MOBILE SUBSTATION ITEMS THAT NEEDS TO BE GROUNDED BASIC INFORMATION AND TUTORIALS



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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