grounding of raised floor

[RKyleM]

my company is designing the electrical system for a building. this building, an administration and classroom building for a local college, has a raised floor over the entire floor to receive LEED credits and facilitate easier distribution of communication cabling. it is not a dedicated computer room.

does this raised floor need to be grounded? we think it does per nec 250.104.D.2. it appears it does and that this grounding connection is required just at one point for "each separately derived system" and not all through out the floor.

 

[PHovnanian]

I suppose it depends on how one (the AHJ primarily) interprets "interconnected" structural metal and how "the area served by the separately derived system" is defined. For each "interconnected" assembly of structural metal in an area served by a "separately derived system", I'd say only one bond to the grounding point is required. But that structural assembly might be large enough to be served by several separately derived systems (from multiple step down transformers and panels, for example). In this case, there will be one bonding jumper from each system to one point in the floor structure in the area served.

If for some reason the structure is not interconnected, then there are two solutions: Run a separate bonding jumper for each structural assembly (my words, not in the code) to the system grounding point. Or electrically bond the structural sections together. There may be cases in which it is desirable to mechanically isolate structural elements. In this event, flexible bonding jumpers can interconnect them.