Earth Grid (Ground grid) proximity to diesel storage

[ahandle]

Hi,

To comply with the requirements of AS3007 for surface mines, our earthing (grounding) expert has decreed that we need separate earthing for our 3.3kV distribution, away from our 33/3.3kV switchyard.

The issue is, about the only available real estate in the vincinity is about 6m away from a diesel tank of approximately 20,000L. It is understood there is also a diesel pipeline in the area, plus some years ago there was a large spill, leaking diesel all through the surrounding soil.

Does anyone know if there is any danger (or any regulations) by having the earth grid near the diesel tank (in terms of ignition)? Is there any chance of ignition if the grid is installed in diesel-soaked soil? Is there any issues by having the earth grid near the diesel pipeline?

Fault current is limited to 50A by an NER, and the earth grid is 500mm below ground level (which is asphalt). As I see it, any fumes from the diesel storage tank are above ground and thus not affected by the ground grid, and there is no oxygen in the soil to ignite the diesel-soaked soil. I can see a danger by bonding to the diesel pipeline as the transferred voltage may ignite the diesel at some other point, but if the earth grid is just near but not bonded, is this an issue?

Is anyone able to provide any thoughts on the above please? Can diesel ignite below ground?

Many thanks

 

[alehman]

Ignition required air. It seems unlikely there would be sufficient air underground for the fuel to burn. It could conceivably be ignited if present at the surface.

Seems like you need to clean up the diesel-soaked soil however.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[juleselec]

There is no danger with an earthing grid near the diesel tanks for at least two reasons:

1) The earthing system is not considered an ignition source. If anything, you should be more concerned about static earthing and equipotential bonding than transferred potentials causing a spark due to a fault. In the oil and gas industry, earthing grids are pretty much installed anywhere in the presence of flammable atmospheres.

2) Diesel fuel is generally not considered to be hazardous as long as it is handled below its flashpoint of 55 deg C - refer to IP 15 (3rd edition) Annex A1.3, Class III(1) materials can be classified non-hazardous.

 

[cuky2000]

The earth conductor should be sized such as the maximum temperature during fault event could prevent fire hazardous conditions.

The IEC and IEEE standards use similar approach to determine the grounding (earthing) conductor temperature during fault scenario.

The enclose excerpt document is suggested as a starting point to provide the engineering support to justify a safe earthing grid for this application.