Under Rated Switchgear SCCR

[Live2learn]

I did a coordination study for an existing system and found a switchgear assembly rated for 30kA and it is a 480/277V 2000A and has the ability to recieve over 90kA of fault current.

This isn't safe and I assume needs to be fixed immediatly.

The solution I have come up with is provide CLF and size them to limit the the let through current to >30kA or less.

Correct approach - any ideas..

 

[rbulsara]

Only if the fuse is "series rated" with breakers in the switchgear assembly for 90kA or more and labeled so. (Assuming the ANSI world).

Applying "up, over and down" method to determine the let through current and using that as "available SCC" is not permitted by any IEEE standard. This has been discussed a few times on this site. Do a search.

I would be very careful as to how I address this, especially if I were the engineer of record.

Rafiq Bulsara

http://www.srengineersct.com

 

[peebee]

I wouldn't address it at all as the EOR -- I'd get a fuse vendor or the switchgear vendor to address it. But the vendors will probably refuse to address the issue using fuses due to the issues rbulsara has pointed out.

You may find that, unless you can find some listed combination of series rated OPD to protect your gear as suggested by rbulsara, your only available options are to increase the upstream impedance (eg, via reactors), or to replace the gear with properly rated gear.

 

[dpc]

I wouldn't address it at all as the EOR

Not quite sure what you mean, but if he determines that the equipment is underrated, he's duty-bound to advise the owner.

 

[Live2learn]

Yeah I am calling the owner now. Funny thing the shops say the bus is braced for 65kAIC, but it is rated at 30kAIC based upon the lowest breaker AIC rating according to the shop drawings.
I just went through the entire breaker line-up and no breaker is rated under 65kAIC and nothing has changed at all since the delivery of the gear.

What else in switchgear corresponds to the AIC rating, besides bus bracing, ckt bkr rating. PT's, CT's...?

 

[dpc]

Generally, the integrated SC rating is based on the lowest rated device. You might verify that there is not a series rating based on the main breaker and the feeder breakers and/or an upstream breaker. Also, maybe the 30 kA breaker was replaced at some point.

In the old days, the bus bracing was often listed separately and each breaker had a SC rating. There was no overall SC rating for the assembly.

I'd also make certain the 90 kA of fault current is accurate. That's a lot. Sometimes with multiple sources, interlocks are provided to prevent energizing from multiple sources (at the same time).

David Castor

http://www.cvoes.com

 

[ScottyUK]

Agree with David - 90kA fault on a 2000A system is unusually high, I'd have expected about half that value or a little less. You're not looking at assymmetric peak current are you?


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If we learn from our mistakes I'm getting a great education!

 

[Live2learn]

No its legit. It's a 20 story high rise data center with over 600,000 sf and half dc power plants and half computers. I have 4 utility circuits onto a mm-t-mm with all breakers closed and all sources sync'd and 10 MVA of xfmr (10/(480/1.73)/.0575)so to be honest the 90kA is down the line a ways. All breakers are on to prevent losing half of the utility and using a transfer scheme to close the tie.

 

[rbulsara]

10 MVA , 480V transformer?? That is 12,000A FLA needing at least 16,000A bus! Is that what you have? Something is missing here.

10 MVA xfmr will have much greater impedance than 5.75%. I bet this is not the only issue this design has. Have an extra pair of experienced eye take a look at the whole issue.

Rafiq Bulsara

http://www.srengineersct.com

 

[davidbeach]

Sounds like a spot network. If so, the numbers are in the right ball park.

 

[Live2learn]

I have a 10,000 amp bus with bus CT's and PT's prevent overheating that bus and I have integrated that into the control system - If we are drawing more than 6000A's I will have bigger problems - meaning they added a lot of stuff without me knowing ie chillers, new data center.

This is a complete redundant system with (4) 2500kVA so I only need (2) but all the breakers will be closed so if we lose one utility the others pick it up with no problem my thought was like a closed transition transfer.

Also have a complete redundant generator plant that is setup for a closed transition via ATS's

I am also directly next to a substation, but have no x/r issues from that.

I do need some experienced eyes, but they keep on retiring and seem to congregate on these sites.

I am very young but fortunate enough to work with some of the brightest and best in this industry, I have read all of the IEEE color series, but that doesn't come close to experience that you gentlemen have.

Doing my best with what I got - anybody wanna check my work for some $ LOL

 

[rbulsara]

Now 90kA seems low..but I am not challenging it. That was not your question.

Just a comment (based on your description), 4 transformers connected to the same bus all the time, when you need 2, does not make it a complete redundant system. Just N+2. Complete redundancy would be a N+N system, imho. But again, that is a subplot not the main question, which has bee adequately answered in earlier posts.

Rafiq Bulsara

http://www.srengineersct.com

 

[Live2learn]

It is low I knew if I said 120kA like it actual is we would get into a fault current debate

 

[dpc]

If you had explained the service was taken from a spot network, I don't think there would have been much debate.

David Castor

http://www.cvoes.com

 

[Live2learn]

Very Very True.
Learn something new everyday - now I can go home
LOL