ERICO Practical Guide to Electrical Grounding - Manual (Page 116)

Practical Guide to Electrical Grounding
most commonly used is No.6 copperbonded steel, 30%
conductivity. CADWELD connections are used to connect
the conductor between the switch handle and the mat.
(Fig. 6-45).
FENCES AND GATES
Where fences surround electrical facilities or areas where a
fence could be energized from a fault, either from within
the facility or one transferred in from attached fences or
other metallic connections, they must be grounded to
protect both the worker in the facility and the general public
who may touch it from the outside. The normal scheme for
grounding the fence is to ground all corner posts and one
line post every 50 feet (15 m). There are two methods used
in designing the fence grounding system, especially at an
electrical facility:
1. Electrically connect the fence grounding system to
the facility ground system (Fig. 6-46). This method
must be used when the fence is within or close to
the facility ground grid.
2. Use a separate grounding system for the fence,
isolated from the facility ground system (Fig. 6-47).
When the fence is tied to the grid, this increases the grid
size which reduces both the grid resistance and the ground
grid voltage rise. However, the internal and perimeter
gradients must be kept within safe limits because the fence
is also at the full potential rise. This can often be
accomplished by burying a perimeter conductor 3 to 4 feet
outside the fence and bonding the fence and the perimeter
conductor together at frequent intervals (Fig 6-48). The
conductor could be buried under the fence line if one is
unable to place it outside. But the touch potential for a
person standing one meter outside the fence would be about
60% greater than if the perimeter conductor were buried
one meter outside (see Note 1).
108
Fig. 6-45
Note 1. Based on IEEE Std 80-1986 (16.2 and Appendix 1, example
1) with a grid spacing of 8 m and conductor burial of 0.5 m.
Fig. 6-44
1/2 M + 2"
M M M
Fence
Grid
Grid
Perimeter Conductor
Fence
Grid
Grid
Fig. 6-46
Fig. 6-47
Grounding Book 4/14/99 10/5/99 6:02 PM Page 108 (Black plate)