Fence Isolation Joint

[cuky2000]

Usually isolation joints on substation fence are installed under overhead lines. This comprises in a fence panel not grounded and isolated for the rest of the grounded fence with post insulator often rated for 95 kV BIL.

This appears to be used in the event of the falling of an overhead live line to prevent additional hazards such as transferred potential to remote location.

Since the grounded fence is coupled to the earth and also a possible arc in the post insulator, I do not see how effective this isolation panel work.

Does anybody have a technical explanation of the effectiveness of the fence isolation joint panel?

Thanks

 

[waross]

Good to see you back, cuky2000.
Substations that I have worked on have typically used a buried grounding conductor that was connected to the main grid, 4/0 or larger, parallel to the fence and connected to the fence mesh and the top barbed wires at 20 ft to 50 ft intervals. Should there be a concern that induced currents may circulate in the fence and the grounding grid, isolation may provide some mitigation.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jghrist]

I have not seen this done in substations except at power plants where the substation fence is sometimes isolated from the rest of the plant fence. I don't see that it would prevent any hazards as long as all sections of the fence are bonded to the substation ground grid. If both sections are bonded to the grid, then they are not isolated. If one side is not bonded to the grid, then there is a possibility of high touch-potentials to the fence for a ground fault in the substation.

 

[Inrushman]

I understand that the fence continuity must be sustained for personnel safety & fault identification in case of line dropped on SS fence!

 

[stevenal]

Cuky,

To control touch potential, the ss fence is bonded to the ground mat which extends a few feet beyond the fence. The problem occurs when there is an adjacent fence. We must either extend the bonding and mat to cover the entire adjacent fence, or isolate. If we provide a small air gap, a person touching both fence posts during a ground fault may be shocked. The air gap must be extended to exceed an arm span, so now the adjacent property has a security issue. The solution is an isolated fence section between the ss fence and the adjacent fence. Between fences you now have two of these 95BIL insulators to prevent touch potentials on the order of hundreds of volts.