pad-mounted transformer grounding

[alehman]

I have a outdoor utility-owned pad-mounted transformer, 480Y/277 4-wire secondary, serving a switchboard in a building via cable in underground conduit. The transformer is located very close to the building.

The utility requires a ground ring surrounding the transformer, which is bonded to the neutral at the transformer as required by code. There is also a buried ground ring surrounding the building which is bonded to the service disconnect ground bus.

My concern is that we have two grounding electrode systems with a few feet of each other that are not connected except via the service neutral conductors between the transformer and service disconnect. This seems wrong. Should I be concerned about step or touch potential between the transformer and the building?

If we connect the building and transformer ground rings directly, then the grounding system conductors will be effectively in parallel with the neutral. This project is in the U.S. and under NEC rules.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[PHovnanian]

The utility requires a ground ring surrounding the transformer, which is bonded to the neutral at the transformer as required by code.

There are two neutrals at the padmount, primary and secondary. And the 'code' that governs utility practices is the NESC rather than the NEC.

Those points aside, what you describe is common utility practice. And it can produce unwanted circulating currents in the customers grounding system due to the utilities multi-grounded neutral. The neutral return current 'thinks' that the customer's ground ring is just another return path to the substation.

n the event that these currents produce unwanted or hazardous conditions in the customer premesis, the utility is required to mitigate the situation, often by breaking the connection between their primary ground ring and the secondary neutral. They don't like doing this, as it can create hazardous conditions for their personnel in the event that the service neutral opens (the only remaining tie to any sort of safety ground on the secondary side).

Dairy farms are one instance in which these utility generated ground loops can cause real problems. Cows, standing on wet concrete floors don't like to be connected to milking machines, effectively placing themselves in parallel with the utilities' neutral return path.

 

[jghrist]

Because the neutral connects the two grounding electrode systems with a low impedance, the two systems will be at the same potentential. It is unecessary, but acceptable to bond the two systems. There will be no step- and touch-potential problems cause by bonding the systems or leaving them unbonded.

 

[dpc]

In most cases, we've just relied on the secondary neutral conductor to bond the two systems together, as far as I can recall.

 

[rbulsara]

All of the above!

As dpc said, the two grounding electrodes are bonded through the neutral, so they are interconnected to meet the code.

But I see no harm in bonding two ground rings together. Neutral paths through ground, on 'line side' of the 'main service disconnect' is common as PHovaninan mentioned.

It is common in on privately owned campus distribution systems as well. This is not get confused with separately derived system downstream of the main disconnect.

Rafiq Bulsara

http://www.srengineersct.com

 

[Kiribanda]

alehman,

1)As alraedy pointed out, it is not necessary but no harm in bonding the two systems by an additional ground conductor.
2)There is no any touch or step potential issue since the fault currents are local.
3)If the cable is provided with sensitive ground fault protection with a DONUT type CT,then it will not see any fault current and will not respond if you use such a bonding conductor between the two buildings.

 

[alehman]

Thanks for the replies. In the past I have usually connected the two grounding electrode systems if they are in close proximity.

I now have a client who is insisting that they not be connected, even though they are right next to each other. Their reasoning is that neutral current would be shared by the grounding conductors. I agree that it would be. But I'm not sure I see any harm in that.

Both grounding electrodes would see earth-return fault current. One would have a longer path that the other, so I suppose there could be a potential difference, but it is probably negligible.

PHovnanian - I think NEC 250.24 requires a neutral to earth connection at transformers located outside of the building.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[rbulsara]

alehman:

I generally bond the two grounding electrode systems in close proximity, with each other too. In fact, that is general guidance of NEC and IEEE.



Rafiq Bulsara

http://www.srengineersct.com

 

[PHovnanian]

The key here is 'utility-owned'. The NEC no longer applies. The governing code is the NESC.

 

[Electic]

Not an answer to the question but comment on an earlier posting: Disconnecting the secondary from primary neutral is not an option for the utility, and the hazard will extend to the entire property, not only the utility personnel.

If the secondary and primary neutral need to be separated (which should not be the case for this situation) then a 1:1 turns ratio isolating transformer or voltage controlled grounding switch should be employed. These are not neccessarily utility obligation as some neutral voltage is inherent in the delivery of electricity.