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Single phase sub-main

GA

Evening, I'm seeking some advise pls. I have a 4core 25mm2 SWA and was going to use this for two single phase submains running from and to a common point, where the circuits will then go into separate consumer units. limited loads a garage and stable. The cable will have DP isolation and RCD protection on each circuit (2cores L+N per sub main) at the supply point. I'm also looking to run a separate earth cable of 25mm2 which would be common to both circuits and armouring of the SWA cable is also earthed and common. My question is I believe this to be complaint with 17th Ed; As I can't see anything saying its not. Second question, could I common the neutral for both submains and therefore use the fourth conductor in the SWA as an earth which would save installing the separate earth cable? Again, I can't see anything incorrect as limited loads to a couple of sockets and a few lights.  thanks for assistance. GA

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  • 0

    Zoomup: 
    Hello G A, We are using the 18th edition nowadays. Many times the armour is adequate to use as a C.P.C.

    Use SWA as CPC, a guide to the acceptability of steel wire armour use as CPC. (gadsolutions.biz).

    Z.

    Just to note that the calculation method for loop impedance shown in Table 4 on this site, whilst still demonstrated in the latest (2018) IET Electrical Installation Design Guide, should really be replaced for cables with line conductor csa 16 sq mm and above (where reactance comes into play more) by the method shown in Annex NA of PD IEC/TR 50480:2011 - specifically sections NA 4.4 (armour alone used as cpc) and NA 4.5 (external cpc). 

    For the case where the armour is used as cpc, the total loop impedance contribution of the SWA cable for a line to earth fault is given by:
     

    eb8a783f45515722fccaf8272b9d31e1-original-image.png

    R1 is the DC resistance milliohms per metre) of the line conductor, R2 is the DC resistance (milliohms per metre) of the SWA.
     

    Don't forget the above is in milliohms per metre, so multiply by length, divide by 1000. 


    This leads to a slightly higher overall DC resistance per metre but a slightly lower overall reactance per metre.

     

     

Reply
  • 0

    Zoomup: 
    Hello G A, We are using the 18th edition nowadays. Many times the armour is adequate to use as a C.P.C.

    Use SWA as CPC, a guide to the acceptability of steel wire armour use as CPC. (gadsolutions.biz).

    Z.

    Just to note that the calculation method for loop impedance shown in Table 4 on this site, whilst still demonstrated in the latest (2018) IET Electrical Installation Design Guide, should really be replaced for cables with line conductor csa 16 sq mm and above (where reactance comes into play more) by the method shown in Annex NA of PD IEC/TR 50480:2011 - specifically sections NA 4.4 (armour alone used as cpc) and NA 4.5 (external cpc). 

    For the case where the armour is used as cpc, the total loop impedance contribution of the SWA cable for a line to earth fault is given by:
     

    eb8a783f45515722fccaf8272b9d31e1-original-image.png

    R1 is the DC resistance milliohms per metre) of the line conductor, R2 is the DC resistance (milliohms per metre) of the SWA.
     

    Don't forget the above is in milliohms per metre, so multiply by length, divide by 1000. 


    This leads to a slightly higher overall DC resistance per metre but a slightly lower overall reactance per metre.

     

     

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