Calculating R1 R2 and finding a suitable CPC

Discussion in 'Electricians' Talk' started by RDB85, Jun 5, 2019.

  1. RDB85

    RDB85 Member

    I am doing some college work at the moment and I am just after some advice. The question is as follows:

    Determine for one of the circuits supplying the fridges which are 16A the minimum possible csa of cpc will satisfy the requirements disconnection as ADS under earth fault conditions and the adiabatic equations.

    Ze is 0.11 ohms

    Zs for a 16A Type B breaker is 2.73 ohms

    Any advice on this would be great.

     
  2. Coloumb

    Coloumb Well-Known Member

    I don't really see how you could work it out. You would need to know what the Zs is at the end of the cable run. The only way you could get any kind of answer would be use the given Zs figure, which I presume to be the max allowed, so it would be a bit pointless. Are you sure you have written the question correctly?
     
  3. RDB85

    RDB85 Member

    Determine, for one of the circuits supplying the fridges, the minimum possible cross-sectional area of cpc which will satisfy the requirements disconnection as ADS under earth fault conditions and the

    adiabatic equation as Regulation 543.1.3 in BS 7671.

    HINT: A two part question. You first need to establish the max (R1+R2) for the circuit based on the Zs for a 16A protective device. From that you can then select a suitable Line & CPC combination for the circuit. The selected size then needs to be confirmed using the adiabatic equation.

    Design Spec:

    Four Fridges in Kitchen Area, Supplied by 2 x 16A circuits.
     
    Last edited: Jun 6, 2019
  4. Coloumb

    Coloumb Well-Known Member

    It still doesn't make a lot of sense mate.

    In order to find the correct CPC size using an adiabatic equation you would have to determine the fault current at the end of the 16a ctt. In order to do this you would have to select a cable, say in this case 2.5mm, and, by knowing the length of the run required, find out it's R1 + R2 by looking up the ohms per meter. One you have R1 + R2 (total ohms) you add Ze to get your Zs which you can then plug into your equation. If you selected say, 1mm, then you would quite likely find the cpc is too small.

    In this case it seems as if they want you to figure out the cpc size by using the maximum Zs which wouldn't really give you the right answer and in any case you simply would not design a ctt. on this basis. I would suggest you go and ask your teacher what the deal is with this or I would say there is still some information missing.
     
  5. Lucky13

    Lucky13 Member

    ZS = ZE + R1 + R2

    ZS - 2.73
    ZE - 0.11

    2.73 - 0.11 = R1+R2 2.62

    Does the question not give you the length of the circuit ? As that will determine which cable to use in order to meet the max R1 R2 value
     
    Last edited: Jun 6, 2019
  6. RDB85

    RDB85 Member

    It is confusing. I have asked the teacher for advice and they have said all the info is there to answer the question.

    • The cable is PVC surface conduit with 70 degrees C thermoplastic single core non-sheathed cable.
    • Calculate CPC using Circuit Breakers Zs
    • Cable Run Max is 30M
    • Ze is 0.11 Ω
    • Zs for a 16A Type B breaker is 2.73Ω
    • So R1+R2 would be 2.73 - 0.11 = 2.62Ω @70°C
    • Correction Factor is 1.20

    I have done this:

    • Ib 16A
    • In 20A
    • Iz 26.59A
    • CSA: 4mm² Ref Method B 32A Volt Drop 11
    • Actual VD: 5.28 Volts
    • Max Volt Drop: 11.5V
    • Zs: 2.73 ohms
    • Ze: 0.11 ohms
     
  7. RDB85

    RDB85 Member

    The Max Length of the Circuits are 30M.
     
  8. Lucky13

    Lucky13 Member

    In - is the CPD size, Ib - is the design current. In your original question you said the MCB is 16a that is the In value. In you workings you have In as 20a

    Since you are given the In rating 16a the design current is <16a so you need to find the cable that has a CCC (Iz) >16a based on the installation method you have been given


    On a side note it’s useful when you ask for help with something you provide all the information in the question ;)
     
    Last edited: Jun 6, 2019
    RDB85 likes this.
  9. RDB85

    RDB85 Member

    I always thought In had to be greater than it equal to Ib. Okay go with 16a as that’s what rating is given in the question.
     
  10. RDB85

    RDB85 Member

    Thanks for the help. I will work out my Iz based upon the rating factors.
     
  11. Lucky13

    Lucky13 Member

    The MCB / Fuse (In) should be greater then Ib but in the question they specify the MCB (In) is 16A so base the cable size on that.
     
  12. Coloumb

    Coloumb Well-Known Member

    I still don't understand why you teachers want you to use the Max Zs for the ctt. If you do the fault current will be really small, less than 100a, which will mean CPC size that is really, really small. It would make more sense if they are telling you the length, Ze and choosen breaker.
     
  13. Lucky13

    Lucky13 Member

    The length is 30m, he listed the full circuit design in a post above
     
  14. Coloumb

    Coloumb Well-Known Member

    Then why is the Zs even listed? What is this, some kind of trick question?

    There is still something not right about any of this. I still think the o.p. is confused and not giving the correct information.
     
  15. Lucky13

    Lucky13 Member

    You need to know max ZS of a CPD to be able to calculate the correct CPC sizes.

    Cables incorporating and earth will generally always be oversized so it’s not an issue But you still have to know how to do the calculations
     
    Last edited: Jun 6, 2019
  16. Coloumb

    Coloumb Well-Known Member

    Which you can't calculate until you have selected a cable size and know the Ze. The o.p. has already given the Zs. Ergo, the cpc size comes out stupidly small with the o.ps given values.
     
  17. Lucky13

    Lucky13 Member

    You only need to know the maximum ZS Value in order to calculate the minimum CPC size required for a circuit, and yes when using the calculation it will nearly always come out with a tiny CPC compared to the standard CPC sizes found in cables.

    • The cable is PVC surface conduit with 70 degrees C thermoplastic single core non-sheathed cable.
    • Calculate CPC using Circuit Breakers Zs
    • Cable Run Max is 30M
    • Ze is 0.11 Ω
    • Zs for a 16A Type B breaker is 2.73Ω
    That's all the information you need to calculate both the CPC and Line conductor sizes.
     
  18. Coloumb

    Coloumb Well-Known Member

    I still don't see what the Zs for the breaker is there for.

    Anyway, I make it a minimum cpc of .78mm presuming I haven't messed anything up.

    Fault current is 283a using a disconnect time of 0.1ms

    This is on the basis that 30m of 2.5mm is used, Ze .11, 230 volts, correction factor 1.2, k value 115.
     
  19. RDB85

    RDB85 Member

    Thanks I will ask the teacher again, but he did say that all the information is given in the question.
     
  20. Coloumb

    Coloumb Well-Known Member

    Ask him if he expects you to use the Zs he has given you or if it needs to be calculated by using the cable length of 30m. I has to be one or the other.
     

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