Results 1 to 10 of 10

Thread: maximum through fault current

  1. #1
    Join Date
    Jan 2010
    Location
    AFG
    Posts
    781

    maximum through fault current

    Greeting all,
    Can anybody explain me the difference between point number one and three. I know regarding number one but number three is a new word for me.


    i. maximum fault current
    ii. minimum fault current
    iii. maximum through fault current

  2. #2
    Join Date
    Jul 2009
    Location
    Ware Shoals, South Carolina
    Posts
    1,086
    Not knowing the context, I believe the first refers to the maximum fault current available. The last refers to the maximum fault current that actually flows through the OCPD before it opens. The device may be current limiting.

  3. #3
    Join Date
    Feb 2003
    Location
    New York, 40.7514,-73.9925
    Posts
    3,346
    For iii. do you mean let-through fault current?
    That would be the fault current that a current limiting protective device passes when acting it is current limiting activation region.

    Regular max fault current is the calculated value if a fault should occur at the point of analysis.
    Ron

  4. #4
    Join Date
    Feb 2003
    Location
    New York, 40.7514,-73.9925
    Posts
    3,346
    http://artikel-software.com/file/04-...lculations.pdf
    Hameedulla,
    What book is this a copy from?
    I guess this is what you are studying, as the reference is exact.
    Ron

  5. #5
    Join Date
    Sep 2009
    Location
    Los Angeles, Ca
    Posts
    646
    I believe it may be referring to amount of fault current that will be let through before the protective device trips - this will also depend on the reaction time of the device. It would be helpful to know the project you're looking at. Also I don't remember exactly but I believe I heard this term used in the context of transformer damage curves but I cant remember where and when I heard it (which means I may very well be wrong!).

  6. #6
    Join Date
    Jan 2010
    Location
    AFG
    Posts
    781
    Quote Originally Posted by ron View Post
    http://artikel-software.com/file/04-...lculations.pdf
    Hameedulla,
    What book is this a copy from?
    I guess this is what you are studying, as the reference is exact.
    please see this link

    http://www.areva-td.com/scripts/solu...sp?P=1228&L=US

    Chapter-4 fault current calculation page-1 pdf format.

    It is regarding to protection system and I found some explaination regarding this in google book please check below link.

    http://books.google.com/books?id=mow...urrent&f=false

  7. #7
    Join Date
    Jul 2009
    Location
    Ware Shoals, South Carolina
    Posts
    1,086
    From the Fundamentals of Power System Protection iii is defined as the maximum fault current that can flow without causing a malfunction in the selective coordination of the system. The example given shows how it is possible for a differential scheme to trip on a external fault that is passing through the differential relays zone of protection and should have been ignored.

  8. #8
    Join Date
    Jan 2010
    Location
    AFG
    Posts
    781
    Quote Originally Posted by SG-1 View Post
    From the Fundamentals of Power System Protection iii is defined as the maximum fault current that can flow without causing a malfunction in the selective coordination of the system. The example given shows how it is possible for a differential scheme to trip on a external fault that is passing through the differential relays zone of protection and should have been ignored.
    yeah, I had no idea regarding this before.

  9. #9
    Join Date
    Jul 2009
    Location
    Ware Shoals, South Carolina
    Posts
    1,086
    Quote Originally Posted by Hameedulla-Ekhlas View Post
    yeah, I had no idea regarding this before.
    Me either, but I now see the possibility that it can happen. Actually it does happen in low voltage where molded case circuit breakers are used. I just never considered a cause beyond overlapping trip curves. Remember, I am a technician not an engineer.

  10. #10
    Join Date
    May 2005
    Location
    Redmond, WA
    Posts
    784
    Through fault current usually refers to fault current that flows through an unfaulted portion of the system to a fault in another location. You need to check that the protection for a certain zone will not trip on through fault current to a downstream zone.

    Usually this comes up in bus or transformer differential relaying. The CT signals measuring current at the inlet and outlet of the protected zone are compared by the differential relay to determine if the fault is in or out of the zone. Since fault currents are large, the CT's might saturate and cause false tripping.

    Example: A facility is fed by 4000A, 480V switchgear. To reduce arc flash hazard the engineer installs a full bus differential protection system with 4000:5 A CT's on the Main and every feeder breaker. The bus differential relay monitors the currents in and out and trips the Main quickly for a fault in the switchgear. Fault current is 50,000A at a downstream MCC fed by a feeder breaker. For a fault on the MCC bus, through fault current will flow through the Main and the feeder breaker CT’s. Only the feeder breaker should trip on its short circuit protection. The bus differential should not trip the Main Breaker (or any other breaker) because the fault is not in the switchgear. The engineer has to check the CT circuit and relay characteristics at that maximum through fault current to verify that a false trip will not occur due to CT saturation.
    Bob Wilson

Posting Permissions

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