Substation grounding bus bar

[BillMar]

Where I work, we have 8 substations that are very old. most of them have a 1/4 x 2" bus bar running along the lower part of the block walls (not all the way around) which is grounded via ground rods driven outside the building at all 4 corners with a ground wire from each to the ground bus. I was told by an engineer that the ground bus should be a closed loop all the around the substation although on one wall the sliding metal door goes from floor to ceiling and can't continue it all the way around. Is this in the code somewhere that this practice should be done? Does the bar have to be in a closed loop?

Thanks

 

[Hameedulla-Ekhlas]

Yes, your engineer is completely right and it is very important.

16. Design of grounding system
16.1 Design criteria
As stated in 4.1, there are two main design goals to be achieved by any substation ground system under
normal as well as fault conditions. These goals are
a) To provide means to dissipate electric currents into the earth without exceeding any operating and
equipment limits.
b) To assure that a person in the vicinity of grounded facilities is not exposed to the danger of critical
electric shock.
The design procedures described in the following subclauses are aimed at achieving safety from dangerous
step and touch voltages within a substation. It is pointed out in 8.2 that it is possible for transferred potentials
to exceed the GPR of the substation during fault conditions. Clause 17 discusses some of the methods used
to protect personnel and equipment from these transferred potentials. Thus, the design procedure described
here is based on assuring safety from dangerous step and touch voltages within, and immediately outside,
the substation fenced area. Because the mesh voltage is usually the worst possible touch voltage inside the
substation (excluding transferred potentials), the mesh voltage will be used as the basis of this design
procedure.
Step voltages are inherently less dangerous than mesh voltages. If, however, safety within the grounded area
is achieved with the assistance of a high resistivity surface layer (surface material), which does not extend
outside the fence, then step voltages may be dangerous. In any event, the computed step voltages should be
compared with the permissible step voltage after a grid has been designed that satisfies the touch voltage
criterion.
For equally spaced ground grids, the mesh voltage will increase along meshes from the center to the corner
of the grid. The rate of this increase will depend on the size of the grid, number and location of ground rods,
spacing of parallel conductors, diameter and depth of the conductors, and the resistivity profile of the soil. In
a computer study of three typical grounding grids in uniform soil resistivity, the data shown in Table 11 were
obtained. These grids were all symmetrically shaped square grids with no ground rods and equal parallel
conductor spacing. The corner Em was computed at the center of the corner mesh. The actual worst case Em
occurs slightly off-center (toward the corner of the grid), but is only slightly higher than the Em at the center
of the mesh.


Please See for more detail.

IEEE Std 80-2000 - IEEE guide for safety in AC substation grounding