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 Post subject: Equipment Protection?
PostPosted: Tue Dec 07, 2010 9:58 am 
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This may be the wrong forum for this but figured I would ask, When looking at a coordination / Arc-Flash study in order to achieve coordination the main switchgear (30cycle rated) main has the inst. function disabled, it is also disabled on the feeder breakers. My question is the equipment the feeder breaker is protecting is a switch and fuse main lug only switchboard panel (UL891) UL891 switchboards are rated at 3-cylces? Is this a concern? The fault rating of the equipment is 65Kaic where the actual available fault current is 27.6KA. The clearing time of the upstream devices is approximately 8-10 cycles picked up by the short time portion of the TCC. If the equipment is rated at 65kaic and the actual is 27.6KA is it acceptable to say that it can handle less fault current for a longer duration? Is there any literature, or formulas on this or how to calculate it? I have looked at various IEEE and other publications I have access to but cannot find anything related to this. They have also in other areas of the study "dialed" up the inst. portion to be out of range above the available fault current to achieve this same thing? Is this acceptable?


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PostPosted: Tue Dec 07, 2010 5:30 pm 
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Mike-
If the downstream panel isn't 30 cycle rated, I don't believe that the feeder breakers are permitted to have the instantaneous trip turned off.

i'd take a closer look at the feeder breakers. Sometimes the same ICCB breaker is capable of being used in UL 891 or UL 1558 gear, but for the UL 891 apps such as your feeder breaker, an instantaneous override should be in effect. This means there will be an instantaneous trip, even though the dial has a physical "off" position.

These ICCB's blur the lines between 891/1558 and 491/1066. hopefully someone else on the board can explain it better than I have.


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PostPosted: Tue Dec 07, 2010 7:23 pm 
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Lvpcb

the breakers in the main switchgear are low voltage power circuit breakers, and where this occurs down stream (dialed up beyond avaliable), my understanding is that depending on the manufacturer on most UL489 breakers wether MCCB or ICCB they do have a hidden instaneous override usually (manufacturer dependant) at 10-14X amp rating of the breaker, however if dialed up the avaliable fault current will never reach this setting. As I originally indicated the main switchbaord breakers are ANSI1559(?)UL1006 rated devices they have INST. disabled protectint an 891 switchboard rated 3 cycles, the AIC raiting of the Swbd. is 65Kaic with a avaliable of 27.6 with the OCPD deice opening at approx 8-cycles at 27.6Ka, I cannont find any literature that allows this, yet on the other hand cannot find any that disputes this either. thanks again.


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PostPosted: Tue Dec 07, 2010 9:18 pm 
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Switchgear incorporates only LVPCBs, short circuit tested at 15% power factor for full 30 cycles, while switchboards are tested at 20% power factor for only 3 cycles. The higher 30-cycle delay time and lower power factor test point require the switchgear structure, including bus, be built to withstand harsher electrical and mechanical stresses for a longer time, thus, LVPCBÔÇÖs can be built without an instantaneous trip function. This is true for the main as well as the feeder breakers. Without an instantaneous the main, feeders, and possible other downstream devices can more fully coordinate with each other.. Because of the limited 3-cycle short circuit test duration applied to most switchboards, all of its protective devices must have instantaneous trip functions to maintain the UL 891 label, limiting selectivity between its main and feeder circuits.

Having said that, I recommend not taking out instantaneous due to increased arc flash hazard, generally I would favour protection over selectivity.


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PostPosted: Wed Dec 08, 2010 12:14 pm 
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You are correct that the switchboards and panelboards should be able to take a lower fault current for a longer period of time. Presently the equipment is tested for XXX fault current for 3 cycles. This is a point (dot) of current and time on a time current curve. This is a problem with the UL testing and NEMA standards. There is a similar problem with testing of ATS switches, MCCs, Switchgear, Panelboards....etc.

We (protection engineers) need an I2T damage line similar to conductors. I am also looking for information. Can anybody help Mike01 and I out?

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PostPosted: Wed Dec 08, 2010 12:46 pm 
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The problem is that switchgear and switchboard fault ratings are based on the ability to withstand short circuit forces, as well as on thermal characteristics. There is no simple relationship between time and current for withstanding short circuit forces.


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PostPosted: Wed Dec 08, 2010 1:15 pm 
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jghrist wrote:
The problem is that switchgear and switchboard fault ratings are based on the ability to withstand short circuit forces, as well as on thermal characteristics. There is no simple relationship between time and current for withstanding short circuit forces.


Are you saying that equipment rated at 35 kA for 3 cycles can then carry 34 kA indefinitely?

I assume not. Then there should be an I2T curve for Thermal and an I2T or vertical line for withstand. Together, they would produce a damage curve for the equipment. Using a dot in space (on the time current curve) to represent an equipment damage point is not very useful. It really restricts what the breaker settings the protection engineers can apply.

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PostPosted: Wed Dec 08, 2010 4:54 pm 
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Exactly

Mr. Fuhr Well said, a better way to explain what I was attempting to in a simpler more direct approach. Better understandint the thermal and magnetic forces to as you indicate create a damage curve similar to a transfomrer or cable damage curve. Have there been any test for this type of application? I do not have access to the blue book at this time but I believe the test for a breaker (UL489) states longer than 3 cycles the the breaker can handle but the queston then is the bus structure and assembly, For some reason I find it had to believe that this is a linear equation but an not sure if anyone, (manufacturers or testing labatories) have done this type of study, and if so I have not been able to find any types of results. I do not have access to what many others may have access so was hoping to reach out to the experience, knolewege, and widom of others who are much smarter than I. Looking at the study in front of me I am going to pose the question back to the equipment manufacturer who provided the study and get their response, Mr. Fuhr I will keep you and everyone else on the fourm up to date on what I discover. Thanks again for everyones time on this topic.

Robertefuhr wrote:
Are you saying that equipment rated at 35 kA for 3 cycles can then carry 34 kA indefinitely?

I assume not. Then there should be an I2T curve for Thermal and an I2T or vertical line for withstand. Together, they would produce a damage curve for the equipment. Using a dot in space (on the time current curve) to represent an equipment damage point is not very useful. It really restricts what the breaker settings the protection engineers can apply.


Robertefuhr wrote:
You are correct that the switchboards and panelboards should be able to take a lower fault current for a longer period of time. Presently the equipment is tested for XXX fault current for 3 cycles. This is a point (dot) of current and time on a time current curve. This is a problem with the UL testing and NEMA standards. There is a similar problem with testing of ATS switches, MCCs, Switchgear, Panelboards....etc.

We (protection engineers) need an I2T damage line similar to conductors. I am also looking for information. Can anybody help Mike01 and I out?


ATS's always seem to be the culpurit when it comes to withstand, and interrupt ratings, espically when fed directly from a LVPCB if the Inst. function is off it's a huge problem as most ATS's are not 30 cycle rated (some are), but also the fact that they test at x/r ratios of UL489 brkrs and not ANSI brkrs.


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