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Earthing question

Earthing question

Earthing question

(OP)
I have come across an earthing design (see attached) where the earthgrid does not cover the complete substation surface area. A reason I was given is that the uncovered area is not frequented by people. I am no earthing guru and so would like some opinions on the following:

I have drawn a person who I believe may experience a touch voltage should an EF occur and the person is touching the fence as shown. The pink/black line is meant to show the approximate voltage gradient. Note that the section of fence being touch is electrically isolated from the rest of the fence.

Is there any merit to my concern?

Thanks.

RE: Earthing question

You have a valid concern and the situation could be worse than you show. If a fence post of the new fence is close to the ground grid, the potential of the fence may be close to that of the grid, not the potential of undisturbed earth outside the existing grid. The touch potential may then be the difference between a point one meter inside or outside the new fence to close to full grid potential. One way to know is to model the fence posts as connected at the same potential, but isolated from the grid. Another way is to build the fence, isolate the grid and inject current to measure the potential of the new fence and points on the earth near the fence.

RE: Earthing question

I don't like "isolated" substation fence: you never know if a part of it is not –somewhere-in contact direct
or indirect with the grounded grid. I think the fence has to be "isolated" and well "insulated". The posts have to
be also insulated and a 0.08–0.15 m (3–6 in) layer of high resistivity material, such as gravel, has to be spread 1m before
and after the fence.

RE: Earthing question

Not in Canada. That is a clear violation of the Canadian Electrical Code. Note that there is also a danger to persons outside the fence and outside the control of the station authority.
36-312 Grounding of metallic fence enclosures of outdoor stations
(1) The fence shall be located at least 1 m inside the perimeter of the station ground electrode area.
(2) The station ground electrode shall be connected to the fence by a tap conductor at each end post, corner
post, and gate post, and at intermediate posts at intervals not exceeding 12 m by a conductor of not less
than No. 2/0 AWG copper.
(3) The tap conductor at each hinge gate post shall be clamped or bonded to the gate frame by a copper braid
or a flexible copper conductor of at least No. 2/0 AWG.
(4) The tap conductor shall be connected to the fence post, the bottom tension wire, the fence fabric (for
which the conductor may be woven in at least two places), the top rail, and each strand of barbed wire,
with the connection to the bottom tension wire, the fence fabric, and barbed wire strands made with
bolted or equivalent connectors, and with the top rail connections bonded at every joint by a jumper
equivalent to No. 2/0 AWG copper.
(5) When there is a metal boundary fence in proximity to the station fence, the touch voltages within 1 m of
all parts of the boundary fence shall not exceed the tolerable values specified in Table 52.

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

RE: Earthing question

(OP)
Thanks for the comments so far. Good food for thought. I have found a section in IEEE-80 which deals with the various scenarios when it comes to Fences. Will give it a good read and take it from there.

Thanks and regards.

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