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Discuss High PFC in the Commercial Electrical Talk area at ElectrciansForums.co.uk.

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  1. eckersecker
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    eckersecker Guest

    I came a high PFC in a domestic installation a few weeks ago and the consensus on the forum was that it was ok because the main fuse was rated at 33KA so the mcbs (rated at 6KA) would be ok.

    Came across similar situation in factory. A PIR had been carried out by another company and I was asked to check out the code 1 and 2s. One of the faults was high PFC at a DB. If there is a fused breaker upstream capable of taking the PFC does this make the DB ok?
     
  2. Welchyboy
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    Welchyboy Guest

    sorry wrong topic
     
    Last edited by a moderator: Feb 22, 2012
  3. wallyanker
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    wallyanker Guest

    Totally agree with the above..

    I always measure the PFC at the origin whilst doing Ze reading, then make sure all mcb's in DB have a high enough rating stated on them.
     
  4. telectrix
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    telectrix Scouser and Proud of It Trusted Advisor

    Location:
    cheshire/staffordshire
    Business Name:
    Telectrix
    so what do you do when the PSCC is over the 6kA rating of the MCBs? they would only have to take that magnitude of fault current if the fault was close to the MCB. circuit resistance would reduce the value the further from the MCB. it's mainly the kA rating of the OCPD at source that needs to withstand the PSCC at source.
     
  5. Engineer54
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    Engineer54 Guest

    As an example, the only real occasions i have stipulated high breaking capacity MCB breakers, is at Final circuit DBs within sub-station rooms, or just outside such locations where the TX's that are supplying the DB are present.... Depending on the KVA size of the TX, determines what breaking capacity i would stipulate for the final circuit MCBs.... And that could be up to 22KA ...lol!!
     
  6. wallyanker
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    wallyanker Guest

    "they would only have to take that magnitude of fault current if the fault was close to the MCB."

    Fully understand what you are saying regarding circuit resistance reducing the PSCC the further away from the DB..... but.... your sentence quoted above mentioned the IF word. I would always plan and design a circuit to be safe at its worst case scenario. So if a fault DID occur very close to the MCB, it would do its job as it should.
     
    Last edited by a moderator: Feb 22, 2012
  7. spinlondon
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    spinlondon Trusted Advisor

    Location:
    Harlow Essex
    CUs are type tested, and rated at 16.5kA, even though they contain devices rated at 3kA, 6kA or even 10kA.
    DBs are not necessarily type tested, and are often only rated as per the installed devices.
    It is possible to obtain MCBs rated at 10kA and even 15kA, otherwise some method will have to be used to limit the PFC, perhaps a switch fuse installed between the cut-out and the DB.
     
  8. eckersecker
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    eckersecker Guest

    So back to my question, is the DB ok if theres a switch fuse upstream of it?
     
  9. R4BOE
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    R4BOE Guest

    i had a disccustion with NICEIC technical about this last week answer was as some have mentioned above, for example the PFC at the socket below D/B is 9 ka you would need to use for example a 10 ka mcb, if there was a socket installed say 30m away from the D/B and the PFC at this socket was 4 ka then a 6KA would be suffice of this this circuit. this was the explination given to me i hope it helps.
     
  10. spinlondon
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    spinlondon Trusted Advisor

    Location:
    Harlow Essex
    Only if the 'peak let through current' of the switch fuse is less than the breaking capacity of the downstream equipment.
     
  11. Inteificio
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    Inteificio Guest

    Just swap a bit of the earth wire with a paperclip, that will up your Zs to a safe level. Make sure you use a good brand paperclip as safety is always paramount.
     
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  12. Engineer54
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    Engineer54 Guest

    MG and other up market manufacturers, have quite an extensive ranges of 22KA or more rated MCBs. As you say DB's are normally site built affairs, with either an isolator or MCCB as the main switch. If installing a DB in a sub-station, you would need to check the breaking capacity and let-through of the supplying breaker at main switchboard, to decide on what if any, protection is required at the Final circuit DB....
     
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