12 kV Substation Grounding Question

[JohnB85]

Hi all,

I have a ground problem that I believe I have solved, just not sure how to prove it. We have installed some manual fire suppression sprinklers in a cable pulling area of a sub - basement area where XLPE cables enter sub. Sub is a 138 to 12 kv yard with 24 feeders and 3-4 transformers.

The sprinkler is tied to a bus bar running the perimeter of the cable pulling area in two places. If the fire fighters goes to connect to the FDC and a fault occurs, would they be subject to touch potential (see diagram). They could be standing inside the yard (withing the ground grid) or outside the yard, as the building wall acts as a fence.

I did not have a large concern the touch potential shock harard, but to remove any touch potential issues, I am installing isolation flanges used for cathotic protection. I was told they were good for 500 volts (Drake Specialty Products has data sheets - listed as 500 vpm). I felt 500 V was an approriate value, since the phone line out in the first manhole is suppose to be 300 volts or less.

Am I looking at this correctly? Is the fire fighter at risk to begin with? Does the flange help? Any advice on how to better prove my theory.

Thanks,

John B.

 

[jghrist]

If I understand the scheme correctly, the "yard" is a small area around the building which has a perimeter ground wire. All equipment, 138 kV and 12 kV is indoors (power transformers also???). The fire suppression equipment is bonded to the perimeter ground.

If this is the case, then I don't think there is any difference between the fireman touching the FDC (abbreviation for what?) and anyone outside the building touching the building. The building and the fire suppression equipment are both at the station grid potential. If the difference between the surface potential outside the building at a point where someone could touch the building or the equipment exceeds the allowable touch voltage, there is a problem. The flange may protect against transferring the station ground potential to a point further away from the building, like to a tanker truck, but if the fire hose is non-conductive, this would make no difference. The conductivity of the water in the hose is not affected by the flange.

 

[davidbeach]

FDC=Fire Department Connection.

 

[JohnB85]

Yes the transformers sit outside in the yard. The yard has a latice ground grid.

The yard also consist of crushed limestone. So if the connection point is in the yard, you are also standing on crushed limstone. We have 19 of these sites and at some of the locations, the FDC is not in the yard, is outside, as the building is flush with the property line.

FDC = Fire Department Connection. The 4 inch fire hose connects to this.