250.122 and Upsizing for Available Fault Current

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Smart $ said:
Is there any ground fault detection schema which would mitigate the need for a larger EGC?
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Not answering for david ,of course. Just my take:

No really. The GF doesn't limit the fault current. Most are set up failrly long time to ride threough any transients.

IEEE 242 also has a pretty good discussion on the subject. Current limiting fuses are considered a mittigation.

As noted earlier, one must get the OCP trip inside of the cable damage curve - bigger wire or faster trip are about it.

ice
 
kwired said:
So if supplying the circuits in my example from the panel in your example - I guess you are talking about the supply end of the feeder, but if feeder is pretty short, most of that 50kA will still be available at the panel, then the 15 amp circuit is pretty much impossible to supply unless you can come up with a 15 amp device with lugs that will accept 1/0, and the 100 amp circuit will require 1/0 conductors minimum.
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Disclaimer: I do very little large 208V. Most anything I deal with that has large SCC wil be 480V - and nowdays, most all of that will be HRG so ground fault is not an issue. Could be that large commercial 208V has different issues.

Its rare to have a panel with 15A CBs to have 50KA available SCC. Also, it takes only a few feet of #14 to severly limit the available SCC. The examples you list just don't come up. If they did, my first suggestion would be: We don't need a 400A panel, filled with 15A cbs sitting on on the xfm. Move it to the other side of the room and put in a pair of 225A panels.

Most all the time one can just arrange the equipment and feeder sizes to get the available SCC down to a reasonable value.

ps: this is one of the reasons I don't like 480 xfm bigger than 2000kva, 5.7%

ice
 
iceworm said:
Disclaimer: I do very little large 208V. Most anything I deal with that has large SCC wil be 480V - and nowdays, most all of that will be HRG so ground fault is not an issue. Could be that large commercial 208V has different issues.

Its rare to have a panel with 15A CBs to have 50KA available SCC. Also, it takes only a few feet of #14 to severly limit the available SCC. The examples you list just don't come up. If they did, my first suggestion would be: We don't need a 400A panel, filled with 15A cbs sitting on on the xfm. Move it to the other side of the room and put in a pair of 225A panels.

Most all the time one can just arrange the equipment and feeder sizes to get the available SCC down to a reasonable value.

ps: this is one of the reasons I don't like 480 xfm bigger than 2000kva, 5.7%

ice
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I agree with and kind of thought of a lot of what you said when I was posting before. Not likely you will have a 15 amp circuit supplied from a panel with 50kA available. But then there probably are the occasions where there is a lot of high power equipment fed and then someone says we need a convenience receptacle or a light here.
 
kwired said:
I agree with and kind of thought of a lot of what you said when I was posting before. Not likely you will have a 15 amp circuit supplied from a panel with 50kA available. But then there probably are the occasions where there is a lot of high power equipment fed and then someone says we need a convenience receptacle or a light here.
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Response: Okay, we can put in a hotel load panel and feed it from one of the 600A distribution panels/(MCCs).
Manager: Its going to cost $10K to put a receptacle in here?
Response: Yes. 120V, 1 pole, 15A, 65KA, CBs are a bit expensive.
Manager: ^#@%$^(*&
Response: Yes, that's true.

ice
 
Generally, my expectation for older obsolete equipment is to open in ten cycles or less for faults in the instantaneous trip rating of the OCPD. 166 milliseconds or less. Ten cycles is my number for equipment I consider to be slow opening under fault and unsafe. At ten cycles of fault current, there will certainly be catastrophic (blast) damage at and near the fault location, and it's a minor miracle the breaker can open under fault without exploding itself. Ten cycles of fault current is unsafe. Something after the high SCCR bus must have current limiting capability and open faster, unless it's old and junk.

My guesstimate would be after ten cycles the breaker would be open but not able to extinguish the arc, if it needs a half second to clear. A faulty OCPD.

I appreciate the discussion but I don't see where any equipment is expected to pass 30 cycles of service entrance high SCCR fault current. I would see the OCPD's at fault if they did not open fast, not the conductors for being under rated.

Is that 1/2 second clearing time a standard that some equipment must have that withstand rating? I would think the utility primary fuses would open before then.

edit: oh, 1/2 second or 1/2 cycle. I should be napping. 1/2 cycle blast damage will still likely be catastrophic at high SCCR, but not to the egc.
 
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capybara said:
Interesting read and what it says is that if the phase conductors are fuse-protected, the EGC is ok. If the phase conductors are on CBs, the EGC is NOT adequately protected. Read it carefully - this is summary only !!!
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Remember that the document was published by a fuse manufacturer. Fuses have a current limiting range just like breakers do. Either can limit the current and protect the EGC, if correctly applied.
 
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