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 Post subject: Table vs Incident Energy method
PostPosted: Mon Jan 10, 2022 5:40 am 

Joined: Mon Mar 09, 2020 9:27 am
Posts: 25
Is the choice to use the table method vs the incident energy method up to the person doing the study? It does not seem to say in NFPA 70e that incident energy method should be used, but if such and such use the table method. It seems to be an either/or assuming you can meet the conditions in the table method.

For example, I have a building that has never had a study done. The building is super critical, shutting down is "out of the question" and opening the equipment involved would not be safe while energized. The equipment seems to fit into the table method criteria, but I am getting a lot of pushback from more experienced Arc Flash Study folks about even considering the table method.

Is there something I am missing as to why it is not desirable?


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 Post subject: Re: Table vs Incident Energy method
PostPosted: Mon Jan 10, 2022 5:55 am 
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Joined: Wed May 07, 2008 5:00 pm
Posts: 863
Location: Rutland, VT
Seems if the building is super critical, there should be an up to date set of electrical drawings that may have a lot of information and if "super critical" seems like a study would have been done on the electrical system to determine such things as short circuit currents and protection/coordination.

That being said, how do you know the parameters (fault current and clearing time) are being met to use the tables without a study?

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 Post subject: Re: Table vs Incident Energy method
PostPosted: Mon Jan 10, 2022 6:07 am 

Joined: Mon Mar 09, 2020 9:27 am
Posts: 25
There is an up to date set of drawings but 1584 says field verification is necessary. Our policy has been to put eyes on everything we are putting in the study.

The only things I can't get from the front of the panels is the wire size. I can see breaker types, most settings, etc. With breaker models I can get max clearing times.

The loop feeding this building was studied this year by an engineering firm, so i have available fault current at the transformer.


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 Post subject: Re: Table vs Incident Energy method
PostPosted: Mon Jan 10, 2022 6:13 am 

Joined: Mon Mar 09, 2020 9:27 am
Posts: 25
If you went by the prints alone. Would you assume the trip settings are at highest setting?


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 Post subject: Re: Table vs Incident Energy method
PostPosted: Mon Jan 10, 2022 7:00 am 

Joined: Mon Mar 09, 2020 9:27 am
Posts: 25
Someone brought up that you would not actually know the fault clearing time of each sub breaker, especially if the fault is small. Also, you can't determine what the available fault current is downstream without a lot of work. So i guess using the prints and listing assumptions is the best method?


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 Post subject: Re: Table vs Incident Energy method
PostPosted: Mon Jan 10, 2022 7:58 am 
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Joined: Thu Apr 18, 2019 11:42 am
Posts: 63
At the end of the day, I think you will have to decide which method fits your particular situation. As Barry mentioned, there are a lot of assumptions using the table method, and depending on where you are evaluating, it will take time to calculate/determine some of the base data needed to make an evaluation. Further, without TCCs, you won't know when the breaker will operate, so that is why you are getting push-back from some of your colleagues/consultants.

Ideally, a full study should be performed at a minimum, to give you and your maintenance staff a little more confidence in the evaluation. Then it's a matter of risk assessing the work.

Mike


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 Post subject: Re: Table vs Incident Energy method
PostPosted: Mon Jan 10, 2022 4:14 pm 
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Joined: Tue Jan 19, 2010 2:35 pm
Posts: 155
In my view, the table method is meant to satisfy folks with smaller electrical systems. If your building is as critical as you say, you should get a full study done as well as a coordination study.

Most of the time, the really high hazards are at the main or mains. Past the mains, the hazards usually fall off a lot. Remember, I’m not considering step down transformers of any size. 480 to 120/208 volt transformers can produce high arc flash hazards.

When you say your building is super critical, several questions come to mind. For example, is there a second switchgear and feeder to the facility? Critical facilities like WWTP or hospitals almost always have two utility feeders, two services and a tie breaker to connect the two services together. If not, then your owner is not treating the building as critical or does not know there is stuff they should do to help insure the power is kept on. It's been my experience that people will often say it's critical, but won't spend the money to have real switchgear, two utilities and pay for the maintenance to keep such a critical plant properly running.

Do you have multiple generators and UPS to back up the critical processes? If not, it’s not nearly as critical as the owner would have you believe.
If you do get a full arc flash study, you’ll likely find a few “land mines” waiting to explode. By land mines I mean things could take your site down for hours or longer.


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 Post subject: Re: Table vs Incident Energy method
PostPosted: Tue Jan 11, 2022 1:05 pm 
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Joined: Mon Jan 18, 2010 11:35 am
Posts: 551
Location: Wisconsin
aguywithfeet wrote:
There is an up to date set of drawings but 1584 says field verification is necessary. Our policy has been to put eyes on everything we are putting in the study.

The only things I can't get from the front of the panels is the wire size. I can see breaker types, most settings, etc. With breaker models I can get max clearing times.


Was the wiring verified to comply with the drawings during installation? Maybe signed check-off lists are available.
Have you considered doing the study using multiple conditions such as, with the expected wire size, with one size smaller than expected, and one size larger?


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