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 Post subject: Two Questions on 70E-12’s Table 130.7(C)(15)(a) - Clearing Times & Note #5 …
PostPosted: Sat Dec 01, 2012 10:17 am 
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With regard to comprehending the information contained within NFPA 70E-12's Table 130.7(C)(15)(a) for installations of 600Vand below; various clearing times are provided for specific equipment criteria on which (table noted) tasks are to be performed on said specific energized equipment.

Such as:
ÚPanelboards or Other Equipment Rated 240Vand below(top of page 33)
Parameters:
Max 25kAshort circuit current
Max 2 cycle (0.03 sec) clearing time


Ú600VClass Motor Control Centers (MCCs)(top of page 34)
Parameters:
Max 42kAshort circuit current
Max 20 cycle (0.33 sec) clearing time


Table Note #5 addresses both of these clearing times (as trip time) with the following information:

ÚWhen 2 cycle (0.03 sec) trip time was used, that indicated MCC orpanelboard equipment protected by a molded-case circuit breaker. Workingdistance used was 18 in. Arc gap used was 1¼ in. (32 mm) for switchgearand 1.0 in. (25 mm) for MCC and protective device type 0 for all.

ÚWhen 20 cycle (0.33 sec)-or-30 cycle (0.5 sec) trip time was used, thatindicated a LVPCB (drawout circuit breaker) in switchgear. Workingdistance was 18 in (corrected by errata) . Arc gap used was 1¼ in.(32 mm) and protective device type 0 for all.


Two Questions:

Is 2 cycle clearing time reasonable for a molded-case circuit breaker ?

… This seems (to me) like an extremely short clearing time for an MCCB, as I would be expecting something in the range of 8 cycles … (in my limited understanding of this, 2 cycles is at the lower end of the instantaneous region of the TCC ..[for a typical MCCB]..) ...


What is meant by the language: ‘protective device type 0’ ?

… I am unfamiliar with this designation for circuit breakers and I have found no manufacturer literature which refers to any CB designations as this … This also implies that there are other designations, other than device type 0, to which the table note is NOT referring to …

Can anyone shed some explanatory light on this for me, and can you direct me to literature which references CBs by their ‘device type’ designations (as refrenced in the standard 70E) ?
OK, sorry … 3 questions …

Any help would be appreciated …

mweaver


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PostPosted: Tue Dec 04, 2012 6:14 pm 
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AS I remember from my days in a laboratory, the smaller MMCCb's will trip in less then a cycle but the Manufacturers generally do not publish the shortest time as these times are dependent on the instantaneous current and the time of the cycle the current is applied,


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PostPosted: Tue Dec 04, 2012 6:15 pm 
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AS I remember from my days in a laboratory, the smaller MMCCb's will trip in less then a cycle but the Manufacturers generally do not publish the shortest time as these times are dependent on the instantaneous current and the time of the cycle the current is applied


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PostPosted: Sat Dec 08, 2012 8:20 am 
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Not sure what protective device type means. Small molded case breakers are often 1-2 cycles. Vacuum 2-3. SF6 and large air/puffer units are much slower... 8-12 cycles or more is not unusual. On modern designs using single function relays, lockout relay stages, and AC signaling can really add several cycles. Best approach is DC with one multifunction relay at least in the fast trip paths.


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PostPosted: Mon Dec 10, 2012 9:28 am 
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Jay & Paul,

Thanks for your responses. I appreciate your feedback and assistance.

Paul,
In response to your feedback, I would like to ask if in your AFH studies .you would actually utilize a total clearing time to fault interruption .of 1 to 2 cycles for a generic MCCB in the less tha 600v .200A to 400A frame size as exampled in the Table ?

With regard to the lack of feedback so far .on my question: What is meant by [color=#ff0000]'device type 0' for all ? ...[/color]
[INDENT=1]This Note #5 published CB designation must obviously mean that there is at least one other 'device type' designation to which the Table in question would NOT apply ... What is that & where does it come into play ?[/INDENT]

What gives me considerable pause with this is:
... when no one [color=#999999]yet can elaborate on what this actually means, how does anyone actually know whether or not one can simply discount this language when attempting to understand the Table information and its subsequent implementation? This language was obviously important enough to include in the Table note as a factor of consideration ... othewise it should be removed ... via proposal, which is too late for the upcomming revision cycle, plus one must substantiate same ...[/color]

I am of the firm belief that:
In our industry, no one has the luxury of just: 'reading over' [color=#ffffff].things they do not understand. [/color]
[INDENT=2]What we do in our[color=#808080] (electrical) industry can have fatal consequences ...[/color][/INDENT]

If anyone in the future can elaborate on the 'device type 0' designation I would appreciate your input.

I will continue my quest on this and will also post back if I can get to the bottom of this ...

Thanks again Jay & Paul for your input on this.

mweaver


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PostPosted: Tue Dec 11, 2012 7:37 am 
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May not be the answer you are looking for but I just use the total clearing times. And only for one scenario. Over 15kv, IEEE 1584 is useless. We are limited to a table, ArcPro, or Lee. Lee blows up and ArcPro doesn't stand their ground on validity of the result. So I check total clearing times and use the table (H/RC 3). Wish someone would do the testing for > 15kv.


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PostPosted: Tue Dec 11, 2012 8:11 am 
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PaulEngr wrote:
May not be the answer you are looking for but I just use the total clearing times. And only for one scenario. Over 15kv, IEEE 1584 is useless. We are limited to a table, ArcPro, or Lee. Lee blows up and ArcPro doesn't stand their ground on validity of the result. So I check total clearing times and use the table (H/RC 3). Wish someone would do the testing for > 15kv.


Thanks Paul

mweaver


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