Equipment Bonding

[PhaseShift]

This is a situation that usually comes up in our facility.

We have a room in which we have several pieces of equipment which are basically several 480V motor mounted on skids with other piping etc. There are two of these skids on one side of the room and two on the other. The skids themselves sit on a concrete slab floor. All 480V branch circuits to skids have EGC's as well as rigid conduit which are all bonded to skid.

The concrete slab consists of rebar which is tied into the plant grounding, building steel etc.. On one side of the room there is a 4/0 bare ground conductor coming up from the slab. This ground can easily be connected to the skids on one side of the room. It has been discussed running conduit all the way over to the other side of the room just to tie this ground into the other skids. My questioning is weather or not this is necessary.

With the EGC's providing a fault clearing path, I see the purpose of this bare ground as providing bonding in order to provide an equal potential surface across all areas of the room. However with this ground all the way on the other side of the room I dont know if its necessary to run it all the way over to the equipment on the other side of the room in order to create this equal potential bonding. Here are some quesions I have.

With the skids sitting on the concrete slab and the rebar within the slab all being bonded will the resistance between the skid and the slab be small enough to consider the skid bonded to the slab?

Since we are only concerned with a step and touch potential within reach can we simply bond everthing on one side of the room together and not really be concerned with bonding it to equipment on the other side of the room which cant be reached? Does the NEC provide distances for bonding?

How do we know if additional bonding is even required? Can we take resistance measurements between skids to see if there is a small enough resistance? Can we take the measuremtns between skid and slab?

As long as we find something near the other side of the room to bond skid to (column, floor, etc..) would this provide an equal potential plane since all of the infastructure is bonded together?

Would the fact that all conduit and EGC's go back to a common pull box, and MCC be sufficient for bonding everthing together and therefore bond the whole slab togeter.

What is typically the rule of thumb for equipement bonding. People here are always quick to say that these skids need to be bonded to ground wire but in any given area of facitly there are disconnects, lighting, fans, etc.. that all have EGS's but are not necessaryily bonded to other equipment. I find it hard to believe that every piece of equipment in any given area has to be bonded together. What is typically the rule of thumb here? For instance if I touch a fan or disconnect on the wall that lost its EGC and is not bonded to slab what is done to eliminate this touch potential?

I appreciate any help on any of these questions.

 

[kingpb]

First, the NEC is not going to give you design parameters as it is a document for minimum requirements for safety. The practices that you need to follow are in IEEE 142 (The Green Book). Measuring is coverd in IEEE 81. The issue of step and touch potentials is covered in IEEE 80, although this is for Substation Grounding. There is some discussion in IEEE 142 about this but it references back to IEEE 80.

As far as your questions, the way I am reading it, sounds like there may be concern that the steel in the slab is not tied to building steel? By connecting the equipment back to the same ground conductor it may be that they are wanting to guarantee it is grounded, and it is not necessarily for bonding purposes, although tied to the same point effectively does both. I would think, the way you describe it, is that the skid connected to building steel would work just fine.

This question probably belongs in grounding versus bonding. Perhaps a moderator can move it.

 

[IslandSparky]

If you are indirectly bonded through the piping, my preference is to bond the equipment as well. My mentor was of the "Ground the L out of It" school of thought. But, if you are concerned with circulating currents, you can rationalize your way out of it, but watch for bearing damage.

Either way, not legally required.

 

[PhaseShift]

I came across this diagram in section 250.50. for a grounding electrode system. Does this mean that every device, switch, panel, etc... needs to have a grounding electrode connected to the ground grid?

 

[PhaseShift]

Forgot attachment!