Indoor Unit Substation Grounding

[mhuckaby]

We have an industrial customer that wants to put an indoor unit substation indoors. The specs are:

Square-D unit substation
13,200V primary fused switch
13,200V-->480/277V 2500kVA transformer
3000A main switchboard (42kA interrupting rating)

This equipment will be located on the 2nd floor of a steel structure. The unit substation itself would be placed on a concrete housekeeping pad (over a concrete floor).

Do we need to place a ground mat (embedded in the concrete) under this equipment in order to minimize any touch potential (per IEEE 80)?

 

[dpc]

Probably, if it's solidly-grounded. The reinforced concrete floor is a pretty good conductor and will help some, but you'll have to run out the numbers. A 2500 kVA 480 winding can produce a heck of a lot of fault current.

Is this an existing floor? I doubt that the housekeeping pad will be large enough to eliminate touch potential concerns.

Assuming a 5.5% transformer, I get about 54,000 A rms at 480V, without considering motor contribution. I'd be concerned about the 42kA rated breakers.

 

[cuky2000]

Beware that the allowable step and touch potential standing in a concrete (50 Ohm-m) pad will be less than other high resistance surface such as crush rock (3000 Ohm-m).

Even if you determine that the current injected to the ground is significantly less than 100%t, still will be a good idea to provide a ground ring loop around separated 3ft from any metalic surface and connected in 2 diagonal points to ground to create an equipotential surface.

Steel collumns will be a good path to ground requiring the reinforce any bolted connection with jumpers, but you probably will be questioned by the local approval authority.

The Electrical inspector probably will be more confortable with two independent ground path with cooper conductor and ground rods.

Good luck

 

[peebee]

While it's a good idea, I'd be surprised if any inspector required it.

Also, if this is new construction, you might be able to use the rebar & building steel to accomplish the same thing.

 

[alehman]

Since it's installed on the 2nd floor, the concrete may be poured on a steel deck. If so, bonding the steel deck and associated structure (preferrable in two or more placed) would most likely be adequate. Otherwise connections to reinforcing steel would be common for this type of installtion. I would recommend also bonding to the service entrance equipment gounding electrode system if possible, esp. if there is no continuous steel structure. If there is more than one substation, they should be bonded with a common grounding conductor.

 

[jbartos]

Suggestion: The IEEE Std 80 could be followed, e.g. Annex D with simplified step and mesh equations

 

[roachy]

Assuming a new slab is beeing poured.

1 All reinforcing mesh in floor slab to be welded together prior to pour.

2 All reinforcing mesh in wall slab/structure to be welded together prior to pour (if applicable).

3 Must have at least ONE, but recommend TWO, connections from items 1 & 2 above to main station earth bar. Proprietry physical connections are available for transition from reinforcing mesh to copper/aluminium earth conductor.


4 All conductive structural steel shall be equipotentially bonded and ultimately terminate at the main station earth bar.

5 The above items are NOT a replacement for the main earth electrode system which you must have a minimum of two electrodes connected to main earth bar.

6 The main station earth bar should be installed within the switchroom to which the above conductors are connected.

7 A grading ring 1 metre outside the edge of the building should be installed at ground level.

Hope this helps.