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Thread: Location used for calculating utility available fault current

  1. #1
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    Location used for calculating utility available fault current

    In Coned service territory I have, after written requests, received available fault current data from Coned calculated at a point in THEIR system located upstream and not at the service disconnect. The fault current information came but without their feeder size and length. Coned's own documentation says the value they will provide is at the service entrance.

    I questioned the Coned field engineer with, really? that much available fault current at my service?

    It was then that I learned their error.

    It makes me wonder.... How many services and equipment ratings have been needlessly oversized in order to meet their faulty data. (No pun intended)

  2. #2
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    What is the data?



  3. #3
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    The source is characterized by available fault current amps and impedance.

  4. #4
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    Quote Originally Posted by Natfuelbilll View Post
    In Coned service territory I have, after written requests, received available fault current data from Coned calculated at a point in THEIR system located upstream and not at the service disconnect. The fault current information came but without their feeder size and length. Coned's own documentation says the value they will provide is at the service entrance.

    I questioned the Coned field engineer with, really? that much available fault current at my service?

    It was then that I learned their error.

    It makes me wonder.... How many services and equipment ratings have been needlessly oversized in order to meet their faulty data. (No pun intended)
    There is usually more fault current available at the service than upstream on the utility primary.
    BB+/BB=?

  5. #5
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    For a system supplying many motors - yes.

  6. #6
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    Quote Originally Posted by mivey View Post
    There is usually more fault current available at the service than upstream on the utility primary.

    Yes. Because transformers are built for trading the combination of voltage and current, in a similar concept to the way a gear system trades speed and torque. As a result, they also amplify the fault current from their primary side to their secondary side.

  7. #7
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    What are you looking to do with that information?

    If it is for sizing equipment to the fault current, then usually the customer service department will provide the fault current on the secondary side of the transformer with an infinite bus. This will provide the max fault current.

    On the other hand, if you are looking for available fault current to perform an incident energy analysis, then it is best to get the fault current at the primary protection point. Usually this would be the riser pole fuses and then obtain the fuse data, conductor data from riser pole to padmount txf and txf data. Since this is usually utility owned, they should be able to provide that. Then since the secondary is customer owned, get that data and you can calculate the available fault current on the secondary side. The riser fuse or possibly the Bay-O-Net fuse in txf would be the clearing device for a fault on the immediate secondary distribution equipment and would give you the fault time for incident energy result.

  8. #8
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    We need the utility information at the service disconnect for two issues: to properly specify the kaic of the customers equipment and to perform arc flash calculations.

  9. #9
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    Quote Originally Posted by wbdvt View Post
    What are you looking to do with that information?

    If it is for sizing equipment to the fault current, then usually the customer service department will provide the fault current on the secondary side of the transformer with an infinite bus. This will provide the max fault current.

    On the other hand, if you are looking for available fault current to perform an incident energy analysis, then it is best to get the fault current at the primary protection point. Usually this would be the riser pole fuses and then obtain the fuse data, conductor data from riser pole to padmount txf and txf data. Since this is usually utility owned, they should be able to provide that. Then since the secondary is customer owned, get that data and you can calculate the available fault current on the secondary side. The riser fuse or possibly the Bay-O-Net fuse in txf would be the clearing device for a fault on the immediate secondary distribution equipment and would give you the fault time for incident energy result.
    Any particular reason you would recommend using different data for each application?

  10. #10
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    Quote Originally Posted by Carultch View Post
    Any particular reason you would recommend using different data for each application?
    For equipment sizing you want to allow for system configuration changes including kVA upsizing, as well as lower impedance due to transformer replacement, primary wiring upgrades/feed changes, substation upgrades, transmission/generation changes, etc.

    For arc flash you want the current system state.
    BB+/BB=?

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