It is currently Thu Feb 22, 2018 3:53 am



Post new topic Reply to topic
Author Message
ekstra   ara
 Post subject: NFPA70E tables vs NESC tables
PostPosted: Wed Nov 07, 2012 11:54 am 
Offline

Joined: Fri Feb 04, 2011 3:02 pm
Posts: 2
I just review the NESC 410 tables and it seems there is a significant difference compared to the NFPA 70E tables? It seems they should be similar regardless of the method used. Will the upcoming code updates address the significant difference? Are there any white papers descibing how the 2 different tables were developed and the pros and cons?


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Wed Nov 07, 2012 1:08 pm 
Offline
Arc Level

Joined: Mon Jan 18, 2010 11:35 am
Posts: 445
Location: Wisconsin
Apples and oranges.
Different rules apply to different conditions.

Effectively, NESC tables are for 'open air' work (e.g. the distance between arcing surfaces is large) and 70E is for all other locations, including inside of 'switch houses'.

From NESC:
011. Scope
A. These rules cover..... utility in the exercise of its function as a utility.
D. NESC rules do not cover .... utilization wiring except as covered in Parts 1 and 3.

Basically, when a person employed by a utility is doing tasks governed by NFPA70E, they should be using the PPE from 70E.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Thu Nov 08, 2012 12:54 pm 
Offline

Joined: Fri Feb 04, 2011 3:02 pm
Posts: 2
What about pad mount transformers? Those are used both by utilities and industry. Same equipment, but very different PPE requirements depending on whether the NESC or NFPA70E tables are used. Why the difference?


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Sat Nov 10, 2012 6:39 am 
Offline
Plasma Level
User avatar

Joined: Tue Oct 26, 2010 9:08 am
Posts: 1878
Location: North Carolina
us267460 wrote:
What about pad mount transformers? Those are used both by utilities and industry. Same equipment, but very different PPE requirements depending on whether the NESC or NFPA70E tables are used. Why the difference?


Two reasons. First, you had a totally different committee looking at it and they used their experiences which would be very different. I've seen a footnoted version of NESC 2012 and it contains tons of information on how and why the values are what they are. Not so with 70E.

Second, as I understand it the starting calculation information for NESC tables is ArcPro. There is some discrepancy whether the values in the NFPA 70E table come from IEEE 1584 or not. I've seen at least one ROP in the last revision cycle that definitely used IEEE 1584. These two calculation methods will give different results.

For similar reasons NESC uses (or at least used to use) a 2 cal/cm^2 cutoff for PPE requirements whereas NFPA 70E uses 1.2 cal/cm^2. Both based it on available technical information as well as field experience.

Either way, on generation/transmission/distribution equipment, you MUST use NESC as NFPA 70E makes this type of equipment out of scope. For instance energizing by attaching jumpers hot is not allowed under NFPA 70E rules (must use a "disconnect") but is acceptable under NESC and is common practice in many areas. Under 70E, you have a choice. You can use the tables in 70E OR you can use the tables in NESC as they are referenced from 70E in Annex D. However if you are going to use those tables from NESC be sure that they are truly applicable to your situation. I have distribution gear in my "plant". We have our own 50 MW generator, we have overhead distribution systems, we have numerous outdoor as well as indoor substations, and lots of equipment operating well over 1 kV which is where the NESC rules are frequently more appropriate.


Top
 Profile Send private message  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 4 posts ] 

All times are UTC - 7 hours


Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Jump to:  
© 2017 Arcflash Forum / Brainfiller, Inc. | P.O. Box 12024 | Scottsdale, AZ 85267 USA | 800-874-8883