Results 1 to 10 of 10

Thread: 70e tables

  1. #1
    Join Date
    Jul 2011
    Location
    stanton
    Posts
    121

    70e tables

    To use the tables in 70e it saids that we have to use the notes. In the notes it asks for two things.
    1. we have to find the maximum ka scc available
    2. maximum fault clearing time.
    We find the ka ratings on the circuit breakers and the clearing times should be in the manufacturing company catalog that sells breakers .
    Would this be a way of solving those two requirements ? Or are there calculations or other means for those two requirements ?

  2. #2
    Join Date
    Feb 2003
    Location
    Illinois
    Posts
    18,589
    You have to know the available fault current of the circuit, not the rating of the breaker.
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  3. #3
    Join Date
    Dec 2010
    Posts
    322
    Quote Originally Posted by don_resqcapt19 View Post
    You have to know the available fault current of the circuit, not the rating of the breaker.
    Absolutely. I have seen 10K breakers installed on systems with over 42K available.

  4. #4
    Join Date
    Feb 2003
    Location
    New York, 40.7514,-73.9925
    Posts
    3,180
    Quote Originally Posted by dkidd View Post
    Absolutely. I have seen 10K breakers installed on systems with over 42K available.
    And equally bad for arc flash application is 42kA breakers with 10kA available. The 10kA will take a long time to get the breaker open compared to 42kA.
    Ron

  5. #5
    Join Date
    Feb 2003
    Location
    Illinois
    Posts
    18,589
    Quote Originally Posted by ron View Post
    And equally bad for arc flash application is 42kA breakers with 10kA available. The 10kA will take a long time to get the breaker open compared to 42kA.
    I thought the time to open was only a function of the fault current. Do the time trip curves change as you get breakers with a higher interrupting rating?
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  6. #6
    Join Date
    Feb 2005
    Location
    Charlotte, NC
    Posts
    6,292
    Quote Originally Posted by don_resqcapt19 View Post
    I thought the time to open was only a function of the fault current. Do the time trip curves change as you get breakers with a higher interrupting rating?
    I think he means if you assume a 42kA fault current based on the 42KAIC rating on the breaker and your actual fault current is only 10kA your clearing time will be longer and may exceed the clearing time limit of the tables.

  7. #7
    Join Date
    Feb 2003
    Location
    Wisconsin
    Posts
    8,582
    Quote Originally Posted by don_resqcapt19 View Post
    Do the time trip curves change as you get breakers with a higher interrupting rating?
    Sometimes.
    One of the ways to get a high AIC rating is to have the breaker start opening sooner so that it finishes clearing before the fault reaches the maximum available.
    Just because you can, doesn't mean you should.

  8. #8
    Join Date
    Feb 2003
    Location
    Illinois
    Posts
    18,589
    Quote Originally Posted by zog View Post
    I think he means if you assume a 42kA fault current based on the 42KAIC rating on the breaker and your actual fault current is only 10kA your clearing time will be longer and may exceed the clearing time limit of the tables.
    That makes a lot more sense than the way I was reading it.
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  9. #9
    Join Date
    Jul 2006
    Location
    San Francisco, CA, USA
    Posts
    3,353
    Quote Originally Posted by hawkeye23 View Post
    To use the tables in 70e it saids that we have to use the notes. In the notes it asks for two things.
    1. we have to find the maximum ka scc available 2. maximum fault clearing time.
    We find the ka ratings on the circuit breakers and the clearing times should be in the manufacturing company catalog that sells breakers .
    Would this be a way of solving those two requirements ? Or are there calculations or other means for those two requirements ?
    hawkeye,
    These two highlighted statements from your original post are the problem. These are NOT the same thing. If you do not know how to determine the maximum short circuit current available, that's where you need to start.

    The kAIC ratings of the breakers are selceted by someone who USED that information, but the breaker ratings do not tell you what the value is IN THE CIRCUIT. So for example if the breakers are rated 25kAIC, the maximum fault current in the circuit should* be something less than that, but it could be ANYTHING less than that, right down to 1000A. Worse yet under some circumstances it may even be more than that. You cannot use the numbers on devices for this purpose.

    *Should, but see dkidd's comment above.
    __________________________________________________ ____________________________
    "Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
    — Kilgore Trout (via Kurt Vonnegut)

  10. #10
    Join Date
    Feb 2003
    Location
    New York, 40.7514,-73.9925
    Posts
    3,180
    Quote Originally Posted by don_resqcapt19 View Post
    That makes a lot more sense than the way I was reading it.
    Then that must be what I meant.

    Actually, that is what I meant.
    Ron

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •