Under Rated Switchgear SCCR
Under Rated Switchgear SCCR
(OP)
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..
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..






RE: Under Rated Switchgear SCCR
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
RE: Under Rated Switchgear SCCR
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.
RE: Under Rated Switchgear SCCR
Not quite sure what you mean, but if he determines that the equipment is underrated, he's duty-bound to advise the owner.
RE: Under Rated Switchgear SCCR
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...?
RE: Under Rated Switchgear SCCR
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
www.cvoes.com
RE: Under Rated Switchgear SCCR
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If we learn from our mistakes I'm getting a great education!
RE: Under Rated Switchgear SCCR
RE: Under Rated Switchgear SCCR
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
RE: Under Rated Switchgear SCCR
RE: Under Rated Switchgear SCCR
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
RE: Under Rated Switchgear SCCR
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
RE: Under Rated Switchgear SCCR
RE: Under Rated Switchgear SCCR
David Castor
www.cvoes.com
RE: Under Rated Switchgear SCCR
Learn something new everyday - now I can go home
LOL