Neutral Grounding Practice for Utility Pad Mount Transformers

[rockman7892]

I was wondering what standard practice was for grounding of neutral connections on utility pad mount transformers served from and overhead distribution line that contains a neutral. Question applies 1-phase pad mount fed from OH lateral feeder but similar question also applies to 3-phase with wye-wye padmount. .

For 1-phase transformers I usually see these as single bushing transformers with that have neutral internally bonded to tank/ground. In these applications are concentric neutral cables used with the concentric neutral connection to grounding point in transformer with intent for the neutral current to flow back to the overhead line neutral via the concentric neutral? Or is this grounding point in transformer only connected to earth through ground rod or ground rings with the intended neutral path to flow via ground to the nearest pole where it finds pole ground and travels up to overhead neutral?

If concentric neutral is typical means, is transformer also typically grounded to earth through ground rod or ground ring around padmount?

Thanks

 

[stevenal]

For 1-phase transformers I usually see these as single bushing transformers with that have neutral internally bonded to tank/ground. In these applications are concentric neutral cables used with the concentric neutral connection to grounding point in transformer with intent for the neutral current to flow back to the overhead line neutral via the concentric neutral?

Yes

Or is this grounding point in transformer only connected to earth through ground rod or ground rings with the intended neutral path to flow via ground to the nearest pole where it finds pole ground and travels up to overhead neutral?

No

If concentric neutral is typical means, is transformer also typically grounded to earth through ground rod or ground ring around padmount?

Thanks

Yes, one needs to control the touch potential.

 

[wcaseyharman]

I understand my utility does exactly as you describe - neutral is tied to the transformer tank and to the vault ground, as well as the customer neutral (and by extension the customer’s ground)

 

[rockman7892]

Thanks for responses. Even is this wasn't a utility pad mount transformer but rather a pad mount transformer within the customer premises I'm assuming the answers would be the same in that the concentric neutral could be used to carry neural current and fault current in system? Or would a separate equipment ground conductor (EGC) be required and the neutral not bonded to ground at the transformer if bonded at fist POCO disconnect.

Yes, one needs to control the touch potential.

So it the ground rod strictly to control touch potential around equipment and not to provide any sort of intentional path through earth back to pole, source or elsewhere? On most distribution systems is the neutral wire intended to carry all fault current back to substation and thus neutral and this distribution equipment on poles and pad mounts always have grounds bonded to neutral?

What about if a distribution system does not have a neutral?

So in terms of step and touch potential and ground grids is that really only applicable to transmission systems where the intended fault path back to source is through the earth?

 

[waross]

It is common for the service neutral to be bonded to ground at the transformer and at the customer service equipment.
The ground path may be metalic but is often from ground electrode to ground electrode.
Typically the neutral has a lower impedance than the ground path even with a metalic ground path and the bulk of the neutral current and the bulk of any faullt current flows in the neutral conductor.
In a grounded distribution system, multiple grounds are typically used.
In the case of distant faults, the neutral conductor will carry a large fault current close to the fault location.
At each multiple ground location upstream some of the fault current will be transferred to the ground through the grounding electrode.
Looking at a distance perspective, at both the source end and at the fault location, the majority of both the neutral current and the fault current will be in the neutral conductor.
The portion of the current in the neutral conductor will decrease to a minimum approximately midway between the source and the fault.
Due to the relative conductor spacing the neutral conductor may always have a lower impedance than the ground conduction.
This makes a simple resistance based solution unreliable.
It depends.
Lots of variables.

 

[rockman7892]

It is common for the service neutral to be bonded to ground at the transformer and at the customer service equipment.
The ground path may be metalic but is often from ground electrode to ground electrode.
Typically the neutral has a lower impedance than the ground path even with a metalic ground path and the bulk of the neutral current and the bulk of any faullt current flows in the neutral conductor.
In a grounded distribution system, multiple grounds are typically used.
In the case of distant faults, the neutral conductor will carry a large fault current close to the fault location.
At each multiple ground location upstream some of the fault current will be transferred to the ground through the grounding electrode.
Looking at a distance perspective, at both the source end and at the fault location, the majority of both the neutral current and the fault current will be in the neutral conductor.
The portion of the current in the neutral conductor will decrease to a minimum approximately midway between the source and the fault.
Due to the relative conductor spacing the neutral conductor may always have a lower impedance than the ground conduction.
This makes a simple resistance based solution unreliable.
It depends.
Lots of variables.

Thanks.

In my application I have a 4-wire distribution service coming into site and assuming distribution is a Mult grounded system. This incoming line hits a pole mounted disconnect, then padmounted recloser before it finally hits wye side of interconnection transformer. From reading the NEC it appears that at the service point (pole mounted switch) I would need to bond the switch to the incoming neutral wire? Do i need a separate ground wire or EGC from that point to carry through to the next downstream devices or can i simply bond the incoming switch to neutral and carry the neutral to recloser and transformer (via concentric cable neutral) and bond both recloser gnd and transformer neutral to ground? In this case concentric neutral would be serving both neutral and EGC purpose.

Also there is a single station service transformer tapped off incoming line after service point with a L-N connection with the neutral being tied back to incoming neutral as well as bonded to ground at transformer.

I'm used to in the NEC world having a single N-G bond at the service and then separate Equpment ground downstream of that. Not sure if that's necessary in this type of application?