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Discuss Any quick calcs to get a rough min Zs allowable in the Electrical Wiring, Theories and Regulations area at ElectrciansForums.co.uk.

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  1. rich3009
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    rich3009 Regular EF Member

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    Lincolnshire
    Any quick calcs to get a rough estimate min Zs allowable for protective devices MCB, BS-88 etc, Im thinking in an exam saves time even looking at the tables if there's a fast ball park check rule of thumb.
     
  2. Leesparkykent
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    Leesparkykent Super Moderator Staff Member Super Moderator

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    Kent
    Do you mean max Zs?
     
  3. rich3009
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    rich3009 Regular EF Member

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    Lincolnshire
    yes sorry of course
     
  4. westward10
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    westward10 In echoed steps I walk across an empty dream. Staff Member Moderator

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    Takes seconds to look at the Tables.
     
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  5. Leesparkykent
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    Leesparkykent Super Moderator Staff Member Super Moderator

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    (UoxCmin)/Ia
     
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  6. rich3009
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    rich3009 Regular EF Member

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    Lincolnshire
    It's a good check though
     
  7. telectrix
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    telectrix Scouser and Proud of It Respected Member

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    Zs < V/ 5In for B type. e.g.for a 32A . Zs < 230/(5 x32) i.e. 230/160 = 1.4375. ( then apply correction factors).
    Zs < V/10In for C type.
    Zs < V/20In for D type.
     
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  8. rich3009
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    rich3009 Regular EF Member

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    Lincolnshire
    I knew I'd seen something like this and I remember rule x10 for C and D types, anything for BS88
     
  9. westward10
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    westward10 In echoed steps I walk across an empty dream. Staff Member Moderator

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    Not sure how these quick calculations will save you time if you have the Tables to hand, it is just something else to remember.
     
  10. Zdb
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    Zdb Regular EF Member

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    UK
    Quicker just to turn to page 58 in the big yellow book.
     
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  11. Ian1981
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    Ian1981 Forum Mentor

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    North east
    You’ve forgotten cmin so it’s 1.37ohms for 32 amp type B and not 1.44 :)
     
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  12. telectrix
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    telectrix Scouser and Proud of It Respected Member

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    not forgot. stated in post to apply correction factors.

    anyway, cmin is to allow for voltages below 230V. what about when the voltage is > 230V. ????
     
  13. Ian1981
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    Ian1981 Forum Mentor

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    North east
    Thought you meant the 0.8
    I’ll apologise
    Type D tho is 230x cmin /10In for 5 second disconnection time circuits and 20In for 0.4 second circuits
     
  14. telectrix
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    telectrix Scouser and Proud of It Respected Member

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    agreed. was just considering the 0.4 secs, as most circuits (apart from distribution ccts) are 0.4,
     
  15. Ian1981
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    Ian1981 Forum Mentor

    Location:
    North east
    We have to use the worst case scenario as given in the regulations or use manufacturers data.
    As we know the voltage measured could be anywhere between 230-250 v or whatever really.
    You know what they are like regarding changing regulations and moving the goalposts
     
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