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CT1 and CT2

lyledunn

The connection of SPDs using CT1 appears to provide protection only in common mode. That seems be the normal method employed. However, looking at CT2 am I correct in assuming that the clamp voltage would need to be very much less as the protection is provided across the conductors supplying load components? It seems to me that CT1 is for lightning and CT2 more for overvoltages derived from switching???

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    As I see it both CT1 and CT2 limit voltages both between live conductors and between live conductors and Earth - I agree the arrangement is different - so in some circumstances you have two SPDs in series to get a particular effect (L-N clamping in CT1 and L-PE clamping in CT2) but as long as the resulting 'let-through' voltages are still acceptable the protection should still be there. The alternative would be to use three SPD triangle fashion directly between all thee wires (L/N/PE), but that doesn't seem to be necessary.

    For me the difference in usage between CT1 and CT2 is the consideration of the result of the L SPD failing to short circuit - if it's connected L-PE and the short can't cause ADS (e.g. because it's TT and upstream of the first RCD) things can turn nasty - so that's avoided with the CT2 arrangement.

       - Andy.

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  • 0

    As I see it both CT1 and CT2 limit voltages both between live conductors and between live conductors and Earth - I agree the arrangement is different - so in some circumstances you have two SPDs in series to get a particular effect (L-N clamping in CT1 and L-PE clamping in CT2) but as long as the resulting 'let-through' voltages are still acceptable the protection should still be there. The alternative would be to use three SPD triangle fashion directly between all thee wires (L/N/PE), but that doesn't seem to be necessary.

    For me the difference in usage between CT1 and CT2 is the consideration of the result of the L SPD failing to short circuit - if it's connected L-PE and the short can't cause ADS (e.g. because it's TT and upstream of the first RCD) things can turn nasty - so that's avoided with the CT2 arrangement.

       - Andy.

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