High Voltage Ground Grid Modifications 1

[kmh1]

I would like to obtain some suggestions on addressing possible safety hazards associated with making modifications to high voltage (115kV) switchyard buried grounding grids such as when new equipment is planned to be installed and/or the yard expanded. Common sense tells me that any connections between bare copper conductors should be made with the yard de-energized or while wearing voltage rated gloves. Does anyone know of any specific procedures for this type of work? I don't see anything within NFPA 70E.

 

[mc5w]

If you connect a 4/0 copper temporary bonding jumper around the ground wire that you are tapping or splicing, I do not see a problem. You would want to be standing on a rubber mat and most definitely do NOT be doing this when it is raining or the cloud ceiling is very low. 97% of lightning damage in a telephone cable plant comes from silent lightning and invisible lightning. When a Big One does strike, you have about 100 side strikes over a 100 yard radius. I also know somebody who had their truck tires incinerated by clear weather lightning during a drought in 1987.

Also, this would be a good opportunity to check the condition of ground rods and the grounding grid and fix anything that is wrong. A lot of things can deteriorate. There are some places here in Ohio where the well water eats copper tubing and all water heater warranties are VOID! The acid water is caused mostly by a source of sulphur and sulphur-metabolizing bacteria that convert hydrogen sulfide into sulfuric acid. Yet, a mile away or so there is nothing wrong with the ground water.

Using exothermic welding to make the tap or splice would be the best way for several reasons:

1. The amount of time spent handling the existing conductor is minimized which helps with safety problems.

2. Crimping really only makes contact with the outside wire strand layer. Exothermic welding gets around that problem particularly with older cables.

3. You do not have to cut the existing ground cable when doing exothermic welding. I would run a temporary bypass cable just in case a ground fault occurs while the joint is cooling.

Hot line gloves would help but you also need an insulated rubber mat when setting up the exothermic welding mold. For your situation, there could be up to 40 KV of voltage drop if the existing ground grid has old age defects.

 

[jghrist]

If an existing conductor is broken or cut, there might be a voltage between the two ends if one part is isolated from the rest of the grid. In this situation, and where you are making the first connection between a new grid and the existing grid, a temporary ground jumper should be installed. See IEEE Std-80 ¶18.5.

In general, the buried ground wire is at the same potential as all of the substation steel, the fence, and equipment frames. Would you use insulated mats and gloves, or de-energize the station when working on steel structures or the fence from the ground?

 

[kmh1]

One concern that I have is working on the new exposed grid after it has been bonded to the existing grid. The touch potential with no copper in the ground beneath your feet or proper surface covering may be high.

 

[IASMike]

A major consideration when working on the grond grid is your step and touch potential. One thing to keep in mind is that these potential are allowed to reach the defibillartion level under fault conditions on your system. So with tis in mind you should treat them as if they were live wires.

 

[jghrist]

Are there any published guidelines for working on grounding systems in energized substations? It is true that you might not have the designed insulating crushed rock surface beneath you when working on the new grid conductors, and may not even be standing if making a connection to a grid conductor in a trench. Highest touch potentials in a substation are near the center of meshes, however. When working on a grid conductor, you will be by necessity near the grid conductor instead of in the center of a mesh. You most likely would be in more danger from touch potentials while working on a grounded steel structure in the middle of a mesh before the final crushed rock surfacing is applied to the new area.

I may be wrong, but I don't think that most electricians would use high voltage insulating gloves while laying and connecting ground grid conductors.

 

[jghrist]

I asked our Harger Lightning & Grounding rep about adding ground grid to an existing station. Harger's chief engineer believes that high voltage gloves should be worn for personnel safety. A lesser alternative would be to not connect the existing grid to the grid extension until the very last step, after all work is completed. This would greatly lessen the chance of a direct shock through the existing grid.