SPD on TT system

I’m glad you have asked this question, I tried asking during a webinar Q&A session a couple of years ago and still don’t feel I have fully grasped it.


In my minds eye I see voltage on either live or neutral being dumped to true earth in a TT or TNS installation, but with neutral and earth being combined in a TNCS installation a SPD connected between neutral and “earth” can’t do anything as they are the same conductor.


Andy B

Only true to say that N and E are at the same voltage if you are close to the point of junction, and exactly what close means depends on the pulse duration, and to a lesser extent what else is connected to earth but not to the neutral that will increase the capacitance to terra firma. But if you are more than perhaps 6 feet away the they can be 1000+ volts apart for a few microseconds during a kA surge.


But in the TT suppression there is a spark gap or at least a gas discharge device in the earth path, as semiconductors may fail short, but not fire the ADS properly In a TN system the semiconductors can go to ground and fuses will handle it.

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.