5kVA 480V to 120V single phase transformer blowing fuses, and shouldn't be.

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sparkie1 said:
1) Some engineer sat there and designed this particular setup with the transformer and the power supplies and put the fuse in there so it can realistically be fed with whatever size of circuit breaker went into the bucket.

2)Using a larger fuse exceeds the 250% allowed for a transformer with both secondary and primary protection.

I see what you are saying on #2. I'm removing it to get the fuses out of there. Just something to blow and cost the company more money and cause down time with no real reason to be there.
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Have you examined the short circuit current rating? Fuses could have been installed - if they are appropriately rated - as current limiting devices to reduce the short circuit current rating. This is obviously highly subjective and would depend on the one-line diagram, available fault current, etc. There certainly are applications where fuses are installed for short circuit protection, rather than overcurrent protection.
 
winnie said:
1) If the purpose of the fuse is to permit a system that works with any size breaker, then in the case where the breaker is suitably sized, then I agree it is fine to eliminate the fuse.

2) The FLA of a 5 kVA 480V primary is about 10A. 250% is 25A unless I am missing something.

Jon
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It's 5000/(480*1.73). I think you forgot the root three there.

Twoblocked and Beck693, that's good thinking and something I hadn't considered. There are no ratings on these panels. They are marked as 24VDC, which is a massive oversight in my opinion. We have a big 480V primary transformer and the 120V secondary in those MCC sections, so it could be the case. I did speak with an engineer that was in on the construction of this facility. He said that these were just kind of thrown together as an afterthought when they were building the building and decided they wanted all field IO to be 24VDC and all control power to be 24VDC. There isn't even a print or documentation for them anywhere, and we have a pretty large database of prints for just about anything you can think of.

That being said, reducing incident energy might be a good reason to keep the fuses in place, even if they are too large.
 
I went back to the original post considering the sqrt(3) confusion.

I checked the calculation, and 250% is allowed with primary and secondary protection, but only primary is required here, so for primary-only, 10A would be the fuse, and 15A with primary and secondary. The secondary protection is the same style of fuse, but at 30A. It's sufficient for our load.
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The title says 'single phase', but those numbers clearly have a three phase factor. 5000 kVA at 120V single phase is 42A, but 3 phase is 24A, perfectly aligned with a 30A fuse. However a 120V L-L three phase transformer is an uncommon beast.

Is it possible that single phase transformers were used, but 3 phase calculations used originally, and the same calculations repeated?

Jon
 
sparkie1 said:
It's 5000/(480*1.73). I think you forgot the root three there.

Twoblocked and Beck693, that's good thinking and something I hadn't considered. There are no ratings on these panels. They are marked as 24VDC, which is a massive oversight in my opinion. We have a big 480V primary transformer and the 120V secondary in those MCC sections, so it could be the case. I did speak with an engineer that was in on the construction of this facility. He said that these were just kind of thrown together as an afterthought when they were building the building and decided they wanted all field IO to be 24VDC and all control power to be 24VDC. There isn't even a print or documentation for them anywhere, and we have a pretty large database of prints for just about anything you can think of.

That being said, reducing incident energy might be a good reason to keep the fuses in place, even if they are too large.
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I've spent probably 1,000 hours over the past two years drawing up schematics for a customer's machines that don't have a scrap of documentation. It drives me up the wall how common it is in the industry to have no electrical docs.
 
winnie said:
1) If the purpose of the fuse is to permit a system that works with any size breaker, then in the case where the breaker is suitably sized, then I agree it is fine to eliminate the fuse.

2) The FLA of a 5 kVA 480V primary is about 10A. 250% is 25A unless I am missing something.

Jon
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Others are willing to take the road less taken there. When 250% works, still others think that's too much.
Me, I don't like to change fuses more often than needed.
 
winnie said:
I went back to the original post considering the sqrt(3) confusion.



The title says 'single phase', but those numbers clearly have a three phase factor. 5000 kVA at 120V single phase is 42A, but 3 phase is 24A, perfectly aligned with a 30A fuse. However a 120V L-L three phase transformer is an uncommon beast.

Is it possible that single phase transformers were used, but 3 phase calculations used originally, and the same calculations repeated?

Jon
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You're right, it is a single phase transformer. The primary is two lines of three-phase, which would be single phase. Sorry for my confusion there.
By those numbers, 5000 / 480 * 2.5 ~ 26 should mean that up to a 30A fuse is acceptable.

I'm okay with that. This whole time I've been adding the root three, when I don't think I should have been. One of you all better check that too.
 
sparkie1 said:
You're right, it is a single phase transformer. The primary is two lines of three-phase, which would be single phase. Sorry for my confusion there.
By those numbers, 5000 / 480 * 2.5 ~ 26 should mean that up to a 30A fuse is acceptable.

I'm okay with that. This whole time I've been adding the root three, when I don't think I should have been. One of you all better check that too.
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You don't have to go 2.5. The 20s are working and most likely will.
 
ptonsparky said:
You don't have to go 2.5. The 20s are working and most likely will.
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The important thing is to know what you CAN go to. Knowing that these can go up to 30 means I don't have to go check every set of fuses, and I know that the ones I've put 20's in are fine. I also know that I can label the fuse holders with 30A max labels when I get into them for the mechanics.

This has been great, though. Now we know where the mistake came from and largely why it is the way it is.
 
topgone said:
Others are willing to take the road less taken there. When 250% works, still others think that's too much.
Me, I don't like to change fuses more often than needed.
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Usually if I'm not throwing the max in, it's due to convenience. These transformers aren't actually really heavily loaded (for the most part), so smaller primary fuses are fine.
 
230411-0857 EDT

What determines peak inrush current to a ferromagnetic magnetic core transformer is the residual flux in the core from the last turn off, and the turn on phase angle of the turn on voltage.

..
 
I'm pretty sure that the 'round up to the next standard fuse size' does not apply for transformer primary protection. So you would be limited to 25A fuses, not 30.

Jon
 
My understanding: for primary protection only you are permitted 125% or next larger standard size.

For primary and secondary protection you are permitted 250% maximum on the primary (no round up) along with 125% or next larger standard size on the secondary.

Jon
 
sparkie1 said:
The important thing is to know what you CAN go to. Knowing that these can go up to 30 means I don't have to go check every set of fuses, and I know that the ones I've put 20's in are fine. I also know that I can label the fuse holders with 30A max labels when I get into them for the mechanics.

This has been great, though. Now we know where the mistake came from and largely why it is the way it is.
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30 amp fuse holders naturally won't really accept over a 30 amp fuse, not without some creativity to get them installed;)
 
winnie said:
My understanding: for primary protection only you are permitted 125% or next larger standard size.

For primary and secondary protection you are permitted 250% maximum on the primary (no round up) along with 125% or next larger standard size on the secondary.

Jon
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That is the way I read it, now that you have brought it to our attention.

Not that it matters anymore, but my SS has been updated.
 
Thanks for the help on this everyone. As far as the next size up rule, it's a bit of a tricky one. From Table 450.5(B), the primary can't use the next size up rule, but the secondary can. As an aside, does anyone know what this "coordinated thermal overload" is? I've never actually looked into it, and a quick google just gives me a bunch of code cites rather than the definition.
 
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