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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
    This all hinges on the failure modes of the solid state surge arrestors, which tend to fail to a low resistance (near short) condition. These then need a series fuse or thermal cut-out or similar which will only blow if there is enough current available. On TT there may not be if Zs is not low enough.,


    Gas discharge surge arrestors tend to fail in the sense that either the electrodes burn back, or the gas escapes,  and both of these result in the striking voltage eventually rising so far they do not operate at all.

    A gas discharge device is not something to be left accross the mains however, as once struck by the impulse of a few kV it will stay lit, like a neon lamp without the limiter resistor, or like striking an arc with a welder - once the arc has struck it needs the voltage to fall enough for it to extinguish. If there is an unlimited source of current at low voltage available it will overheat and fail in short order.

    Hence the slightly more complex internals of the TT type surge arrestor with both types of device.

    The regs do not explain this at all, and make the description of  how to do surge protection far more complex than it needs to be (and ambiguous...).


    Mike.
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  • 0
    This all hinges on the failure modes of the solid state surge arrestors, which tend to fail to a low resistance (near short) condition. These then need a series fuse or thermal cut-out or similar which will only blow if there is enough current available. On TT there may not be if Zs is not low enough.,


    Gas discharge surge arrestors tend to fail in the sense that either the electrodes burn back, or the gas escapes,  and both of these result in the striking voltage eventually rising so far they do not operate at all.

    A gas discharge device is not something to be left accross the mains however, as once struck by the impulse of a few kV it will stay lit, like a neon lamp without the limiter resistor, or like striking an arc with a welder - once the arc has struck it needs the voltage to fall enough for it to extinguish. If there is an unlimited source of current at low voltage available it will overheat and fail in short order.

    Hence the slightly more complex internals of the TT type surge arrestor with both types of device.

    The regs do not explain this at all, and make the description of  how to do surge protection far more complex than it needs to be (and ambiguous...).


    Mike.
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