Fault Clearing Time

[timm33333]

For arc flash calculations of an MCC, should we only calculate the fault clearing time of the upstream breaker on the primary side of the transformer? Or should we calculate the fault clearing times for breakers on both primary and secondary sides of transformer, and then choose the faster fault clearing time to calculate incident energy? Thanks.

 

[timm33333]

LionelHutz, it looks that you are a bigger safety risk than even me. My question was clear, anyway as implied by IEEE-1584-2002 section 4.1, if the main incoming breaker of MCC is isolated from the bus, then it can be selected as fault clearing device in arc flash calculations.

 

[LionelHutz]

Your question was not clear. It was vague as hell. You need to be actually be clear before you post insulting crap.

You never specified where the secondary breaker is installed. There are many installations where the transformer feeds the main breaker inside the first section of a MCC. Does that not make the main breaker cell part of an MCC? If you understood what I posted then anything after the first cell can include the secondary breaker.

Desrod posted that you want to co-ordinate the protection for good reasons. There was no need for your insulting response to him. The co-ordination might be to prevent you from having to call in the utility to change your primary protection fuses. I'm sure there are other reasons to co-ordinate as well. You didn't provide enough specific information to know if that is the case or not but it's a possibility.

You're here posting question without ever contributing back and then insulting the members who try to help you. Nothing more then a borderline troll. I really hope you don't kill someone, but if it happens I hope you live in a part of the world where they use criminal prosecution in cases such as this.

 

[timm33333]

Sorry I did not mean to be rude or somehitng, I was only kidding. And you are right that if the main breaker is inside the first cabin of MCC then it should be part of MCC. But I have read in ETAP help file that if the main breaker is separated by sheet metal, then it will be considered "isolated". It is my understanding that most MCC's are made of sheet metal. So I understand that an MCC in a separate first cabin of the MCC will be condsidered "isolated".

 

[ScottyUK]

He did declare the rbeaker location, although it took me a second read of the post to pick up on it:

"I have utility at the top, then transformer primary fuse, then transformer (only one transformer, not transformers in parallel), then only [red]one transformer secondary breaker[/red] which is acting as the main breaker of the MCC, then MCC, then three motors each of which has its own breaker."

Maybe you should all have a beer and calm down before writing any more comments?

 

[timm33333]

In above comment in last line I meant to say: “””… I understand that a main breaker in a separate first cabin of the MCC should be considered “isolated””"

 

[LionelHutz]

Scotty - that sure sounds like a description of how it is laid out in E-tap which doesn't have a damn thing to do with the physical location of the parts.

You have to open the doors and look. You can't consider the compartments isolated when there are big gaping holes between them or thin plastic barriers capable of only preventing touch access.

 

[timm33333]

I am not trying to challenge anybody, but attached sheet from ETAP help file says that sheet-metal is a sufficient barrier for arc flash calculations. (Please see second paragraph under the heading of "protective device isolation.” I understand that most MCC's are made of sheet metal so a main breaker in the first cabinet of the MCC should be considered isolated.

 

[ScottyUK]

Lionel - fair point.

 

[LionelHutz]

Big holes and plastic is not sheet metal. If there is a full metal barrier then yes it's isolated. The bus compartments would need isolation with bushings or some other substantial barrier for bus pass through.

 

[veritas]

In SKM's Powertools for Windows (PTW) there is the option for a line side or load side study at the incomer. I always use the line side as it relies on the HV breaker to clear the fault. This is the more onerous condition and is a reality if the LV breaker fails to clear the fault. Note also, that LV faults are typically in the HV protection curves inverse zone whilst the LV breaker will trip for the fault in it's short-time zone.

Almost always, I set the HV breaker to grade with the LV breaker if there is sufficient grading time and it is safe to do so. Agreed that for a trfr tripping of the HV or LV breaker makes no difference to the load but I deem it wise to always trip closest to the fault first. Sometimes it has the aided benefit of informing that the fault is not in the trfr. Useful if the trfr has no unit protection. Some trfrs also have auxiliary trfr at the LV side feeding station supplies.

Regards.

 

[dpc]

timm33333,

I think you are reading what you want to see into that ETAP documentation. Unless the barrier between the main breaker and the rest of the equipment has been tested as arc-resistant, it is not 100% certain that the arc will not propagate. Since arcing faults have blown doors completely off enclosures, it is not possible to claim that a "sheet metal" barrier will always be sufficient. There is nothing in IEEE 1584 or NFPA 70E that is going to give you a certain answer either. The decision on whether to include or exclude the (local) main breaker from the arc flash calculation is a decision that you must make for every piece of equipment that you deal with. It is always conservative to exclude the main.

 

[timm33333]

That makes sense Dpc. The way I was looking at it is that as per table 1 of IEEE-1584-2002, it only takes 3 cycles (0.05 seconds) for a LV main breaker to trip. So by the time the arc flash explosion will blow the sheet metal and make its way to approach the main breaker, 0.05 seconds would have already elapsed and the breaker would have already tripped. But you are right, there is no proof that this will happen this way.

 

[dpc]

timm,

It will likely happen that way, but there is no way to be completely sure. Also, even if you include the main breaker for determining the bus arc-flash, you will still have a separate (higher) incident energy for the main breaker section.

 

[timm33333]

In continuation to last post, the speed of arc flash explosion is about 600 mile per hour so it will reach the main breaker in no time. But maybe if there is barrier of sheet metal, it will delay the process and it might take more than 0.05 seconds to reach the main breaker and by that time the breaker would have already tripped. But this is only assumption with no backing.