ProMbrooke:

gkenyon:

That's interesting.


So, how should we approach TT systems?

Delayed RCD main in a plastic enclosure with 30ma sub-main RCD. However, clearing longer than 0.035 seconds of the main RCD may not be fast enough in areas of lowered skin conductivity.  

Sub-main only requires 2 s disconnection time. Also, in TT systems, even in final circuits you're not limited to 30 mA RCD for some circuits, provided earth electrode resistance is sufficiently low to ensure 2x residual current rating. You can achieve 0.04 s disconnection with any non-delayed RCD if you can achieve 5x residual current rating.

In all honesty I'd like developed countries to move away from TT supplies. TT, TN-C-S and PME are simply a choice between cons in a world where DNOs do not want to provide more conductors than it takes to get the lights on. TN-S provides all the advantages of superior fault clearing given by TN-C-S, but without parallel earth currents or the risk of exposed parts becoming live should the neutral be lost. 

In PME systems, the main issue we have is when the PEN breaks.

If we use a 4-core cable, with the armour or outer wrap as earth, we will have the same problem in TN-S systems. It will be a broken PE rather than PEN - although this time you won't know about it until you need it. At least with broken PEN, most of the time it's known about before it's a real issue.

There are things you can do to monitor PE - but there are also things you can do to monitor PEN (perhaps less reliable, though, if you have a single-phase installation in a three-phase network).


Because of this, the only advantage I can see that TN-S has over TN-C-S is that there are no diverted neutral currents in "normal operation". Diverted neutral currents can cause "tingles" in certain conditions, and of course not recommended in locations with explosive atmospheres. Where it may cause tingles, or livestock are involved, a buried earth grid can help control voltage effects of diverted neutral currents.


 

Stray voltage is the byproduct of TN-C-S.


Any idea why the UK started off with TN-S but moved away from it?

cost

ProMbrooke:

gkenyon:

That's interesting.


So, how should we approach TT systems?

Delayed RCD main in a plastic enclosure with 30ma sub-main RCD. However, clearing longer than 0.035 seconds of the main RCD may not be fast enough in areas of lowered skin conductivity.  

Sub-main only requires 2 s disconnection time. Also, in TT systems, even in final circuits you're not limited to 30 mA RCD for some circuits, provided earth electrode resistance is sufficiently low to ensure 2x residual current rating. You can achieve 0.04 s disconnection with any non-delayed RCD if you can achieve 5x residual current rating.

In all honesty I'd like developed countries to move away from TT supplies. TT, TN-C-S and PME are simply a choice between cons in a world where DNOs do not want to provide more conductors than it takes to get the lights on. TN-S provides all the advantages of superior fault clearing given by TN-C-S, but without parallel earth currents or the risk of exposed parts becoming live should the neutral be lost. 

In PME systems, the main issue we have is when the PEN breaks.

If we use a 4-core cable, with the armour or outer wrap as earth, we will have the same problem in TN-S systems. It will be a broken PE rather than PEN - although this time you won't know about it until you need it. At least with broken PEN, most of the time it's known about before it's a real issue.

There are things you can do to monitor PE - but there are also things you can do to monitor PEN (perhaps less reliable, though, if you have a single-phase installation in a three-phase network).


Because of this, the only advantage I can see that TN-S has over TN-C-S is that there are no diverted neutral currents in "normal operation". Diverted neutral currents can cause "tingles" in certain conditions, and of course not recommended in locations with explosive atmospheres. Where it may cause tingles, or livestock are involved, a buried earth grid can help control voltage effects of diverted neutral currents.


 

Stray voltage is the byproduct of TN-C-S.


Any idea why the UK started off with TN-S but moved away from it?

cost