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TT CT2 SPDs and TN requirement

wallywombat
534.4.6 says that for TT systems where an SPD is upstream of the RCD, then a CT2 SPD must be used (fair enough), but also says that "the conditions of Regulation 411.4.1 shall be met". But 411.4.1 is about TN systems having a dependable connection to earth. In particular:
411.4 TN system

411.4.1 In a TN system, the integrity of the earthing of the installation depends on the reliable and effective

connection of the PEN or PE conductors to Earth. Where the earthing is provided from a public or other supply

system, compliance with the necessary conditions external to the installation is the responsibility of the distributor.

I don't get why a section about SPDs in a TT system refers to a TN requirement.
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    whjohnson:

    :

    I have a job coming up on a TT supplied installation whereby a new consumer unit will be fitted in addition to the existing set-up. I wish to elect to use a CPFUSEBOX item with a T2 SPD

    :

    The only info I can see is that their SPD is labelled T2.


    If it's this one https://www.consumerunitworld.co.uk/fusebox-spd2ptn-type-2-surge-protection-device-3644-p.asp the description says suitable for TN - so I'd suspect it's configured CT1 (with the bottom of the L SPD connected to PE rather than N) - which means it wouldn't be suitable for connection in TT system upstream of the first RCD.


       - Andy.


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  • 0
    whjohnson:

    :

    I have a job coming up on a TT supplied installation whereby a new consumer unit will be fitted in addition to the existing set-up. I wish to elect to use a CPFUSEBOX item with a T2 SPD

    :

    The only info I can see is that their SPD is labelled T2.


    If it's this one https://www.consumerunitworld.co.uk/fusebox-spd2ptn-type-2-surge-protection-device-3644-p.asp the description says suitable for TN - so I'd suspect it's configured CT1 (with the bottom of the L SPD connected to PE rather than N) - which means it wouldn't be suitable for connection in TT system upstream of the first RCD.


       - Andy.


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