Overcurrent Protection for Transformers

[xptpcrewx]

Trying to get some clarity on transformer protection for situations where there is a circuit breaker installed on the primary side, no circuit breaker or fuse on the secondary side, but CT sensing on the primary and secondary configured to trip the primary circuit breaker. Which NEC rules apply here and is this considered both primary and secondary protection even though the disconnecting means is on the primary feeder only? Thanks in advance.

 

[don_resqcapt19]

I don't think there is anything in 240.21(C) that would permit that CT arranagement to provide the required protection for the transformer secondary conductors.

 

[jim dungar]

Look at 240.92(C)(2)(3).

 

[xptpcrewx]

Look at 240.92(C)(2)(3).

Nice. That hits the nail on the head (for secondary conductors). Looks like this is only permitted for supervised industrial installations...

 

[jim dungar]

I don't think there is anything in 240.21(C) that would permit that CT arranagement to provide the required protection for the transformer secondary conductors.

240.15(A) allows protective relays and CTs

 

[xptpcrewx]

240.15(A) allows protective relays and CTs

Going back to the OP. What about the "transformer" protection itself.

 

[jim dungar]

I read 240.15(A) as treating relays as a subset of circuit breaker. The relay is the sensing element, the breaker contacts are the interrupting device.

 

[xptpcrewx]

I read 240.15(A) as treating relays as a subset of circuit breaker. The relay is the sensing element, the breaker contacts are the interrupting device.

Let me re-phrase the question. Does transformer secondary protection require the "breaker contacts" to be located on the secondary side, or is the "sensing element" located on the secondary side sufficient to constitute secondary protection?

 

[jim dungar]

No.
I have designed many of these installations for arc flash mitigation. I have not had any AHJs question this approach.

 

[xptpcrewx]

No.
I have designed many of these installations for arc flash mitigation. I have not had any AHJs question this approach.

Putting arc flash hazard mitigation, secondary conductors and supervised industrial installations aside and focusing on Article 450 Table 450.3(B), does the OP scenario allow for primary only, secondary only, or primary and secondary protection?

 

[mayanees]

Putting arc flash hazard mitigation, secondary conductors and supervised industrial installations aside and focusing on Article 450 Table 450.3(B), does the OP scenario allow for primary only, secondary only, or primary and secondary protection?

I think it allows for primary and secondary, as long as the primary and secondary settings meet the 450.3 (A) or (B) requirements.

 

[xptpcrewx]

I think it allows for primary and secondary, as long as the primary and secondary settings meet the 450.3 (A) or (B) requirements.

Is there a code reference? Or is it similar to the logic for secondary conductors as in post #2?

 

[don_resqcapt19]

240.15(A) allows protective relays and CTs

I have never read that part ...thanks

 

[synchro]

Are you concerned about installations above 600V?
This is from pg. 6 of the Schneider document at the link below:
...

https://download.schneider-electric.com/files?&p_enDocType=Data%20Bulletin&p_File_Name=0110DB0201.pdf

 

[xptpcrewx]

Are you concerned about installations above 600V?
This is from pg. 6 of the Schneider document at the link below:
...
View attachment 2557445

https://download.schneider-electric.com/files?&p_enDocType=Data%20Bulletin&p_File_Name=0110DB0201.pdf

Thanks for that. Over 600V is easier to deal with because of the whole “engineering supervision” thing. I am particularly looking at below 600V.

For the most part, I agree with everyone’s logic and how to reason through this from an engineering perspective. Just trying to find actual code language that explicitly allows or prohibits this kind of set up. To me it doesn’t make sense to allow it for transformer protection but somehow not allow it for secondary conductor protection (as with post #2). Seems irrational.

 

[jim dungar]

Putting arc flash hazard mitigation, secondary conductors and supervised industrial installations aside and focusing on Article 450 Table 450.3(B), does the OP scenario allow for primary only, secondary only, or primary and secondary protection?

Per 450.3, transformers are always required to have primary side protective devices. Secondary side protection is optional, it's only purpose is to allow a larger size primary device.

Per 240.21(C), transformer secondary conductors must have overcurrent protection in one of several methods.

 

[xptpcrewx]

Per 450.3, transformers are always required to have primary side protective devices. Secondary side protection is optional, it's only purpose is to allow a larger size primary device.

Per 240.21(C), transformer secondary conductors must have overcurrent protection in one of several methods.

Correct. So the set up described above is limited to allowing either "Primary and Secondary" or "Primary" only protection. "Secondary" only is not allowed (I'm not sure why I even mentioned that).

What I am getting from this discussion is that transformer secondary protection (according to the NEC) doesn't really specify a location... and doesn't specify if primary interruption with secondary settings counts as secondary protection. Not sure why that bothers me so much.

Now shifting focus to 240.21(C), the question is will the set up described above allow for secondary conductor protection even if interruption is on the primary feeder?

 

[synchro]

If it's for a "supervised industrial installation" you could consider 240.92(C)(1)(3) and 240.92(C)(2)(3).

 

[xptpcrewx]

If it's for a "supervised industrial installation" you could consider 240.92(C)(1)(3) and 240.92(C)(2)(3).

And if it’s not a supervised industrial installation? Would the scheme somehow be less effective?

 

[jim dungar]

Would the scheme somehow be less effective?

No.