Grounding Issues

plc noob

Member
Join Date
Oct 2007
Location
atlanta , georgia
Posts
258
I have 2 panels for 1 large system. The panels are side by side but the only physical connection between them is 4 1” chase nipples. Panel 1 is fed from the mcc to a isolation transformer which the output of the isolation transformer feeds panel 1. Panel 1 has 3 ground bars that all tie into 1 ground block at the panel disconnect area and connect with the feeder ground from the isolation transformer. The isolation transformers ground connects to the primary side ground which goes all the way back to the mcc. Panel 1 has a paint free steel interior panel which multiple drives are mounted to.
Panel 2 is also fed from the same mcc but does not go through a isolation transformer it’s ground bars also connect to it’s feeder ground t one point near the disconnect.
We are having some noise related issues with the drives and the manufacturer wants us to ground the drive panel to building steel. The machine that both panels feed and control is already connected to building steel.
My question is should both panels be connected top building steel individually or should one panel be connected to building steel and a ground wire ran through the chase nipples and both panels bonded together by means of their ground bars?
Or any other or better method for noise reduction.
 
What type distribution do you have wye or delta. 450 delta is not required to be grounded but wye is.

The system ground point is the place all grounds and bonds should be tied to. The building steel is required to be BONDED to the system ground point. Building steel should not be used as a ground point.

I do not think daisy chaining grounds is a good idea. If you need to ground the building pull a dedicated ground conductor. Do NOT bond the building to the grounds in the panels.

Do NOT tie the grounds in the panels together, leave each panel as stand alone as possible.

I would check the grounds in the two panels for good continuity back to ground point. A short cut to do this is
1. Check for voltage between each ground bus. A Fluke may indicate a volt or two which may or not be OK -- any higher than that I would do a little more investigation before step two.
2. Read resistance between the two ground buses. I would say a reading of 3 to 5 ohms would be good but could be higher due to conductor length. I would venture that a reading of 100 ohm or more indicates poor ground continuity and needs checking of connections.

Dan Bentler
 
The building steel is required to be BONDED to the system ground point. Building steel should not be used as a ground point.
Hmmm...Dan, Have you read Article 250.52 of the 2008 National Electrical Code recently?

"250.52 Grounding Electodes
Electrodes Permitted for Grounding.
.....
(2) Metal Frame of the Building or Structure. The metal frame of the building or structure that is connected to the earth by any of the following methods...
(Three methods are listed.)
...."

In my area of the southern US, steel-framed buildings are commonly bonded to earth with a Ufer grounding system (Method 2 in the Code), so the steel of the building provides a very good ground point and it is legal to use it for that purpose. Other places do it differently.

I have done tests on buildings with a Ufer concrete-encased electrode connected to building steel, and find that the resistance always passes the 25 Ohm test, and usually the resistance to a test rod runs from only 2 to 5 Ohms! I never achieved that low resistance with any type of grounding-rod system.
 
Last edited:
Hmmm...Dan, Have you read Article 250.52 of the 2008 National Electrical Code recently?

"250.52 Grounding Electodes
Electrodes Permitted for Grounding.
.....
(2) Metal Frame of the Building or Structure. The metal frame of the building or structure that is connected to the earth by any of the following methods...
(Three methods are listed.)
...."

In my area of the southern US, steel-framed buildings are commonly bonded to earth with a Ufer grounding system (Method 2 in the Code), so the steel of the building provides a very good ground point and it is legal to use it for that purpose. Other places do it differently.

I have done tests on buildings with a Ufer concrete-encased electrode connected to building steel, and find that the resistance always passes the 25 Ohm test, and usually the resistance to a test rod runs from only 2 to 5 Ohms! I never achieved that low resistance with any type of grounding-rod system.

obviously not recent enough forgot about Ufer also. Maybe I am just old school but I still feel using building steel is not best practice -- but it is just my opinion.

Dan Bentler
 
We are using the ground from the feeders which goes to the main switchgear room. In the switch gear room there is a a connection to the ground bus to building steel.

Bonding to building steel at the panels in addition to the ground conductors is what the oem wants use to do.

We did this today and it did lower the ground resistance down to 1 Ohm.

If there are any issues we can put it back like it was. I was just thinking that it may create a ground loop?
 

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