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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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    It sounds to me that these devices are of limited value and perhaps cause more problems in terms of making an installation potentially unsafe with their presence than real use.

    In the event of a L-N or LE short within the device, I thought the purpose supplied and connected upstream mcb would negate the large fault current, which I thought was why CPFUSEBOX began to include mcbs in their 2nd series of consumer units. The initial consumer units did not have a purpose installed mcb, but CPFUSEBOX stated that any non-mcb equipped units did not require any retrofitting.

    I thought the principle difference between CT1 & CT2 devices was in their ability to handle differerent sized fault currents, with the CT1 device being rated for higher levels than CT2..
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  • 0
    It sounds to me that these devices are of limited value and perhaps cause more problems in terms of making an installation potentially unsafe with their presence than real use.

    In the event of a L-N or LE short within the device, I thought the purpose supplied and connected upstream mcb would negate the large fault current, which I thought was why CPFUSEBOX began to include mcbs in their 2nd series of consumer units. The initial consumer units did not have a purpose installed mcb, but CPFUSEBOX stated that any non-mcb equipped units did not require any retrofitting.

    I thought the principle difference between CT1 & CT2 devices was in their ability to handle differerent sized fault currents, with the CT1 device being rated for higher levels than CT2..
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