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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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  • 0
    As I said earlier, Type and Connection Type are two very different concepts. Type 1,2 or 3 is, very crudely, a measure of how "beefy" they are, while Connection Types 1 and 2 refer to the pattern of connection - whether the PE is treated as common, connected via a device to each line, or where the N is treated as common, with a device connected to each live and to the PE.


    CT2 is needed just for the specific case of a TT system where the SPD is connected upstream of the RCD (which is serving for fault protection rather than just additional protection) and where a fail-to-short on an SPD between a live and PE won't trigger ADS - thus leaving the PE at near live potential.
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  • 0
    As I said earlier, Type and Connection Type are two very different concepts. Type 1,2 or 3 is, very crudely, a measure of how "beefy" they are, while Connection Types 1 and 2 refer to the pattern of connection - whether the PE is treated as common, connected via a device to each line, or where the N is treated as common, with a device connected to each live and to the PE.


    CT2 is needed just for the specific case of a TT system where the SPD is connected upstream of the RCD (which is serving for fault protection rather than just additional protection) and where a fail-to-short on an SPD between a live and PE won't trigger ADS - thus leaving the PE at near live potential.
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