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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
    In principle if the varistor blew SC, there is plenty of LN current, so the fuse will remove it. (or it will remove itself, depending on the design) Steady state once the event has passed N and E are at nearly the same voltage and the arc goes out in the gas, all is well.


    During the zap, the TT earth electrode and wiring will rise quite a way above terra firma earth, perhaps more than a TN system, but to be honest  perhaps not, depending on the wiring inductances.

    The situation is not clear cut.

    M.
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  • 0
    In principle if the varistor blew SC, there is plenty of LN current, so the fuse will remove it. (or it will remove itself, depending on the design) Steady state once the event has passed N and E are at nearly the same voltage and the arc goes out in the gas, all is well.


    During the zap, the TT earth electrode and wiring will rise quite a way above terra firma earth, perhaps more than a TN system, but to be honest  perhaps not, depending on the wiring inductances.

    The situation is not clear cut.

    M.
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