I am not to sure of the advantage of having a TNC-S system over a TNS, i know that the impedance on the TNC-S is higher and you get nuisance tripping but what is the advantage of that system over TNS???????
Whether there is any advantage or not is somewhat academic as TN-C-S (PME) is here and it is here to stay.
Many wax lyrical about the good old days of TN-S but all systems have advantages and disadvantages. Taken across the world the most widely used system is, in fact, TT. It is mainly North America and Northern Europe that favour TN systems.
TT systems require less technical input and have much lower fault currents. However, they do generally rely on an electromechanical device (RCD) to operate under fault conditions.
Without attempting to list all of the advantages and disadvantages of TN-S and TN-C-S - I will just highlight a couple for TN-C-S, since this is now the dominant system.
Advantages
1) Cost – generally cheaper to install
2) Fault monitoring – people shout when the supply neutral breaks, they don’t if the supply earth does.
Disadvantages
It raises the potential of the consumers’ earth terminal above true earth.
Broken supply neutral faults can have serious consequences - but these are rare occurrences.
Another problem is that we cannot recreate the good old TN-S system because we no longer install lead sheathed cables. TN-S was good because that lead sheath provided a very long low impedance earth electrode once the hessian had rotted off
. Lead is far too dangerous and expensive to be used these days.
TN-S formed by a separate copper conductor would be expensive and would be prone to unnoticed failures.
So we are where we are I am afraid.
Please be aware that whilst you may think you have a TN-S supply this may only be true of your service line. DNOs will often convert main distributors to PME if they have to do a repair - because they can’t get the lead sheathed cable.