UPS and Transformer Parallel

[RUWired]

In my attached sketch of this project the engineer is using the bonding jumper in the UPS for the required grounding of the transformer secondary. With the new wording in 250.20(D) is this design a violation?

I would think that a system bonding jumper should be installed in the transformer and a 4-pole bus bar and breakers should be used. I don't know if a 4-pole bus bar is even available.

The 4-wire primary transformer is being used to match the UPS secondary phase angle output from what i understand.

(D) Separately Derived Systems. Separately derived systems, as covered in 250.20(A) or (B), shall be grounded as specified in 250.30(A). Where an alternate source such as an on-site generator is provided with transfer equipment that includes a grounded conductor that is not solidly interconnected to the service-supplied grounded conductor, the alternate source (derived system) shall be grounded in accordance with 250.30(A).

Rick

View attachment 4084

 

[__dan]

Looks good

Looks good

The transformer on the left is a separately derived system. The earth ground symbol is the GEC per 250-30 (A) 3 and it's at a common point of bonding and grounding the system neutral with the bonding jumper.

The transformer on the right is not separately derived. Its neutral is already grounded and bonded by the bolted connection to the shared system neutral.

The drawing looks good to me. That bypass switch is very likely a closed transition switch, so the UPS output is synched to the bypass other source.

I'm not sure is your question was about the "service supplied" neutral wording. The secondary neutral system can have only one bonding to ground connection point, which your diagram shows.

 

[don_resqcapt19]

There is a violation of 250.30(A)(1) as it is shown on the drawing. If the UPS output transformer did not have its own OCPD, you could eliminate the violation by moving the bonding jumper to the transfer equipment, but I am not sure if you can make this system code compliant unless you use transfer equipment that switches the neutral.

 

[RUWired]

Is 250.30(A)1 a violation because 250.20D led you to saying that the transformer is a SDS because the secondary grounded conductors are not tied into the service supplied grounded conductor.

I'm trying to pin point the path to 250.30 because if a generator is tied into a transfer switch with a solid neutral, (utility being the primary source) it is not considered a SDS. But if there are two generators or two transformers as the sources, neither are tied into a service supplied system, 250.20(D) would tell you they are SDS's and shall be wire accordingly.

Rick

 

[__dan]

Try the Green Book

Try the Green Book

In your drawing, the engineer chose to introduce the transformer on the right and shows a floating neutral. I do not know why exactly. The transformer on the right shows as utility fed and its secondary (floating) is the bypass other source.

The UPS output transformer is connected to this, a system shared neutral with no solid ground connection. This makes the UPS output separately derived and requires compliance with 250-30 (A) 1 and 3, N to G bond and the GEC.

If the bypass other source was a solidly grounded system (the service or grounded transformer) the UPS would be the non separately derived unit and there would be no N to G bond at the UPS. There can be only one N to G bond per system to prevent placing the ground paths in parallel as a conductor for neutral currents.

Your drawing, as is, shows a system that is solidly grounded and has no parallel path for neutral current on the ground. I am looking at the circuit, not specifically at the "service supplied".

The bypass other source, the transformer with the floating neutral, is tied into a system that "is" solidly grounded. That transformer is not the SDS.

There's some good reading here:

http://standards.ieee.org/colorbooks/sampler/Greenbook.pdf

The drawings 1-38, 1-39, and 1-40 do not cover your application exactly but there's enough discussion where you should be able to meet the intent, providing a solidly grounded system with only one N to G connection.

imo, 1-39 is the preferred scenario with the UPS input,output, and bypass other source all delta, no neutral, and the load is a solidly grounded stepdown transformer at the PDU.

 

[don_resqcapt19]

Is 250.30(A)1 a violation because 250.20D led you to saying that the transformer is a SDS because the secondary grounded conductors are not tied into the service supplied grounded conductor.

I'm trying to pin point the path to 250.30 because if a generator is tied into a transfer switch with a solid neutral, (utility being the primary source) it is not considered a SDS. But if there are two generators or two transformers as the sources, neither are tied into a service supplied system, 250.20(D) would tell you they are SDS's and shall be wire accordingly.

Rick

You make a good point as to the definition of SDS. It always has been my view that all transformers, other than auto-transformers are SDS, but with your drawing and the definition of SDS, the transformer in this installation can't be a SDS. Given those facts, I don't see a code or any other type of issue with the installation.

 

[RUWired]

Dan thanks for the link, there is a lot of good information in there. I agree that the system will work fine providing the ups is not physically removed when the bypass is in operation because of a failure. I am hung up on applying this 250.20(D) article to this application because there isn't a direct connection to the service supplied grounded conductor.

Don or Dan,
Question is would 250.20(D) be refering to a utility supply and whether a solid or switched neutral is being used or could it be refering to when multiple alternate sources are used without the utility being present or both?

Rick

 

[SG-1]

250.20(D) is in reference to a utility, because it uses the term service supplied. Article 100 defines Service as from a utility to the premises served.

Do not forget Article 700.8(B), 701.9(B), or 702.8(B).
All of these references a service, but would make the installation much safer, especially during maintence of the system. From the schematic four or more signs are needed.