2011 NEC 250.24(C) Grounding neutral in 2 locations - objectionable current?

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shespuzzling

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Hi,

Wondering specifically about the example in the 2011 NEC, figure 250.7. It shows an outdoor transformer with a ground electrode connected to the secondary neutral conductor. As the feeder enters the building, the neutral is grounded again at the service disconnect.

I am assuming that in both the transformer and the main disconnect switch, the neutral is also bonded to the enclosure and there is an equipment grounded conductor connecting the transformer and the main disconnect switch. Wouldn't this mean that under normal conditions, your neutral current could return back to the source (the transformer) via the equipment grounding conductor? The only thing limiting it would be the impedance of the equipment ground conductor pathway.

Thanks for helping!

Figure 250.7.jpg
 

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that transformer on the ground is basically the same as if the utility had a transformer on the pole in front of your home... there would be a ground at the base of the pole, connected to the transformer.
At least there is on the ones in the neighborhoods I have worked in. Usually covered over by a big rubber or plastic cover coming down the pole.
 
shespuzzling said:
Hi,

Wondering specifically about the example in the 2011 NEC, figure 250.7. It shows an outdoor transformer with a ground electrode connected to the secondary neutral conductor. As the feeder enters the building, the neutral is grounded again at the service disconnect.

I am assuming that in both the transformer and the main disconnect switch, the neutral is also bonded to the enclosure and there is an equipment grounded conductor connecting the transformer and the main disconnect switch. Wouldn't this mean that under normal conditions, your neutral current could return back to the source (the transformer) via the equipment grounding conductor? The only thing limiting it would be the impedance of the equipment ground conductor pathway.

Thanks for helping!

View attachment 21431
Click to expand...

There is no equipment grounding conductor from the transformer to the disconnect switch. There is a current path through the earth, via the ground rods. If you have municipality supplied water there’s also a connection between the disconnect neutral bar and the copper waterline which your neighbors will be connected to this can be is a parallel path especially if your supplied from that same transformer


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Thanks all. So is the transformer case not grounded and there's no main bonding jumper? What about the conduit connecting the transformer and the service disconnect as an EGC?
 
roger ober said:
There is no equipment grounding conductor from the transformer to the disconnect switch. There is a current path through the earth, via the ground rods. If you have municipality supplied water there’s also a connection between the disconnect neutral bar and the copper waterline which your neighbors will be connected to this can be is a parallel path especially if your supplied from that same transformer


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Click to expand...
PHP:
PHP:


Water companies have been switching to water meters with rubber in between the metal pipes to prevent that. If you get between the water lines, it could kill you.
 
As a follow-up, what about this figure where you are bonding 2 service switches neutrals to the EGC. I see the same issue here where current could flow through the main bonding jumper, through the enclosure, through the trough and then through the other main bonding jumper to get back to the neutral and back to the source. Isn't this dangerous?
250.8.jpg
 
Buck Parrish said:
PHP:
PHP:


Water companies have been switching to water meters with rubber in between the metal pipes to prevent that. If you get between the water lines, it could kill you.
Click to expand...

That’s OK I still have to Bond my water meter across it so I don’t know what the purpose of that would be. Any waver from the NEC would have to be written and approved by the state at least here in Pennsylvania


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shespuzzling said:
Wouldn't this mean that under normal conditions, your neutral current could return back to the source (the transformer) via the equipment grounding conductor?
Click to expand...
As the others have stated, the service conductors are not accompanied by an EGC; the neutral is the only conductor joining the GECs. The earth is the only thing connecting the two earthing points, which is permissible, and because it's not a metallic pathway, it's no more a parallel current pathway than it is either between buildings or utility power poles.

The power companies normally don't use metal conduit, and when they do, they normally don't bond it at both ends, and when they do, it does become a parallel current pathway. Fortunately, a conduit large enough to house a given set of conductors is large enough to carry the current of the conductors within, which is why we can use it as the EGC.
 
shespuzzling said:
As a follow-up, what about this figure where you are bonding 2 service switches neutrals to the EGC. I see the same issue here where current could flow through the main bonding jumper, through the enclosure, through the trough and then through the other main bonding jumper to get back to the neutral and back to the source. Isn't this dangerous?
View attachment 21432
Click to expand...

The NEC doesn't consider it dangerous to have neutral current flowing on metal parts on the line side of the service disconnecting means.
 
shespuzzling said:
As a follow-up, what about this figure where you are bonding 2 service switches neutrals to the EGC. I see the same issue here where current could flow through the main bonding jumper, through the enclosure, through the trough and then through the other main bonding jumper to get back to the neutral and back to the source. Isn't this dangerous?
Click to expand...
That is why we have the extra bonding requirements in 250.92. I call it the unavoidable objectionable current.
 
Buck Parrish said:
Water companies have been switching to water meters with rubber in between the metal pipes to prevent that. If you get between the water lines, it could kill you.
Click to expand...

Don't water meters require a bonding jumper across them for this reason?
 
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