Substation Design Question

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faresos

Senior Member
Hello everyone:



I’m working on a project involves a design of a substation. The substation area is enclosed by fence and split in half by another fence; one area for the utility metering; the other is for the customer owned 34.5KV GIS switchgear, transformer (34.5KV- 12.47KV), and 12.47KV switchgear. Long story short, the utility company asked us to provide an overhead non fused switch structure in the customer side of the fence before terminating into the GIS 34.5KV switchgear main breaker. My questions:
  1. My understanding, based on this configuration, the switch structure will become the service disconnection mean (since it will be owned by the customer). Per NEC 230.91, this switch must have and OC protection device (fuse), correct? If yes, then it will be an issue for us because the utility company told us the maximum fuse size that can be used which will not work for us because it will limit the capacity of the transformer.
  2. Does the code provide any exception to allow to have a non-fused switch (Isolated switch) in this case since the GIS switch has a main and located within the same substation yard?
  3. Does NEC 110.31 table require us to maintain 15ft clearance from the exposed life part? Does that mean the distance is measured from the actual live part, which is up high, or from the structure pole? We have a very limited space available to install this structure, that’s why a pad mounted gear is not an option.


Your thoughts are really appreciated.
 
faresos said:
My understanding, based on this configuration, the switch structure will become the service disconnection mean (since it will be owned by the customer). Per NEC 230.91, this switch must have and OC protection device (fuse), correct? If yes, then it will be an issue for us because the utility company told us the maximum fuse size that can be used which will not work for us because it will limit the capacity of the transformer.
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230.205. The later parts are for over 1000V.

faresos said:
Does the code provide any exception to allow to have a non-fused switch (Isolated switch) in this case since the GIS switch has a main and located within the same substation yard?
Click to expand...

Yes. It isn't an exception, it is just allowed.

faresos said:
Does NEC 110.31 table require us to maintain 15ft clearance from the exposed life part? Does that mean the distance is measured from the actual live part, which is up high, or from the structure pole? We have a very limited space available to install this structure, that’s why a pad mounted gear is not an option.
Click to expand...

The switch handle is not considered live parts. It is, however, considered grounded metal when looking at table 110.31. Your clearances for overhead live conductor is not from table 110.31. That is for your 34.5kV switchgear. Your distances for energize conductors overhead come from the last part of art. 225 or the utility. It kind of depends on the point of demarcation.


To be honest with you, if they are requiring you to add a switch, I would ask them for specifications. Just say it is outside of the requirements of the electrical code and would need more guidance on how to meet their requirement. I think they make horizonal 34.5kV pole switches but I would want to make sure that is what they mean. And not some substation type switch with remote actuation and arc whips.
 
Thank you for your response. I have a follow up questions, which section allows the use of service disconnect without OCP? Section 230.208, if I'm reading it correctly, requires a short circuit protective device.

My reference to table 110.31 because I was looking at the clearance requirement between the switch structure and the fence, which I believe we will need 15ft if I'm reading it correctly. Thanks,
 
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faresos said:
Thank you for your response. I have a follow up questions, which section allows the use of service disconnect without OCP? Section 230.208, if I'm reading it correctly, requires a short circuit protective
Click to expand...
Your 34.5kv GIS switchgear is the short circuit protectuve equipment for code requirements
 
Deltaforce said:
B

Your 34.5kv GIS switchgear is the short circuit protectuve equipment for code requirements
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Still not following, how the GIS gear provide a short circuit protection if its located downstream of the Isolating switch structure? In another word, what protects the cable between the GIS gear and the non fused switch structure? Thanks,
 
faresos said:
In another word, what protects the cable between the GIS gear and the non fused switch structure? Thanks,
Click to expand...
The same device that protects the conductors on the line side of the isolating switch.

This is why the NEC has language particular to Service Entrance Conductors.
 
faresos said:
Still not following, how the GIS gear provide a short circuit protection if its located downstream of the Isolating switch structure? In another word, what protects the cable between the GIS gear and the non fused switch structure? Thanks,
Click to expand...
Your 34.5KV GIS not protect the conductors from the isolating switch structure, they are protected by utility OCPD.
But if you want such protection, you may consider time delay drop out fuse on isolating switch structure as maximum standard size fuse limit transformer size ie blowing on initial transformer surrge current.
 
Referring to NEC 110.31 table there is a note here:
Informational Note: See ANSI/IEEE C2-2017, National Electrical Safety Code, for clearances of conductors for specific system voltages and typical BIL ratings.
Unfortunately, what I have it is C2-2007.However, according to NESC C2-2007 Table 110-1, for 34.5 kV and BIL 200 the minimum distance it is 3.2 m [10.5 feet].
My edition of NEC [NFPA 70] is from 2023, and the last 230 article item is 95. No other items after then. So, what is 230.205?
 
Sorry! I saw that up to NFPA 70/2020 the article 230 had the last part for high-voltage services. From 2023 it is article 235 destined for above the 1000 V ac branch, feeder, and services.
So, let’s take now art.235.203 and 205 [204 does not exist]. However, nothing new here.
I think the article 245.21 Circuit-Interrupting Devices. permits use of breaker or fuse
as circuit interrupting devices.
 
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