transformer grounding electrode

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faraday2000

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Transformer grounding electrode.

Article 250 state that you need a grounding electrode connected to a transformer. Does this grounding electrode have to be connected to the grounding electrode system?
 
Well there is a 30 kva transformer installed outside the electric room. The transformer is connected to a grounding rod that is installed next to it. The grounding rod is not connected to the grounding electrode system of the building.

250.30 (A)(4)
 
faraday2000 said:
Well there is a 30 kva transformer installed outside the electric room. The transformer is connected to a grounding rod that is installed next to it. The grounding rod is not connected to the grounding electrode system of the building.

250.30 (A)(4)
Click to expand...
Is this a service transformer or a separately derived system transformer supplying power to the building containing the electric room you refer to, or the latter fed from the electric room but supplying another building or structure?
 
faraday2000 said:
It is a derived system transformer supplying power to the building containing the electric room.
Click to expand...
I understand it is a derived system...

But is it a separately derived system transformer or a service transformer? The former would be after the service disconnecting means; the latter before the service disconnecting means.

Note the latter is a derived system, too... just usually not separately derived, however, as the primary grounded conductor is connected to the secondary grounded conductor. At 250V (???) I have to assume it is a grounded system. For this transformer, an additional connection to an outdoor electrode is required per 250.24(A)(2). This electrode is not required to be connected to the building GES with a GEC, as it is connected to the grounded conductor at the transformer. To also connect with a GEC to the building grounding electrode system (GES) would create a parallel path for neutral current.

Where you have a separately derived [grounded] system (i.e. located after the service disconnecting means), it has to meet the requirement of 250.30(A)(4)... but as it is outdoor it can either 1) be connected to the building GES, or 2) have it's own GES.

In your case, it appears to have it's own GES... but is in violation if resistance is greater than 25 ohms ground, where two ground rods would be required.
 
The system is this....The service is 480vac. The transformer is 480 to 120/208 3 phase.
There are 480 motors. The transformer was originally installed in the electric room but was move
Outside. The x0 is connected to a rod but not to the grounding electrode system. 250.50 says all electrodes has to be connected together, I have Mike Holt understanding the nec and in all the pictures show the transformers are connected to the system electrode.
250.30 (A)(4) metion connection to the system, that means the rod should be connected to the building system right?

Thanks for the help:)
 
faraday2000 said:
The system is this....The service is 480vac. The transformer is 480 to 120/208 3 phase.
There are 480 motors. The transformer was originally installed in the electric room but was move
Outside. The x0 is connected to a rod but not to the grounding electrode system. 250.50 says all electrodes has to be connected together, I have Mike Holt understanding the nec and in all the pictures show the transformers are connected to the system electrode.
250.30 (A)(4) metion connection to the system, that means the rod should be connected to the building system right?

Thanks for the help:)
Click to expand...
It's debatable. 250.50 requires electrodes present at each building or structure to be bonded together. Your transformer is outside, in essence a separate structure. As a separate structure, it is required to have its own GES. Additionally, the NEC specifies no criteria to determine whether an outside rod electrode is or is not present at the building.

Since you mentioned no connection to the transformer enclosure, I have to assume the system bonding jumper is in the disconnecting means and the building GES is also connected there. As such, if you connected the rod to the building GES, you would create a parallel path for neutral current... and 250.6 allows you to eliminate the connection to building GES as long as there is an effective fault current path. Electrically speaking, it is still connected to the building GES by way of the grounded conductor.
 
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