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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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    I agree it's not at all clear what it's trying to say (it's obviously confused me too as my copy has a pencil comment of 'Eh? 411.4.1. is about TN systems' against it - presumably from my first reading of it about three years ago).


    I suspect that Z. has  a handle on it - i.e. when the N-PE SPD shorted, the system in effect becomes TN-C-S and so the supply N is acting as a PE - so the supply N should meet the same requirements for earthing as the PE or PEN in a TN system. Otherwise any nasty voltages on the supply N get imposed on the consumer's exposed/extraneous metalwork.


    Does perhaps rather beg some awkward questions where the distribution system doesn't meet PME standards though...


    Or it might be a typo and it should be referring to something else completely.


      - Andy.

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  • 0
    I agree it's not at all clear what it's trying to say (it's obviously confused me too as my copy has a pencil comment of 'Eh? 411.4.1. is about TN systems' against it - presumably from my first reading of it about three years ago).


    I suspect that Z. has  a handle on it - i.e. when the N-PE SPD shorted, the system in effect becomes TN-C-S and so the supply N is acting as a PE - so the supply N should meet the same requirements for earthing as the PE or PEN in a TN system. Otherwise any nasty voltages on the supply N get imposed on the consumer's exposed/extraneous metalwork.


    Does perhaps rather beg some awkward questions where the distribution system doesn't meet PME standards though...


    Or it might be a typo and it should be referring to something else completely.


      - Andy.

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