounding electrode as a result. Just a bond to building steel.

The intent would to bond to building steel for lightning protection (It is a large steel manufacturing/warehouse building), not for use in the electrical ground system. The circuit grounds will still be provided. You have a good point though. We could form a ground mat and bond to the interior columns, then exit at one point near the electrical service ground and connect to the ground ring outside the building.

 

[Smart $]

The intent would to bond to building steel for lightning protection (It is a large steel manufacturing/warehouse building), not for use in the electrical ground system. The circuit grounds will still be provided. You have a good point though. We could form a ground mat and bond to the interior columns, then exit at one point near the electrical service ground and connect to the ground ring outside the building.

A contiguous steel frame structure is only required to be bonded at one point. The structural joints take care of the continuity to the rest of the structure. Now if you have isolated steel framework, I can understand installing bonding jumpers, but unless this structure is supplied with MV/HV and a lot of associated electrical equipment (step potential issues), installing a grounding grid or mat would be overly eccentric IMO.

Also, if the service has a compliant GES, and steel is bonded per code, there is no reason to connect your grid or mat to an exterior ground ring... which from your reply to my first post seems to indicate a ground ring is not even necessary.

 

[hurk27]

The intent would to bond to building steel for lightning protection (It is a large steel manufacturing/warehouse building), not for use in the electrical ground system. The circuit grounds will still be provided. You have a good point though. We could form a ground mat and bond to the interior columns, then exit at one point near the electrical service ground and connect to the ground ring outside the building.

Is this lightning protection system being designed by a engineer or is this something the owner has spec'd

Are you a UL certified lightning protection contractor? I would be careful in how you word this kind of work in your contract as if you are not a UL certified lightning contractor, I would make sure the owners know this as they could come back on you if major damage to electronics was done in the event of a lightning strike if they are expecting protection, I would make sure you put in your contract that the work was spec by the owners or the engineer of charge and that any damage caused by lighting would not be covered.

It would have been nice if you had re-bar expose in these precast walls to bond to as they would offer you more protection by making it into a faraday cage around the building if you could bond each section to the building steel, I would not want to direct the lightning current into the building as even if it does dissipate into the earth it can still produce a EMP pulse into the circuits inside of the building that can damage electronics, you want to draw the energy outside and away from the building, most rings I have installed were 5' to 10' away from the building and at least 24" deep, putting rods at least 20' apart along the ring and using flat braided fine stranded copper cable to bond the ring back to the building metal or its electrical system GES, lightning is a high frequency event and the impedance can be very high using regular cable as the flat cable offers more surface area, keep in mind that the grounding in the NEC is not for lightning protection as many times or GEC's are too long to offer any protection from lightning because the impedance can be so high that lightning can't see or follow these paths.

Also many of these buildings will have multiple electrical paths into or out of the buildings at locations too far apart, to have a good lighting protection system require routing all conductors leaving or entering the building through what is called a ground window which acts like a choke to high frequency's as well as acting as a single conductor to lightning, which current can't flow if you only have one conductor, when we have several conductors entering a building at different locations we give the lightning a loop that its current can flow in, so there is allot to lightning protection and even the best design systems can still have some damage done in a lightning event the best we can do is keep it to a minimum by correctly designing the system and also using protection at the equipment that needs the protection such as good surge protection at both the service entrance and at the point of use.

So if you are not a lightning specialist or UL certified lightning protection system contractor then be very careful in how you offer this service as it can come back and bite you when things go wrong, make sure it is understood that you are offering to install this as spec'd by the owners or engineers and by no means are you guaranteeing it to provide the protection that a qualified lightning specialist would provide.