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Electrical Installation Earthing

Ishan
What are the factors need to be considered when converting an existing TT earthing system to a TN-S earthing system?
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    What are the factors need to be considered when converting an existing TT earthing system to a TN-S earthing system? 



     



    On the assumption it is a true TN-S system (so not a LV supply from a UK DNO).


    Earthing conductor size might need increasing (TT could haven been as small as 2.5mm2 if mechanically protected).


    Bonding sizes might need increasing (not likely, but theoretically possible depending on Zs and DNO's protective device)



    PEFC will almost certainly be higher than under TT - if higher than PSCC then it would be wise to check the breaking capacity of protective devices (although probably not necessary for a domestic installation with a UK style CU with its 16kA rating).


    Small possibility that some reduced c.p.c.s might be undersized, given the larger PEFC - especially where circuit breakers are used rather than fuses.


    Isolation and disconnection times are more onerous for TT than TN-S, so no problems there.


    Consideration might be given to the removal of some RCDs if they're no longer necessary (e.g. ≥100mA units on submains) to reduce maintenance overheads and increase reliability.


       - Andy.
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  • 0


    What are the factors need to be considered when converting an existing TT earthing system to a TN-S earthing system? 



     



    On the assumption it is a true TN-S system (so not a LV supply from a UK DNO).


    Earthing conductor size might need increasing (TT could haven been as small as 2.5mm2 if mechanically protected).


    Bonding sizes might need increasing (not likely, but theoretically possible depending on Zs and DNO's protective device)



    PEFC will almost certainly be higher than under TT - if higher than PSCC then it would be wise to check the breaking capacity of protective devices (although probably not necessary for a domestic installation with a UK style CU with its 16kA rating).


    Small possibility that some reduced c.p.c.s might be undersized, given the larger PEFC - especially where circuit breakers are used rather than fuses.


    Isolation and disconnection times are more onerous for TT than TN-S, so no problems there.


    Consideration might be given to the removal of some RCDs if they're no longer necessary (e.g. ≥100mA units on submains) to reduce maintenance overheads and increase reliability.


       - Andy.
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