MV Switchgear Ground Bus Connections to Ground Grid

[milwaukeebob]

Hello,
Have a question about a line-up of medium voltage (MV) switchgear. This is one of longer line-up's of gear I've encountered in my career measuring almost 40' in length. The main bus is rated 15kV, 2000A, 25kA, 3PH-3W and consists of seven (7) feeder breakers from very small generators and three (3) tie/ring-bus breakers. This is the first time I've ever seen a MV line-up like this with only one (1) ground-grid riser cable connection to the main ground-bus in the switchgear. This single 4/0AWG ground-grid riser connection to the MVSG ground-bus is essentially on the far left side of the line-up. When it comes to short or long MVSG line-ups, my experience suggest two (2) ground-grid connections appropriately positioned/connected to the ground-bus such that fault current associated with any of the sections has the shortest and least resistance path to ground.
I'd appreciate any additional insight on this topic from other forum members. Thanks

 

[che12345]

Mr. milwaukeebob (Electrical)(OP)31 Jul 23 20:19
"...#1. a line-up of (MV) switchgear..... almost 40' in length. #2.the main bus is rated 15kV, 2000A, 25kA, 3PH-3W .... with only one (1) ground-grid riser cable connection to the main ground-bus in the switchgear. #3. This single 4/0AWG ground-grid riser connection to the MVSG ground-bus is essentially on the far left side of the line-up. #4. When it comes to short or long MVSG line-ups, ...... suggest two (2) ground-grid connections appropriately positioned/connected to the ground-bus such that fault current associated with any of the sections has the shortest and least resistance path to ground ".
I look at it as following for your consideration.
1. There is NO limitation on the length of MV switchgear.
2. with only one (1) ground-grid riser cable connection to the main ground-bus in the switchgear is allowed. No standard specifically mentioned that there shall be at least two separate grounding points, irrespective of the length.
3. Attention: The 4/0AWG (copper?) ground-grid riser is UNDER sized! consider the breaker is rated 2kA. Note: Suggest copper > kcmil 250.
4. No standard "insist" or states "shall be" having [two] grounding points.
4.1. It is immaterial whether the grounding point is at the left or right extreme end or middle, as the resistance of the ground bar is << ground resistance.
5. Reference: See NEC Art. 250.....for detail
Che Kuan Yau (Singapore)

 

[waross]

7 Very small generators won't supply much fault current.
Voltage collapse and getting enough current to properly trip may be a bigger problem.
However what does the Code say?
The Canadian Electrical Code specifies a Minimum of 250 MCM for grounding equipment fed by 2000 Amp bus bars.
(The Canadian Electrical Code specifies a Minimum of 1/0 for grounding equipment fed by 2000 Amp cables.)

I hope that each generator is also grounded, the size of grounding conductor determined by the Ampacity of the generator feeder.

Further in the code: If your system has a grounded neutral, this would be the size of the jumper from the system neutral to the ground bud. The code assumes that the greatest part of a ground fault current will flow through the equipment grounding conductor and little or no current will return through the grounding electrodes.
The ground grid serves to develop an equi-potential zone for the protection of persons.
In some cases the conductor from the ground bus to the ground grid need only be #6 AWG copper.

Your installation may be compliant with your local codes.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[milwaukeebob]

Che12345 and Waross, thanks for your feedback. Understood. I've never seen only one (1) ground-grid riser connection to a MVSG bus in my 25+ years. Thus the reason for my question. One important point I need to make is that I'm coming into this project very late as an "observer". I in no way, shape, or form was involved with any of the design or installation.

Waross, thanks for the CEC references...interesting. This is a US project (and a difficult and unique one at that) and unfortunately. Each generator is grounded appropriately as you indicated. Also, yes the generators each have a resistance grounded neutral.

thanks again for your insight.