Utility Transformer Grounding

Status
Not open for further replies.
J

jbrunson

Member
Location
Boise, ID
Gentlemen (and ladies!) -

I recently received this comment from an engineer on a set of drawings and would like some input (the transformer referenced in his comment is a customer-owned utility transformer):

Recommend adding note to ensure transformer ground/neutral NOT bonded at transformer (a lot of the time transformers come from the factory bonded) where this is done in the service disconnect.

Somebody please tell me that I am reading his comment wrong and he is NOT telling me to do the opposite of 2020 NEC 250.24(A)(2)?!?!

Thanks!
 
If the transformer is owned by the customer, then 250.30 is applicable, not 250.24.
 
jbrunson said:
Gentlemen (and ladies!) -

I recently received this comment from an engineer on a set of drawings and would like some input (the transformer referenced in his comment is a customer-owned utility transformer):

Recommend adding note to ensure transformer ground/neutral NOT bonded at transformer (a lot of the time transformers come from the factory bonded) where this is done in the service disconnect.

Somebody please tell me that I am reading his comment wrong and he is NOT telling me to do the opposite of 2020 NEC 250.24(A)(2)?!?!

Thanks!
Click to expand...

1. Main bonding jumper can be at the transformer or at the service panel but not both, which may be the point.
2. There is a requirement for bonding even at service entrance but not necessarily tying X0 to the case.
3. I haven’t seen transformers “automatically” come jumpered. Some have an available jumper but that’s it. I have seen some delta-wyes which are nearly always jumpered not get one. And above 250 V you don’t have to run with a grounded neutral (delta is allowed) although bonding must still be grounded and I don’t recommend it (see for instance FM 10).
4. Utilities among other things typically run grounded wye-wye and are multi-grounded. NEC doesn’t even apply (see Scope in NEC). Even OSHA regulations are 1910.269 as opposed to 1910.300-399. So if it’s truly run as a utility NEC doesn’t apply. But if it’s simply that the point of delivery is a higher voltage then we are still in somewhat of a grey area because it is clearly distribution and outside NEC scope but most people just ignore that technicality because they are unfamiliar with utility rules and follow NEC. Under utility designs you ground everything including every pole.
 
Note: 250.30(A)(1) Exception 2 & (A)(2)
Exception No. 2: If a building or structure is supplied by a feeder from
an outdoor separately derived system, a system bonding jumper at both
the source and the first disconnecting means shall be permitted if doing
so does not establish a parallel path for the grounded conductor.

(2) Supply-Side Bonding Jumper
Exception: A supply-side bonding jumper shall not be required between
enclosures for installations made in compliance with 250.30(A)(1),
Exception No. 2.
 
jbrunson said:
... (the transformer referenced in his comment is a customer-owned utility transformer):
...
Click to expand...
Is the transformer on the line side or load side of the "service point" as that term is defined in the NEC?

If it is on the line side of the service point it is totally up to the utility, but where is it supplying a system with a neutral, the utility will always have a neutral to earth bond at their transformer.

If the transformer is on the load side of the service point, then 250.30 applies and the location of the system bonding jumper is a design issue.
 
paulengr said:
Oh yes it does. Lots of larger plants own their substations. It sometimes saves a lot of money and it’s certainly easier coordinating things.
Click to expand...
Agee adn that would be "customer owned", but as electrofelon states the term "customer owned utility transformer" does raise eyebrows. Kinda like being "almost pregnant" .. it would be one way or the other.. customer owned or utility owned ?
Goes back to don's poiint... where is the "service point"
 
So imagine if you have a 230 kV substation fed from Dukes transmission system. With a 50 MW Cogen(waste heat recovery). Add in 70+ miles of distribution lines, multiple large double ended subs. 7 houses fed from the system to help out a local coop. Multiple chemical plants. Hundreds of portable subs in a mine. Throw in an air strip (for George H Bush back in the day) and two barge slips (ports). This system is the largest customer on the Eastern district of Duke Power.

It would be utterly impractical to treat that system under NEC exclusively. You had MSHA, maritime, and FAA in their areas and the internal distribution system is clearly a private utility. It would be a nightmare trying to apply NEC rules to a generation, transmission, and distribution system. This is one fairly extreme example but there are a lot of similar plants. In fact if you read the NESC and NEC scopes it’s pretty clear the demarcation is not clear. It is a negotiated item. It’s also not uncommon to have utility controlled space within the customer substation for metering.

When you install draw out gear i would challenge you to call that anything but distribution. Running short service drops and light poles is one thing but hot taps to substation bushings is hardly utilization. And once you do, you are in NESC scope and OSHA 1910.269.

As far as multigrounded systems Z0 is zero in a wye-wye transformer. Their faults are your faults and vice versa. There is no electrical separation and this is very common as opposed to delta-wye where Z0 is infinite so ground faults are isolated. Couple this with the fact that substation grounds are vastly better than pole grounds and effectively the customer is supplying localized grounding for the utility so whether you ground H0 and X0 or not is a moot point.

There are good arguments why utilities use this arrangement but it is rightfully despised by end users.
 
Status
Not open for further replies.
Top