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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
    Yeah I understand all that, but i still can't see why 411.4.1 is made an additional requirement. Or to put it another way, suppose I have a TT system which compiles with all the relevant bits of BS 7671 - so there's an earth rod, 30mA RCD at origin etc. Then later, I connect a CT2 SPD upstream of the RCD. What (if any) extra constraint does 534.4.6 impose upon the installation (that wasn't already there by virtue of being a TT system), by requiring me additionally to comply with 411.4.1? And if it doesn't impose any extra constraints, why require it?
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  • 0
    Yeah I understand all that, but i still can't see why 411.4.1 is made an additional requirement. Or to put it another way, suppose I have a TT system which compiles with all the relevant bits of BS 7671 - so there's an earth rod, 30mA RCD at origin etc. Then later, I connect a CT2 SPD upstream of the RCD. What (if any) extra constraint does 534.4.6 impose upon the installation (that wasn't already there by virtue of being a TT system), by requiring me additionally to comply with 411.4.1? And if it doesn't impose any extra constraints, why require it?
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