Industrial Ground Grids

[Electrobe]

In most industrial buildings there is typically an underground ground grid that consists of mutliple ground rods and ground rings. The building steel and other items are typically bonded to this system. What codes determine the layout and size of this grid?

With the above system what needs to be attached to this system? Example: 460V motor. Motor is fed from MCC which has a ground bus bonded to this grid and the WYE transfomer is also bonded to this grid. If using type MC cable (3/C w/gnd) to feed the motor and the ground is attached to the motor and also to the MCC ground bus. Does this motor also need to have a ground ran directly from the ground grid to the motor?

Thanks

 

[SG-1]

In most industrial buildings there is typically an underground ground grid that consists of mutliple ground rods and ground rings. The building steel and other items are typically bonded to this system. What codes determine the layout and size of this grid?

The NEC does not specify the type of grounding electrode system a building gets, an engineer hopefully choose this based on soil resistivity, and other factors.

 

[SG-1]

The EGC (gnd) from the MCC should be sufficient if properly sized & the MCC is properly installed. The MCC ground bus should be connected to the building grounding electrode system. Re-grounding is not necessary or desired. You may create ground loops & provide parallel paths for fault current to travel over that are not desired.

 

[SG-1]

The code does specify in 250.50 that any grounding electrode present must be bonded together to form the Grounding Electrode System. 250.24 specifies the Grounding Electrode System is connected at the service.

 

[drbond24]

In most industrial buildings there is typically an underground ground grid that consists of mutliple ground rods and ground rings. The building steel and other items are typically bonded to this system. What codes determine the layout and size of this grid?

With the above system what needs to be attached to this system? Example: 460V motor. Motor is fed from MCC which has a ground bus bonded to this grid and the WYE transfomer is also bonded to this grid. If using type MC cable (3/C w/gnd) to feed the motor and the ground is attached to the motor and also to the MCC ground bus. Does this motor also need to have a ground ran directly from the ground grid to the motor?

Thanks

One of the IEEE color books provides guidelines for this. Sorry, don't remember which one. I'd like to think it was the grounding book, but it doesn't always work out that way.

 

[__dan]

Earth grounding and equipment grounding

Earth grounding and equipment grounding

Earth grounding and equipment grounding are two separate functions or types of grounding.

The motor ground is an equipment ground and its function is to provide a low impedance path back to the source breaker to facilitate breaker tripping in a ground fault. It goes to the source panel equipment grounding busbar. The lowest impedance path will be the run with the circuit feed.

There is a code reference against haphazard grounding, like going to an earth ground when the correct path is to the source panel run with the circuit feed.

250-4, electrical systems are required to have an earth ground (don't have the code book in front of me).

1. All ground electrodes are required to be connected together. The building steel and interior piping connect to the earth ground system.
2. The system neutral of the transformer secondary connects to the earth ground system with a single point of attachment only per 250-30. At the specific point where 250-30 is satisfied, the equipment ground bonding jumper, grounding electrode (earth ground) conductor, primary side equipment ground, and system neutral single point of attachment all connect together on a common grounding busbar.
3. Antennas require single point connection to the earth ground system
4. Lightning protection requires a single point connection to the earth ground system.
5 The electrical equipment grounds are bonded to the earth ground system (multiple haphazard points of connection). In your example the motor would be bonded to the earth ground but the code requires "equipment grounding". Best bet is to make secure intentional bolted connections rather than laying wire in loose running contact.
6. The earth ground system is also used for a signal reference for electronic equipment, ie a computer rack could have an earth ground attachment in addition to the required equipment ground attachment.
7. Steel additions to the building are connected to the earth ground system

I'm sure there are others

 

[dereckbc]

What codes determine the layout and size of this grid?

None this is an engineers problem to figure out. Grids are useful in several applications. The two I am familiar with are substations and data centers, and they have completely different functions.

In a substation the grid size is determined by the spacing of the equipment, so that structures can be directly bonded to it without laterals. The purpose is strictly life safety to clear high voltage faults. Next time you see a substation take note and look at the switch gear and service points where personal are required to be to operate the equipment. You will notice steel platforms for them to stand on, and think why?

In a data center there are two design principles both based on a signal reference point. One is spacing again for the equipment aisle so the equipment racks can be bonded with the shortest possible length of cable. The other method is designed on a frequency cut-off point based on 1/4 wave lengths. However both these methods are completely antiquated and there usefulness nullified by modern digital and optical signal transmission.

 

[Pierre C Belarge]

If a ground ring is installed, it has to follow the minimum requirements of 250.52(A)(4), and 250.50.

Unless it is being installed to satisfy 250.54.