Grounding Mat

[flengineer]

Does anyone know what a grounding mat is? I can't find any reference to it in the NEC. I have equipment specifications that call for a #4/0 EGC from the "building grounding mat" to an equipment room.

I was providing the 4/0 EGC from a ground bus in the Main Electrical Room tied to the grounding electrodes (two ground rods, cwp, foundation steel), but now think this is referring to some a grid of grounding conductors buried beneath/in the slab.

 

[nakulak]

We have had to connect grounds from anti-static grounding mats in areas where ammo was handled. they have another name for the mat, but I can't think of it off hand. I suppose they could also be talking about an equipotential bonding grid, if that applies. there may be other types of grounding mats, would be helpful to know the application.

 

[flengineer]

This is for a train station. The train is powered by DC guiderails. The specs are calling for three grounds from the building mat.

One is going to a room that contains control equipment. Another grounds the guiderails. The last is going to the equipment room that feeds the UPSs, inverters, etc. to power the train. In the last case, I guess it would actually be a GEC, not an EGC since this is where the mains are located. (Train receives power from a separate substation, apart from the building service.)

They're definitely not talking about an anti-static mat. Is an equipotential bonding grid the same as an equipotential plane? I found references to that in 547 - Agricultural Buildings and 682 - Natural and Artificial Made Bodies of Water, but theres no indication on how to size them.

How is it sized (physical size and gauge of conductors)?

 

[zog]

Grounding mats are used in switchyards all the time. Everything in the yard is bonded to the same point (ground grid), including the fence, transformer, structures, switches, etc. The areas where an operator would operate equipment has a ground mat for them to stand on. IF something bad happens, everything metal in the yard will be at the same potential.

 

[jghrist]

Grounding mats are used in switchyards all the time. Everything in the yard is bonded to the same point (ground grid), including the fence, transformer, structures, switches, etc. The areas where an operator would operate equipment has a ground mat for them to stand on. IF something bad happens, everything metal in the yard will be at the same potential.

I would generally agree with this, but the station ground grid is also sometimes referred to as the "ground mat". From the context of the OP, this is probably the intent. There may not even be a grid of conductors, but connecting the EGC to the building grounding electrode system is what is required, regardless of what comprises the electrode system.