Grounding and Bonding for 4160V distribution 1

[Steve 854]

Planning some work for a small factory. The power company will be replacing and moving a 35KV to 4KV outdoor transformer. They will do all the grounding and bonding required for that work.

However, I'm doing the design for the 4160V distribution after it leaves the power company transformer. An underground feed will run to a new 4160V switchgear located inside the building. 4160V feeders will run from the switchgear to new and relocated dry type transformers/substations. All of these transformers/substations are also inside the building - some at grade and some on mezzanine levels.

I'm trying to figure out the grounding and bonding requirements at each transformer, and also at teh 4160V switchgear. Do I need to specify some type of ground grid at each location? DO I need to bond every metal door and metal object in the room with large wire(like 4/0)?

Or are the requirements for these transformers pretty much the same as for a transformer with 480V or 208V on the secondary?

I guess I confused on the differences between outdoor utility installations, and what is required for interior transformers.

I've tried to read some references, and I see notes about calculating step and touch potentials. DO I need to work through all those calculations?

Thanks;
Steve

 

[Kiribanda]

It depends on whether your 4160V distribution is solidly grounded or HRG and I believe that all these transformers are provided with cables.
That means depending on the neutral grounding system, you have to bond all the cable bonding conductors to the ground side of the NGR
or to the ground terminal if 4160V network is solidly grounded (system ground). At the same time you have to bond all your metallic enclosures of the
transformers and other extraneous conductive parts to the building grounding system (safety ground). Same for 480V & 208 distribution.

 

[Steve 854]

Yes, the 4160V will be solidly grounded?

Do I need to install a ground grid around either the 4160V switches, or the transformers? Or is a typical ground consisting of three ground rods, a concrete encased electrode, and a water pipe enough?

 

[JezNZ]

1. Start with basic functional earthing.
2. Calculate the fault loop impedances.
3. Calculate your earth fault return current.
4. Determine protection operating time.
5. Determine tolerable safety limits.
6. Determine EPR (Earth Fault Return Current x Earth Grid Impedance).
7. If EPR < Safety Limits, done.
8. If not, then you need to start undertaking a detailed earthing study.

 

[Kiribanda]

As mentioned before if you have 5kV cables and if they have bonding conductors which are bonded to system ground
at the source side and to extraneous & exposed conductive parts at the load side, then there is no step & touch voltage issues.
Design a simple grounding system with few rods so that your GF current will not exceed 3000V GPR during a ground fault.

 

[Steve 854]

That's what I kind of understood from my research. That having the ground conductor really reduces the touch and step potential's.

I did a quick calculation using the reactance of the 1/0 ground wire back to the utility transformer,and using the max. fault current. That gave a total voltage drop across the ground wire of about 520 volts.

If I assume that same voltage is distributed evenly through the earth back to the utility transformer, its less than 3 volts per foot.

So I don't think step and touch voltage should be an issue as long as everything in the equipment room is grounded.