bonding xfmr neutral [xo]

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EE: That's right , when the ground mat is down, before the gravel pad is placed, go around the substation and cut a gap in the mat. And all of those MC-HL cables being installed - don't terminate the sheaths or the EGCs.


Elec: Okay, we will do it as speced. But don't you think the Inspector is going to choke over the unterminated MC-HL sheaths and grounding conductors.

But the part not answered yet is: Other than the answer I gave earlier, how come we treat the utility owned transformer different than the customer owned transformer. The electrons won't.

cf

All the substation metal, ground mat, fencing, equipment are bonded together, I would never say otherwise. If case 2 are there is the potential for premises neutral current to flow in parallel on the earth grounding system, some isolation can be created by the use of PVC pipe between the substation and premises service. Intentionally leave only the earth as a parallel path.

I was going to post to clarify - two things:

1. If you treat the transformer as utility substation equipment, there is an assumption that in a lightning strike, for the duration of the pulse, the entire metal grounding system + equipment can go to several thousand volts above ground due to the impedance of passing the strike into the earth.

For this reason it is essential that all substation metal equipment enclosures, fencing, metal in concrete pads, be all bonded together intentionally and substantially. So, when the grounding system goes way up over ground voltage there is no effective difference of potential between metal points, typically, between the pad where the man stands and the switch he throws. Both can be way over ground voltage in a strike but no voltage between them. Both must have metal bonded together.

2. Case 1, single N+G+earth connection is no problem, neutral current cannot find a complete circuit on a shunt path.

Case 2 has a few variables, how far from the transformer to the service and parallel paths other than the earth, ie, feeders in RGS not PVC and I believe your example was metal armored control cable.

If you have a parallel path through the feeder RGS with N+G bonds at both ends, case 1 is preferable, bond the neutral at only one end. Is it possible to only N+G bond at one end. I would look very closely at this with the other stakeholders.

Control circuits and accessory loads definitely in PVC unless they are contained in and part of the 'substation' perimeter.

Some thoughts:
Use the grounded Y secondary but bring 3 ph 3 wire no neutral to the gear

Bring 3 ph 4 wire to the gear but have all the loads 3 wire delta so there are no neutral currents. Lighting, if only a small portion of the load could be through 480 delta to 480 Y transformers with local, clean neutrals.

If you foresee a heavy neutral current on the grounding path, some corrective action will have to be taken.
 
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quote=__dan;1131016
All the substation metal, ground mat, fencing, equipment are bonded together, I would never say otherwise.
Never though you did, or would -cf

If case 2 are there is the potential for premises neutral current to flow in parallel on the earth grounding system, some isolation can be created by the use of PVC pipe between the substation and premises service. Intentionally leave only the earth as a parallel path.
True - but not part of what I pointing out here

I was going to post to clarify - two things:

1. If you treat the transformer as utility substation equipment, there is an assumption that in a lightning strike, for the duration of the pulse, the entire metal grounding system + equipment can go to several thousand volts above ground due to the impedance of passing the strike into the earth.

For this reason it is essential that all substation metal equipment enclosures, fencing, metal in concrete pads, be all bonded together intentionally and substantially. So, when the grounding system goes way up over ground voltage there is no effective difference of potential between metal points, typically, between the pad where the man stands and the switch he throws. Both can be way over ground voltage in a strike but no voltage between them. Both must have metal bonded together.
Why is this different than the same transformer that is customer owned? As I said earlier, the electrons can't tell the difference.

2. Case 1, single N+G+earth connection is no problem, neutral current cannot find a complete circuit on a shunt path.

Case 2 has a few variables, how far from the transformer to the service and parallel paths other than the earth, ie, feeders in RGS not PVC and I believe your example was metal armored control cable.

If you have a parallel path through the feeder RGS with N+G bonds at both ends, case 1 is preferable, bond the neutral at only one end. Is it possible to only N+G bond at one end. I would look very closely at this with the other stakeholders.

Control circuits and accessory loads definitely in PVC unless they are contained in and part of the 'substation' perimeter.

Some thoughts:
Use the grounded Y secondary but bring 3 ph 3 wire no neutral to the gear

Bring 3 ph 4 wire to the gear but have all the loads 3 wire delta so there are no neutral currents. Lighting, if only a small portion of the load could be through 480 delta to 480 Y transformers with local, clean neutrals.

If you foresee a heavy neutral current on the grounding path, some corrective action will have to be taken.

dan -
Every thing you are saying is all true and great ideas. But they are all about how to meet a useless code requirement.

Here is the question: Again, as I asked earlier - Why is a customer owned transformer treated different than the exact same transformer that is utility owned?

cf
 
Here is the question: Again, as I asked earlier - Why is a customer owned transformer treated different than the exact same transformer that is utility owned?[/COLOR]

cf

Maybe because the customers transformer falls under the NEC, and the Utilities doesn't?



FWIW, we bond them all in the transformers only. This is done to keep all the techies and wannabee engineers from going into panels and lifting the bond screw thinking this causes noise. They don't go in transformers, but do play in panels.
 
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