If you need a Zs figure for a type B,C or D ......

Discussion in 'Electrics UK' started by festa, 2 Apr 2007.

  1. festa

    festa

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    If your ever caught short without your `BIG BROWN BIBLE BS7671`and need to know a Z`s figure use this simple formula.....

    For a type B MCB....
    1.Multiply the nominal setting of MCB by 5
    Then using Ohms law...
    2.Divide 240 (volts) by the sum you got in the above calculation.

    I.E for a 25 amp type B

    25A x 5 = 125A
    240V / 125A = 1.92 ohms

    For a type C MCB substitute the 5 for a 10

    25A x 10 = 250A
    240V / 250A = 0.96ohms

    For a type D MCB substitute the 10 for a 20

    25A x 20 =500A
    240V / 500A = 0.48ohms.
     
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  3. Jim2287

    Jim2287

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    Or you can do following, as is shown in table 41B2, BS7671 16th edition


    Type B MCBs BSEN 60898 and RCBOs BSEN 61009

    48 / Current rating of MCB


    Type C MCBs BSEN 60898 and RCBOs BSEN 61009

    24 / Current rating of MCB


    Type D MCBs BSEN 60898 and RCBOs BSEN 61009

    12 / Current rating of MCB
     
  4. Spark123

    Spark123

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    Don't forget to also compensate for conductor temperature when comparing with measured results, when using copper conductors for a temperature drop of 60ºC i.e. from 70ºC to 10ºC the max Zs should be divided by 1+(60x0.004)=1.24 or multiplied by 0.8 (figures in OSG).
     
  5. Jim2287

    Jim2287

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    NIC told us that you dont need to do that, as the ones in regs are closer to real maximums ... dunno if thats right or not

    thats probably just because we do domestic and temps dont change that drastically
     
  6. Spark123

    Spark123

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    Isn't there a chart with maximum values for measured Zs printed on some of the NICEIC pads?
     
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  8. RF Lighting

    RF Lighting

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    Yep, but I don't have one at home :(
     
  9. DESL

    DESL

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    But they don't write the Regs!!!!!!!
     
  10. Spark123

    Spark123

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    I'd think it a bit odd that they go to the trouble of printing the figures on their pads only to say don't use them ;)
     
  11. RobFurn

    RobFurn

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    The OSG quotes figures that have been corrected i.e
    BS7671 (brown book) table 41B2 quotes 1.5ohms for 32A B type MCB but table 2D of the OSG (page 92) quotes 1.2ohms for a B type MCB

    (1.5 x0.8 = 1.2)
     
  12. DIYnot Local

    DIYnot Local

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