Calculating R1 R2 and finding a suitable CPC

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

  1. ajohn

    ajohn Active Member

    So this seems to be the formulae


    The one to use there is the known area but of the cpc only as it's the thinnest and the line wont get as hot.

    So t = 100^2 x 1.5^2 / 982^2 = 0.023 sec

    For the live it would be 100^2 x 2.5^2 / 982^2 = 0.065 sec

    Given a typical limit curve 16x rating would achieve that.

    If the cpc was increased to 2.5mm the fault current would go up 1293 amps

    So t = 100^2 x 2.5^2 / 1293^2 = 0.037 sec

    The formulae in the guide is S^2=I^2 x t / k^2 ;) They don't say what the constants are. I can guess I and K but not S or t. It's mentioned along with disconnect time so may or may not have something to do with that. Earlier I assumed it was.

  2. ajohn

    ajohn Active Member

    :) Stinking fish. It seems as some one mentioned that permissible let through energy is needed. It's seems that for a 16A Type B class 3 mcb A^2s is 35000 for one with a 6KA break capacity and 70000 for ones with 10KA. This is for BS EN 60898 mcb's. It's not spec'd on earlier ones which are regarded as class one. The mcb's should have 10000 or 6000 with a 3 under them actually on them.

    The info I found suggests that the figures are I^2t and for some reason they use A^2s. That sort of thing annoys me - lets change the jargon

    So max I^2t = 35000 for a 6KA part. Max T with the 1.5mm cpc is 35000 / 982^2 = 0.036 sec.

    With a 2.5mm cpc it becomes 35000 / 1293^2 = 0.021 sec

    RCBO's are the same other than max fault current wiring resistance can be based on it's current trip rating.

    The info came from here - sorry about the link, wont open in a tab.

    I haven't got BS 7671 but something smells rather strongly to me in this area as there is no way of knowing where a fault will occur in a cable. If one does trip sounds like they should be replaced.

  3. ajohn

    ajohn Active Member

    :oops: I may need to rethink how max time is calculated from a stated let through max value. eg 35000 I^2t for a 16amp 6KA mcb

    I see it as I^2 x t <= 35000
    so t <= 35000 / I^2

    :) You are all the electricians so tell me. Pretty sure I am correct actually.

    But say a 10KA device is used. It then becomes t <= 100000 / I^2 so as expected the time is longer.

    I did wonder if these times could be used in the adiabatic equation. If the 6K1 figure is used in it A comes out at 1.41mm so maybe this is the figure for t that the OP should use. :) Bit of a problem though if the circuit was a 2.5mm t&e radial socket circuit as it would need to be larger. Then if a 10KA device is used the time goes up so the equation results in much larger areas.

    Leaves me wondering what the sections indicated on this actually say


    :) I needed to take a photo of a plasterboard lift to sell it so thought may as well take another of the guide. :) And then thought I'm in need of getting a life.

  4. RDB85

    RDB85 Member

    So I had another go:
    • Ib 16A
    • In 16A
    • Method of Installation: C
    • Rating Factor: 16/0.94x0.80 = 21.27A
    • It 2.5mm2 24A VD 18
    • Actual VD: 18X12X16/1000 = 3.456V
    • Max VD: 11.5V
    • Max R1+R2 2.73-0.11/1.2x12 = 181.94Ω
    • Calculated R1+R2 1482x1.2x12/1000 = 0.11Ω
    • Calculated Zs 2.73x0.8 = 2.18+0.11 = 2.2Ω
    • Fault Current: 230/2.2 = 104A
    • S = Square Root: 104² x 0.1/115 = 0.28mm²
    2.5mm² CPC is suitable.
  5. ajohn

    ajohn Active Member

    That to me says that it isn't and I can't see where the 104 amps comes from other than via using the Zs of the breaker which appears to be the min resistance for the breaker to trip after ~5sec worst case to ~3sec fastest neither of which is 0.1sec so t in that case should be 5secs. If Zs can mean something else that shouldn't be called Zs. If it happened to be the resistance of the device the volts drop in normal use would be enormous.

    ;) I've been trying to find BS7621 2018 on the web. No chance. Might be able to get one but not quickly. Hopefully it should shed some light on the subject but having used various spec's in the past it may just refer to others.

    I did buy the 2018 guide for specific personal reasons. It's very different to the previous version but the part that interests me, bonding pipes is exactly the same. There is a lot more on mcb's etc but all the mention on breakers is the 3 to 5 times rating to trip and more info on the markings on them. Also a lot on cable lengths that can be used without calculations but some of it is a little odd. Some for 2.5mm for instance is I suspect for a ring but that's not clearly stated.

    :) Noticing what a full set of IET books cost I'd suspect that your answer is in those. £350 - sounds like a money go round to me. Funny thing. If it was a university course worked examples which always help clear things up are easily available and pretty cheap. Schaum produces them. Loads of them several of which have given me severe headaches if I need to pick something up outside my own field.

  6. ajohn

    ajohn Active Member

    :) Not used this plotter before and having some spare time decided to try it. This is the result with details originally posted and a length of 15m. 2,5mm line and cpc varied.


    Checked a couple of values manually and seems to be correct. ;) Done out of interest - ;) I have odd ones at times.


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