2009 NFPA 70E Table 130.7(C)(9) Notes 1

[farmape]

The notes setting the boundaries for table use list a maximum short circuit amps available and a maximum fault clearing time.
Is this "maximum fault clearing time" determined by the "maximum short circuit amps"?
Thanks

 

[Zogzog]

Yes, that is the intent, they clarified this a little in the 2011 version by saying 25kA @ 0.03 sec (2 cycles)

However, it should be noted that the maximum fault current does not always create the largest arc flash hazard, IMO this is a problem with the tables. I would rather see a limit in maximum amp-cycles but that was viewed by the commitee to be too complicated (Basically requires a full analysis)

 

[farmape]

Thank you, good to know it was clarified. I agree with you and your concerns. In my limited arc flash analysis experience, indeed a higher hazard is realized BECAUSE the PD is operating below its INST region - at lower fault currents.

 

[jghrist]

Maximum fault current doesn't always create the largest arc flash hazard, but that's because lower current can result in longer fault clearing time. That's why there is a maximum fault clearing time in the notes.

The problem with the table is that for a system of any size, by the time you've gotten enough data to determine if the table note limits are met, you are more than halfway to a full incident energy analysis.

 

[farmape]

I'm assuming the the intention of the "maximum fault clearing time" was based on the maximum fault current of the specific system (NOT the "maximum fault current" listed in the notes). I think that's what you are saying also so we agree.

 

[jghrist]

The "maximum fault clearing time" is based on the fault current that gives the maximum fault clearing time. This will probably be less than the maximum fault current of the specific system.

 

[Zogzog]

Wouldn't the maximum fault clearing time be a 100% overload with a clearing time of several minutes? I don't think that is the intent. OR are you refering to the minimum acring current, say about 38% of BF current for a 480V system?

While I agree the highest arc flash hazard may very well not be at maximum availabel fault current the notes of the tables are refering to the clearing time at the maximum avilable fault current not to exceed the fault current listed in the tables, right or wrong, that is clearly the intent of the notes.

 

[farmape]

Agree with Zogzog. The term "fault" encompasses a wide range of currents. Some faults currents are below the pickup of a properly sized PD, some faults take minutes to clear, some are instantantaneous.