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Changes for the 18th Edition - Disconnection times for distribution circuits in a TN system

Yadagani Balakrishna
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Hi,

With reference to the post in  "Changes for the 18th Edition", what is the reason for considering 1 Sec (see snap below) disconnection time for Other circutis in TN system.

As per BS 7671:2018 Regulation 411.3.2.3 - In a TN system, a disconnection time not exceeding 5 s is permitted for a distribution circuit and for a circuit not covered by Regulation 411.3.2.2.

So is this supposed to be 5 Sec as per the BS 7671?

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    Hi Shane,
     
    This is Balakrishna, Project engineer working in Aecom Bangalore office.  Thank you for the further information provided in trailing email notification from Bentley Communities.
     
    But the actual issue is not just the display error. In fact the calculation is considering the disconnection time of 1 Sec for other circuits (typically 5 Sec as per BS 7671)  in TN systems. Ideally, when we run a project in both versions the submain cable should not have a problem with regards to the disconnection times as the table 41.1, as defined in clause 411.3.2.2 of BS 7671:2018 is only for final circuits mentioned therein.
     
    Please see the below snaps for calculations performed in both 17th edition (using previous Hevacomp version V8i SS1 SP5) and 18th edition (new version V8i SS1 SP7). For this case the same project has been calculated in both versions and the submain cable results are presented below for verification.
     
    Also, I am wondering why the foot note “Earth fault failure (Ref 411.3)” is referring to a disconnection time of 0.4 Sec instead of 5 Sec for a submain distribution cable? We appreciate your time and effort in resolving the issue.
     
    17th edition calculation:
                                                                                                                                                   
     
     
     
     
    18th edition calculation:
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    Thanks & Regards,

    Balakrishna Y B, MIET
    Project Engineer - E&P, Transportation - Rail
    Yadagani.Balakrishna@aecom.com

    AECOM
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    Survey Number 28, Doddanekundi Main Road, Doddanekundi, Mahadevpura,
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  • 0 in reply to Yadagani Balakrishna

    Hi Balakrishna / Yadagani,

    Many thanks for the clarification above. I am consulting the support manager and development about this issue and will get back to you as soon as I have further information.

    Shane Regan | Principle Building Performance Support Engineer

       

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