What are the substation items that
needs grounding?
Fence
The fences may be either connected to
the mobile substation grid or isolated. A dangerous voltage may exist if an isolated conducting (metallic) fence
and the trailer can be bridged by a person standing between them.
Connecting the fence and trailer together will lower
the voltage between them, even if they may not be touched at the same
time. If an ungrounded conducting temporary fence
is installed adjacent to a grounded substation fence, an isolation
section may be necessary to prevent a transfer
voltage on the temporary fence.
Gate
A conducting gate should be bonded to a
conducting fence. If the gate swings outward, a loop conductor should
be
installed to control the touch voltages
when opening the gate. If the gate opens inward, no special grounding
may be
needed unless the fence is isolated
from the mobile substation’s ground grid.
Trailer frame
The trailer frame and/or trailer ground
bus should be connected to the grid with a conductor of a size
adequate to carry the available fault current. If the trailer has no
ground bus for the equipment connections, all equipment and the
trailer should be connected to the mobile substation ground grid
using separate conductors of proper size for each piece of equipment.
Multiple trailers (e.g., a switch and
fuse trailer used with a mobile transformer trailer) should be
connected via the grid or a direct cable tie to prevent voltages
between them. Caution should be taken while any maintenance is
performed on the trailer (e.g., changing or removing tires and
adjusting jacks) while the unit is in service. Movable (e.g.,
slide-out switch bases, etc.) or removable conducting parts (e.g.,
steps, dollies, etc.) should be connected to the trailer ground bus
or the ground grid, or both, to eliminate voltages between them.
Cable shield grounding
Cable shields should be grounded. If
the cable shield is to be grounded at both ends, the shield must be
sized to conduct fault current or have a separate parallel conductor
to prevent excessive current in the shield. Refer to IEEE Std
525-1992 for further guidance.
Operator platforms or plates
In the absence of a properly designed
ground grid, operator platforms or plates should be installed and
connected to the grid at all switch handles that are accessible from
the ground. In the case of a properly designed ground grid, operator
platforms and plates may be installed as a supplement to the ground
grid and connected to the grid. If the trailer jacks are to be
adjusted while the mobile substation is in service, a platform or
plates connected to the grid should be installed at the jack handle.
Neutral grounding
Neutral conductors of adequate
fault-current capacity should be installed from the mobile
transformer to the grid. Feeder neutrals should be connected to the
provided attachment point (neutral bushing) or directly to the grid.
If possible, transmission line shield wires should be connected to
the grid.
Due to the higher resistance of a
separate mobile grid not connected to the main substation grid, a
high percentage of the fault current will flow on the neutral and
transmission line shield wires. In some cases, the current limits of
these wires may be exceeded by return currents.
When the mobile substation is located
outside the substation fence, the grids should be connected together,
if practical, to lower the ground potential rise. Installation of two
or more ground cables is desirable to reduce the inductive reactance
between the two ground mats and to lower the transient overvoltages.
Temporary equipment
Any temporary equipment connected to
the trailer or equipment on the trailer should be grounded to the
grid or trailer ground bus.
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