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TNCS Condition

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Perhaps you can shed some light on this condition for TNCS systems in the Irish Rules. I can only see that as RE decreases RB becomes more onerous but to be honest I am not really getting the essence.
  • Certainly a very odd set of words.


    If I had to guess (and this really is a guess - more based on what I think it might mean rather than what it says)...


    By 'extraneous-conductive-parts not connected to a protective conductor, through which a fault between line and earth may occur' - I'm guessing they mean the earthing system downstream of the PEN break - which is now just Earthed by some incidental connection with true Earth (bonding to some extraneous-conductive-part say) and separated from the functioning PEN conductor by the fault.


    If there was a L-PE fault downstream of the PEN break then the fault loop would include this 'incidental' resistance to Earth at the consumer/fault end (RE) and the source electrode(s) (RB) at the supply end in series (rather like a TT system) - with practically the entire supply voltage (Uo) shared across both electrodes.


    So I think what they're trying to achieve is that the supply star point isn't raised by more than 50V above true earth - presumably for the benefit of other installations still connected to the PEN.


    The situation sounds not unlike the TT systems with an uncleared earth fault that Alan mentions from time to time.


    Anyhow that's my complete guess for the day. I wouldn't be at all surprised if I'd got it completely wrong.


       - Andy,
  • I am trying to figure out how to draw a simple line diagram to illustrate a layout such as that described, but failing to picture it in my minds eye.


    The only example I can think of would be a steel lighting column with its base buried in the ground and supplied by a TN electrical system without a protective conductor connected to it.


    Which makes me think I am getting this completely wrong.


    Andy Betteridge
  • I suppose a steel lighting column is more likely to be an exposed-conductive part than an extraneous-conductive part, as it depends on the insulation rating of the luminaire.


    So if we consider a steel framed building instead, does that fit the description?


    Andy Betteridge
  • Thank you AJ, that makes sense. There must be some sort of guidance for the DSO in order to establish RE or perhaps a deemed to satisfy provision. If RE was one ohm, for example, then RB would need to be no more than 50/160 ohms when Uo is 230v.
  • I am thinking like an English electrician, not an Irish electrician so it’s good job I am not taking an Irish Regs exam.


    This needs considering in the context that all TNCS-PME earthed installations have to have their own earth rod which is effectively connected to the DNO supply cable PEN neutral.

    Analytical Testing Ireland


    Pages 12 and 13.


    TN-C-S System

    The 230 Volt single phase supply to a premises is taken from the secondary winding of a DSO transformer. One terminal of this secondary winding is connected to earth ( ground ). This terminal becomes the neutral for the system. The neutral conductor is therefore at earth potential ( zero volts ).

    A two core concentric cable is generally used to connect from the supply network to the premises. The centre core is the phase conductor. The outer core performs the functions of neutral and protective conductors. This conductor is known as a PEN conductor. It is terminated at the DSO main fuse unit.

    PEN = PE for protective conductor ( protective earth ), N for neutral conductor.

    From this point on, the neutral and protective conductors are separated and must not be connected together, anywhere throughout the entire installation. The protective conductor is referred as the Main Protective Conductor. It is connected to the Main Earthing Terminal of the installation. This means that the Main Earthing Terminal is solidly connected to the DSO neutral.

    At each installation an earth electrode is provided. The earth electrode is connected to the Main Earthing Terminal and therefore to the DSO neutral. This arrangement provides the consumer with an earth terminal, which is connected to the neutral conductor of the system, thereby providing a low impedance ( low resistance ) path for the return of earth fault currents.

    Impedance is the ratio of AC voltage and current. The Ohm is the unit of impedance. Its symbol is the letter Z.
  • So it's a TNC-S with a supplementary earth electrode.
  • If we took the DNO earth resistance RB as 20 ohms then the consumers earth rod could have a maximum resistance of 5.26 ohms.


    I took 20 ohms as that is the accepted maximum given for a DNO transformer earth in the English books, without guidance I don’t see how you can do the maths and get an answer.


    So I reserve the right to change my mind and change the answer, maybe because I have completely misread the question.


    When installing the earth rod can you test the rod and the DNO earth terminal separately, then use the test results obtained to do the calculation?


     Andy B.
  • Lol.


    What we call the DNO earth is referred to as the DSO earth in figure 8.


    So in for a penny, in for a pound!


    If the DSO earth is 20 ohms the maximum consumer installation earth rod impedance is on the face of it is 5.26 ohms, but it is actually referring to the extraneous-parts, not the rod.


     Andy Betteridge

  • If we took the DNO earth resistance RB as 20 ohms then the consumers earth rod could have a maximum resistance of 5.26 ohms.



    I get 72Ω for a DNO resistance of 20Ω and 3.6Ω for 1Ω - and I reckon they're minimum acceptable values if the condition is to be met. I.e. for safety the customer's extraneous-conductive-parts must have a value of that or more (but someone please check my maths!). The lower the resistance of the consumer's "electrode" the lower the voltage difference around it - and hence the greater the voltage difference around the DNO's electrode(s).


    What puzzles me though is what on earth you're meant to do with this information. If the water or gas pipe happened to have lower resistance to Earth ('cos it's buried under half the town) what are you supposed to do about it? You can hardly leave it unbonded, or increase its resistance to Earth. I presume they don't want you to start introducing deliberate impedances into bonding conductors (which would be defeated by c.p.c.s to boilers etc anyway). You can hardly change the DNO's Earthing either.


       - Andy.
  • I did the sums using RB=20 and RE=100


    Then I decided that the other side of the equation always stays the same, so RB/RE has to greater than or equal to 0.2631ohms, so if RB is 20 ohms RE cannot be more than 5.26 ohms.


    Then again perhaps I should not do sums and maths in public. 


    Andy B 

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