Separately Derived systems and ground bus

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Jlelectricllc

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Location
Sneads Ferry, NC
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Engineering Technician, Electrical Contractor
Currently have a Barracks under construction. Have 1000A 480V MDP fed from pad mount transformer. Terminated at MDP, have the two ground rods, cold water bond, Gas line bond, comm. rack bond and rebar ground. Then we have a step down dry type on all three floors, so copper bus bar has been installed on each floor for the separately derived systems. A grounding conductor has been installed from floor to floor and a grounding conductor has been installed to the dry type transformer. On the first floor a grounding conductor goes directly to grounds rods. The contractor has installed a grounding conductor from the grounding bus bars to the MDP on the first floor to pick up his other bonds inside on the panels ground bar.. I feel that is incorrect and could cause an issue bonding the two systems and also be a path for a fault
Sorry for being confusing, but hard to explain. Any thoughts?
 
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Yes it's a little confusing so if I'm understanding it correctly you have a GES (rods, water pipe, etc. all bonded together). And you have a common ground riser that connects the copper bus bars on each floor and that goes directly to the ground rods? What size conductors are being used?
 
Yes. Prints call for GES to be bonded in the 480v MDP, which that 480v system goes to each floor using the grounding conductor within. Then we have step down to 120/208 on each floor which is using the common ground between floors to ground rods due to separately derived on each floor. I just didn't like the common ground going back in the 480v MDP on first floor. Doesn't seem right. I believe 3/0. Size is not the problem at this point. Just trying to get the right path. Hope this gives a better understanding. I am the AHJ and don't feel comfortable with the install. I am also waiting response from Design Engineer. Sometimes they don't have a good grasp on the grounding and bonding
 
Assuming that the MDP is listed as suitable for use as service equipment and that the feeders to the step down transformers originate in that MDP, Exception 3 to 250.30(A)(6) permits the installation as described.
 
I'm going step out there & make comment also. You've left out the actual construction of building type. I'm assuming this structure was built "cast in place" concrete and rebar construction, with no exposed structural beam(s)..? Would this be correct (?)
 
In all respect, this being "been there & done that situation".

In reading both your posting(s) and not actually walking the structure, it's sounding like your electrical contractor performed good job.

But I'm not the AHJ either..
 
Allowing for possible gaps in your description, it sounds fine and close to ideal. Grounding for all systems on site is supposed to be all connected to each other and then go to actual ground. This does not create additional fault paths, that's part of what separately derived means.
 
I think there's kind of a misunderstanding between where you really do not want to have more than one connection between the equipment grounding conductor and the grounded conductor, and not having more than one connection to the grounding electrode system. There is no way that you can prevent the equipment grounding conductor system from being in contact with the grounding electrode system. It's just going to happen because you're going to have conduit and other metal pieces that are bonded connected to building structure that is in some way going to be connected to the grounding electrode system. There's absolutely nothing wrong with this.

Where you want to have a single fault path is where the equipment grounded conductor is connected to the grounded conductor.
 
The biggest concern I had originally was the fact the common ground bonded inside the MDP. This was without having ground rods attached from the common ground bus. In this case, the grounding conductor being disconnected inside MDP would have left the common ground without electrodes. Adding the grounding conductor from common ground bus to the electrodes and to the MDP seems much safer. Thanks for all the responses, just want it to be safe
 
Broadly speaking, multiple connections to electrodes are neither required nor forbidden by the code. They might be desirable from the redundancy standpoint that you are thinking about, but they might be undesirable from the standpoint of ground loops or pathways for voltage gradients caused by lightning. Many people think that a single grounding electrode conductor (and bonding jumpers between electrodes) is better for the latter reasons. Hopefully anyone working in an MDP understands not to leave a GEC disconnected. (OTOH, some on this forum might say the importance is overestimated.)
 
Thanks for all the replies. Sometimes I can get caught overthinking grounding and bonding. I feel comfortable with the grounding installed. Helps to get other thoughts.
 
Jlelectricllc said:
The biggest concern I had originally was the fact the common ground bonded inside the MDP. This was without having ground rods attached from the common ground bus. In this case, the grounding conductor being disconnected inside MDP would have left the common ground without electrodes. Adding the grounding conductor from common ground bus to the electrodes and to the MDP seems much safer. Thanks for all the responses, just want it to be safe
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Since the grounding electrode system for an SDS within the same building as the SDS feeder circuit really does not do anything for safety or function, it would not bother me in the least that there was no connection to a grounding electrode.
 
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