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 Post subject: 70 E Tables
PostPosted: Fri Aug 05, 2011 7:40 am 

Joined: Tue Feb 17, 2009 11:04 am
Posts: 36
I would like to know the easiest method to determine if the equipment I will be working on, falls within the parameters given in the notes. For example, 25kA, and a 2 cycle clearing time. Any help would be greatly appreciated.

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PostPosted: Mon Aug 08, 2011 1:14 pm 
Sparks Level

Joined: Thu Jul 07, 2011 6:03 am
Posts: 59
Location: Netherlands
The notes take all the 'easy' out of using the tables. You need at least three numbers:

  • maximum bolted fault current
  • minimum arcing current
  • magnetic pickup current of the largest circuit breaker *

* if there are fuses, find the current where it's fast-acting

To determine if your location falls within the note parameters check two things: 1. the maximum bolted fault current isn't larger than the kA value in the note (25kA in your example) and 2. your circuit breaker or fuse will reliably trip within two cycles on an minimum arcing current.

The first can be considered easy if you've done a basic short circuit calculation to check the rating of the equipment. For the second one you need to know the arcing current. I don't know of an easy way to calculate arcing current other than using IEEE 1584's equations and once you've collected all the data for it you'll be halfway done with a full hazard hazard analysis.

If your system is simple enough or has a lot of similar setups you might be able to take some shortcuts, but that requires a pretty thorough understanding of IEEE 1584 and your protective devices.

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PostPosted: Wed Aug 31, 2011 8:34 am 

Joined: Tue Aug 14, 2007 6:25 pm
Posts: 7
It looks like the 2012 edition of 70E will have the short circuit and duration limits directly in each line item of the table along with more specific arc flash protection boundaries for each instead of foot notes. Not that it makes using the limitations easier, just not as easy to overlook.

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