Short Circuit rating @ 480V with M-T-M closed

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jim dungar

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zog said:
The one I have seen had longer times. You would close the Tie (With both mains closed) manually and then have like 30 seconds to open a Main before the system opened a Main for you (Or maybe it opened the Tie, cant recall) but I do know it was an adjustable time and the one I worked on was set at 30 seconds. There are several of these at the Toledo Jeep plant.

But even with this longer time, it is not likely that a bolted fault would occur during the transfer between two energized systems. But, an arcing fault is a different matter.
 

zog

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Charlotte, NC
jim dungar said:
But even with this longer time, it is not likely that a bolted fault would occur during the transfer between two energized systems. But, an arcing fault is a different matter.

Agreed 100%
 

zog

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weressl said:
How much more likely that an arcing fault develop during that <10cycle transition than a bolted fault?

You are switching. Thats one the most likely times for a arcing fault to occur. You close the other main and BOOM.
 

zog

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Location
Charlotte, NC
weressl said:
Would you care to back that up with statistics? I find no evidence of this in the IEEE Gold Book.

Nope, no stats, just personal experience. But dont you think that an arcing fault is more likely to occur when closing a breaker (As I have repaired dozens of following an arc flash) than it is in a static state?

The 70E arc flash requirements apply whenever you are interacting with the equipment because thats when arc flashes occur, during outside interaction.

P.S. Be careful mentioning the IEEE color books in here, they are "witchcraft" to some. Apparantly the Red Book is now My Red book that I was bashed for mentioning as a reference, what do those nerds over at IEEE know anyways ;)
 

jim dungar

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weressl said:
Would you care to back that up with statistics? I find no evidence of this in the IEEE Gold Book.

The most likely time for any switching device to fail is when it is changing state. The mechanism hanging up or contacts failing to seat properly are some of the most common failures.
 

ron

Senior Member
Lazlo,
It would take me quite a while to find it, but an industrial manufacturer attempted to make a proposal a couple of code cycles ago, to modify NEC 110.9 to essentially exempt the need to consider "short" closed transitions when selecting AIC ratings. The proposal was rejected.
I also would like to see the document you refer to as "This was confirmed by NFPA".
 
bob said:
You can install Current Limiters or Current Limiting Reactors that will solve your problem. Google "Current Limiters" an "Current Limiting Reactors" for more information.
If the load is balanced between the two busses you can install a half size tie breaker in series with a reactor between the two busses. Often the tie breaker is only specified for the same size as the mains so that there is an available spare for the mains.
 

kingpb

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SE USA as far as you can go
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Rating the equipment based on one feed with tie closed is universally accepted in the power generation industry, with the design being that the transition is automatic, and both mains cannot be accidentally left closed by the operator.

With arc flash gaining a wider acceptance even amoung utilities that do not have to comply, this practice may need to be revisited.
 

don_resqcapt19

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1-150 Log #2969 NEC-P01


(110-9)​


Final Action: Reject​



Submitter:
David Soffrin, American Petroleum Institute​



Recommendation:​



Add the following after the first sentence:

During the momentary paralleling of an automatically controlled power transfer between two sources, the single-source condition
interrupting rating shall be considered adequate provided there is no intentional time delay and the parallel condition cannot be
maintained. The initiation of the automatically-controlled transfer shall be permitted to be manual or automatic.​


Substantiation:​



The condition identified in the proposal is common in double ended substation arrangements where maintaining power to the loads is​

critical for operation or system safety. It is also a common practice in generating stations where continuity of power flow may be
requried for public safety. These systems are designed for only a momentary parallel condition, typically only long enough for the
closure of the paralleling breaker to initialize the opening of the designated breaker returning the system to single source configuration.
During the brief parallel time (often only a few cycles), the short circuit rating of the switchgear feeder breakers may be exceeded. This
added provision in 110.9 would recognize this arrangement and establish limitations under which it can be applied. IEEE 666, Design
Guide for Electric Power Service Systems for Generating Stations, section 4.6.1 specifically allows this arrangement and equpiment
rating.​


Panel Meeting Action: Reject​


Panel Statement:

The operational practice described is only one of a number of methods that may be permitted under 90.4. This issue is best addressed​

through evaluation of specific installations.​


Number Eligible to Vote: 12​


Ballot Results: Affirmative: 12


 

zog

Senior Member
Location
Charlotte, NC
don_resqcapt19 said:
1-150 Log #2969 NEC-P01


(110-9)​


Final Action: Reject​



Submitter:
David Soffrin, American Petroleum Institute​



Recommendation:​



Add the following after the first sentence:

During the momentary paralleling of an automatically controlled power transfer between two sources, the single-source condition
interrupting rating shall be considered adequate provided there is no intentional time delay and the parallel condition cannot be
maintained. The initiation of the automatically-controlled transfer shall be permitted to be manual or automatic.​


Substantiation:​



The condition identified in the proposal is common in double ended substation arrangements where maintaining power to the loads is​

critical for operation or system safety. It is also a common practice in generating stations where continuity of power flow may be
requried for public safety. These systems are designed for only a momentary parallel condition, typically only long enough for the
closure of the paralleling breaker to initialize the opening of the designated breaker returning the system to single source configuration.
During the brief parallel time (often only a few cycles), the short circuit rating of the switchgear feeder breakers may be exceeded. This
added provision in 110.9 would recognize this arrangement and establish limitations under which it can be applied. IEEE 666, Design
Guide for Electric Power Service Systems for Generating Stations, section 4.6.1 specifically allows this arrangement and equpiment
rating.​


Panel Meeting Action: Reject​


Panel Statement:

The operational practice described is only one of a number of methods that may be permitted under 90.4. This issue is best addressed​

through evaluation of specific installations.​


Number Eligible to Vote: 12​


Ballot Results: Affirmative: 12




Bravo Don, great find.
 

jim dungar

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don_resqcapt19 said:
Panel Statement:

The operational practice described is only one of a number of methods that may be permitted under 90.4. This issue is best addressed through evaluation of specific installations.​

It appears the panel agrees there are limited conditions where this practice may be acceptable and therefore should not always be disallowed.
 
jim dungar said:
It appears the panel agrees there are limited conditions where this practice may be acceptable and therefore should not always be disallowed.

Therfore the prudent action by the panel should have been to allow the exception with a duration limit. But hey, why should they bother, it is not out of their pocket!:mad: Even if it is supported by engineering Standard, such as IEEE.
 
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jdsmith

Senior Member
Location
Ohio
We just single-ended four subs from yesterday morning until this afternoon so I thought I would share our procedure and how our gear is set up. We have an auto/manual switch and a main 1/main 2 selector switch on the tie breaker. With the tie normally open when we wanted to de-energize main 1, we first set the auto/manual switch to manual. The selector switch was set to main 1 so that when the tie breaker was manually closed, main 1 would trip after 4 seconds. My understanding is that 4 seconds was chosen as being long enough to ensure that the tie held in, and short enough to minimize the short circit issues being discussed above. I haven't yet had time to check the rating on the 4160v MCCs and the rating/short circuit current on the 480v gear.
 

zog

Senior Member
Location
Charlotte, NC
jdsmith said:
We just single-ended four subs from yesterday morning until this afternoon so I thought I would share our procedure and how our gear is set up. We have an auto/manual switch and a main 1/main 2 selector switch on the tie breaker. With the tie normally open when we wanted to de-energize main 1, we first set the auto/manual switch to manual. The selector switch was set to main 1 so that when the tie breaker was manually closed, main 1 would trip after 4 seconds. My understanding is that 4 seconds was chosen as being long enough to ensure that the tie held in, and short enough to minimize the short circit issues being discussed above. I haven't yet had time to check the rating on the 4160v MCCs and the rating/short circuit current on the 480v gear.

Is it Cutler Hammer gear by chance? DS?
 

jdsmith

Senior Member
Location
Ohio
zog said:
Is it Cutler Hammer gear by chance? DS?

One sub was Cutler Hammer, I think the other was GE. Interestingly enough, what I was told was that the refinery spec'd the 4 second delay but Cutler Hammer refused to provide it. Not sure how that got messed up in the bidding process, but they had somebody come in a few years later and add the delay.
 
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