SPD on TT system

what the essential difference is between a TN and TT connection and the consequences if it is incorrect.

There are two connection types CT1 and CT2. CT1 puts an SPD element between each live conductor (L or N) and PE, CT2 puts them L-N and then one N-PE.


The big advantage of CT2 is that if an SPD fails short-circuit, it won't short L-PE (and if two or more elements fail short-circuit, you'll get a L-N fault and disconnection should follow regardless of the earth impedance). On the flip side, CT2  possibly has somewhat higher L-PE voltage let-though during a surge as it'll be across two SPD elements rather than just one.


Its' not entirely down to whether it's TN or TT but whether a L-PE short will cause ADS or not. In a TN system or TT after the first RCD then CT1 is OK, but where ADS isn't available - such as TT before the first RCD - then CT2 is the way to go, otherwise a SPD fail to short-circuit could leave the entire earthing system permanently hazardous live.


Some designs of SPD are less likely to short circuit when the fail, so with these a CT1 approach might not be so dangerous - but without detailed information about the particular design it's difficult to be sure.


  - Andy.

what the essential difference is between a TN and TT connection and the consequences if it is incorrect.

There are two connection types CT1 and CT2. CT1 puts an SPD element between each live conductor (L or N) and PE, CT2 puts them L-N and then one N-PE.


The big advantage of CT2 is that if an SPD fails short-circuit, it won't short L-PE (and if two or more elements fail short-circuit, you'll get a L-N fault and disconnection should follow regardless of the earth impedance). On the flip side, CT2  possibly has somewhat higher L-PE voltage let-though during a surge as it'll be across two SPD elements rather than just one.


Its' not entirely down to whether it's TN or TT but whether a L-PE short will cause ADS or not. In a TN system or TT after the first RCD then CT1 is OK, but where ADS isn't available - such as TT before the first RCD - then CT2 is the way to go, otherwise a SPD fail to short-circuit could leave the entire earthing system permanently hazardous live.


Some designs of SPD are less likely to short circuit when the fail, so with these a CT1 approach might not be so dangerous - but without detailed information about the particular design it's difficult to be sure.


  - Andy.