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 Post subject: NFPA 70E Tables- Interim measures
PostPosted: Wed May 09, 2012 10:43 am 
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[INDENT]Posting this thread in this forum also...also posted elsewhere as it pertains to PPE.

As our interim measure while we are process of performing an Arc flash analysis study for our facility we have decided to use the NFPA 70E tables (2012 version) to guide PPE requirements for different types of work/tasks being done on the MCC/ Switchgear. However, the tables are accompanied by parameters which state values such as the available short circuit current, FCT etc. Our gear at the 480V and >1kV level for example, may not necessarily follow these parameters. An example of this is at our 480V level, our FCT for our protective devices are greater than those stated in the parameters. Would it be wise to use these tables given that the parameters do not match ours?

Another issue we have is around the use of the term "exposed energised conductors or circuit parts" in the parameters. If you are familiar with the Freedom 2100 Cutler Hammer MCCs, when the cubicle door is open, there are shutters which guard the bus. Can we say there are "exposed" parts in this case?
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PostPosted: Wed May 09, 2012 2:54 pm 
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1. If it is insulated, guarded, or isolated (as per definition of exposed), then it is not exposed. Note that you can't just draw a line in the sand. In some cases it may change state. For instance with bare overhead lines if I'm on the ground, it's not exposed. When I get in the bucket truck and lift off the cradle, it's exposed.
2. You can't use the tables if your equipment specs are greater than the parameters listed. The tables are formulated using those conditions. There are however readily available alternatives. Some of the required data for some of the calculations used in the annex is not any more detailed than what is required to use the tables. These methods (Lee in particular) generate very conservative results but it's better than nothing. Second, brainfiller's site has another simplified method that again doesn't require an onerous amount of data to use it. It basically turns IEEE 1584 inside out and works it backwards from a given incident energy.
3. Watch out for "simplified" approaches to the actual calculation. It takes a long time to gather data on the various buses and cabling but often makes a dramatic difference in the result. Simply using transformer impedance frequently results in absurdly high values.
4. IEEE 1584 is designed for circuit breakers. Some modeling software incorrectly models current limiting fuses, something that the Neal/Doughty method captures better.


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PostPosted: Mon May 14, 2012 4:19 am 
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Thanks Paul! Will investigate some of the other methods you discussed above.


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PostPosted: Tue May 22, 2012 10:33 am 
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PaulEngr wrote:
...
4. IEEE 1584 is designed for circuit breakers. Some modeling software incorrectly models current limiting fuses, something that the Neal/Doughty method captures better.


Paul,
Are you interested in sharing which software incorrectly models current limiting fuses? Is it any of the big three (SKM PTW, ESA EasyPower or ETAP)?

Aleesha,
I want to expand on what Paul said regarding the table parameters and "simplified" approaches.

Don't use the task tables outside of the "Parameters". For example, if the parameters include a fault clearing time of 0.03 seconds, you must be sure that that location will clear the fault in 0.03 seconds. This value can't be known simply by looking at a panel. The system must be analyzed to determine this value. (What circuit breaker or fuse is upstream? What is the arcing fault? How long will it take the upstream device to operate under these conditions?) If you use task tables without examining for this information, I feel you are likely putting (unqualified and even many qualified) people at risk because they likely don't have the information necessary to use the table. The table is useless without the maximum fault current and the maximum clearing time. By the time you've examined the tripping characteristics and short circuit current available, you are basically done performing a full arc flash hazard analysis.

As stated in the code "... for power systems with greater than the assumed maximum short-circuit current capacity or with longer than the assumed maximum fault clearing times, an arc flash hazard analysis shall be required in accordance with 130.3."

Be aware that there are some flow charts floating around which try to over simplify the hazards of arc flash. You must always be aware of the parameters. The word "parameters" is used in the 2012 edition of NFPA 70E directly after the bold text task description. The previous edition of the code addressed these same issues using the phrase "Specific Notes" in small text at the very end of the tables. As a side note, I want to commend the code for increasing the visibility of these parameters (or specific notes) although I still believe the tables are misleading. Be aware that if you see a table or flow chart with "Specific Notes" that you may be looking at outdated material.

In my experience, the task tables are not applicable at the main panel in a building or at large distribution panels which are the locations where a significant number of tasks are very common. Therefore, in my opinion the tables shouldn't be used because they may likely be misapplied.

I should concede that in my experience, the task tables are applicable to the majority of locations in a building. The tables are likely applicable to all or most lighting panels. The tables are likely applicable to all or most small disconnect switches. Since lighting panels and disconnect switches far outnumber distribution panels, the tables are likely applicable to the majority of your facility. However, as someone who is morally and legally liable, I can't in good conscience recommend the use of task tables due to the reasons listed above.

I should also share that I'm paid to do arc flash hazard studies, so some may say I'm biased, which would be true. But at the end of the day, I believe my opinions are valid for the reasons listed above.


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