Transformer Secondary Protection

[diakonos1984]

I'm working on a system design that has a 4160V Delta - 480V Wye transformer with a latching fused contactor on the primary side. The secondary feeds two breakers, through cables bolted onto the secondary bushings. One breaker is somewhat larger than the secondary FLA of the transformer, the second is less than half of the secondary FLA of the transformer.

I understand that NEC Article 240.21(C)(3) could and should apply, but this requires very large cables to a fairly small breaker, and several interested parties don't like this idea.

My question is:
If:
I put CTs on both sets of feeder cables at the transformer secondary,
I connect those CTs to a protective relay with individualized protective functions for each feeder cable,
and I use the protective relay to trip the primary side latching contactor
Then:
Does this qualify as "overcurrent protection...located at the point where the conductors receive their supply"? (Article 240.21)

Thanks!

 

[augie47]

I think my brain is in gear when asking where the utiliy supply ends.
It will help make a determination if these are customer feeder taps or service conductors.

 

[diakonos1984]

Good question, thanks!
This is pretty deep into a large industrial facility (actually a "mining" operation). Not a utility supply.

 

[diakonos1984]

The big question in my mind is since my sensing device (the CT) is located "at the point where the conductors receive their supply" (the xfmr secondary) is it okay that the interrupting device (the contactor) is further upstream (the xfmr primary)? It seems to me that it should be, but I want to make sure I'm not missing anything.

 

[augie47]

I'm going to have to yield to someone with greater experience and knowledge. On the surface, it would appear if you qualified as a supervised industrial supervising, what you have MIGHT comply with 240.92.
That said, if you are actually in a mining situation, per 90.2(B) the NEC might not even apply.
I'm sure there are folks here that can answer but you may need to be patient

 

[Smart $]

IMO it would serve the purpose. However, my opinion don't count :roll:

My literal interpretation of "shall be located at the point where the conductors receive their supply" is conductors must be connected directly to device which deenergizes them. That is not the case here.

This brings up another question which I have wondered about from time to time. 240.21(C)(3) is for industrial installations only and provides requirements to be in compliance. Does this mean an industrial facility must comply to these requirements... or ...does it mean if the requirements are not met that a non-industrial application of code has to be used, such as 240.21(C)(6)? As I see it, 240.21(C)(6) is easier to comply with...

 

[diakonos1984]

Thank you both, I think we're in the clear now...

240.92(C)(2)(1) does NOT apply, since we're terminating two sets of conductors with two different breakers.
BUT
240.92(C)(2)(3) and (4) do apply since all secondary conductor current is sensed and upstream devices will be opened.

Regarding the mining situation, this installation is in the processing facility of a surface mine or quarry, so it is under MSHA, but the 90.2(B) exception doesn't apply.

As to Smart $'s question, it seems to me that it would be required for an industrial installation, and not optional. I do wonder why they have that exception there when as Augie47 pointed out there is an entire section 240 Part VIII for Supervised Industrial Installations.

I also want to know why 240.92(E) allows secondary protection by the primary overcurrent device with no limitations when 240.4(F) and 240.21(C)(1) both make it clear that Delta-Wye transformers are NOT allowed to use primary devices for secondary protection?!?!