Calculating R1 R2 and finding a suitable CPC

Discussion in 'Electricians' Talk' started by Deleted member 176520, Jun 5, 2019.

  1. ajohn

    ajohn Screwfix Select

    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 Screwfix Select

    :) 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 Screwfix Select

    :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. 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 Screwfix Select

    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 Screwfix Select

    :) 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.

  7. ajohn

    ajohn Screwfix Select

    Had a nose at 2018 so pretty sure that the following is correct. Also use the method indicated in the guide where an initial trial size is used as the other way that calculates size isn't sensible as the current is unknown and is set by the cpc wire size.

    Ze is 0.11 ohms
    Zs for a 16A Type B breaker is 2.73 ohms

    Resistance factor for temperature rise from 20 to 160C is 1.2. K for 70C PVC is 115. Assume 12m long. Anything longer will be "better"

    2.5mm line. Method doesn't matter for this as that is part of the actual cable rating - voltage drop etc.

    So fixed R with 2.5mm = ( 12 x 0.00741 x 1.2 ) + 0.11 = 0.217

    R from 12m of 1.5mm = 12 x 0.0251 x 1.2 = 0.362

    R from 12m of 2.5mm = 12 x 0.00741 x 1.2 = 0.106

    Max supply voltage is 230v + 10% = 253v

    Short circuit with 1.5mm = 253 / ( 0.217 + 0.362 ) = 437A

    Short circuit with 2.5mm = 253 / ( 0.217 + 0.106 ) = 783A

    Max t for 1.5mm cpc = 115^2 x 1.5^2 / 437^2 = 0.155sec

    Max t for 2.5mm cpc = 115^2 x 2.5^2 / 783^2 = 0.135sec

    There is a table in 2018 on FiG3A4 that states the 0.1 to 5sec break time. For a 16amp it gives 80amps. Curves too showing the same thing except at >80amps the break time is 0.1 sec. Taking the worst interpretation of that the short circuit current needs to exceed 80 amps to break in 0.1sec. it does so either size of cable could be used but 2.5 T&E would be used - ;) I'd hope so if I was paying for it.

    Zs at lowest mains gives 230v - 5% / 2.73 = 218v / 2.73 ohms = which ;) oddly enough comes out at 80 amps. The circuit resistance is way way lower so 80 amps is bound to be exceeded so once again it will trip on the magnetics. The guide suggests using residual current if this is a problem.

    2018 also mentions the woolly bits. Use the manufacturers figures if available. :) Great most just say meets the BS what ever for the type of device and don't give figures. It also mentions using manufacturers let through figures for times less than 0.1sec. Extremely useful as they generally don't give these either. Some one could fork out a lot of money for this particular standard but the manufacturers may exceed what ever it states.

  8. sparky Si-Fi

    sparky Si-Fi Screwfix Select

    In the real world, this example of a test question is completely pointless:

    two 16A radials on a two 16A breakers, whatever size line/neutral conductors, your going to pull in the same CSA cpc inside that conduit
  9. ajohn

    ajohn Screwfix Select

    Did you notice that the disconnect time needs to be shorter for the larger cable ? Some one could finish up with a disconnect time that needs the manufacturers data. Anyway the standard says that the calculation will be done. ;) When the spec writers eventually get at it they will probably change will to shall. Spec writers love their shalls. No doubt another edition will come out.

    To be honest in terms of a document people are intended to actually to work to 7671 is probably the worst I have ever seen. By work to I mean people who actually do it. They have even wasted ink by printing page numbers in it and not making any use of them at all. It's essentially a jumble and not laid out at all well for people actually doing the job. The guide isn't much better really but I recollect pages of cable ratings and lengths that can be used without calculation with a 0.4sec disconnect time. Interesting time that. Cable temperature looks to be moderate in that but why tables where cable temperature will be 40C over ambient. I'd guess this would shorten the life of the cable.

    The 0.4sec seems to crop up due to the possible need for an rcbo as the circuit resistance is too high for an mcb. Switching time is circa 300mSec at it's rating - 30ma etc. However the ones I looked at reduce this to 40msec at 5x the rating. So why say on some wiring lengths a type C can't be used. Looked at that way it probably can be. Also what max length would be ok with a breaker and 0.1 sec trip time - have to work it out. ;) Maybe reels should be fitted with a length gizmo so that known lengths are pulled out when cable is installed. ;) Hope there isn't a section of the table for mcb's and trip times of 0.1sec or less. I haven't noticed one but surely it would be more useful.

    The thing that confused me about this area was the disconnect times mentioned. Say 0.1 to 5sec. What that doesn't state clearly is that it's due to variations in breakers. They can break quickly at 3 to 5 times their rating so at 5 times some supposedly could take 5 sec but faster ones will break in 0.1sec or less in practice. Then if some one looks at actual data the 5 sec break point is 4.8 times the rating not 5. However greater than 5 guarantees 0.1sec or less.

  10. sparky Si-Fi

    sparky Si-Fi Screwfix Select

    I guess from the amount of info typed here, your a retired sparks?

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