WHATS THE DIFFERENCE BETWEEN TNS and TNC-S

[Joeyoc2006]

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 nusance tripping but what is the advantage of that system over TNS???????? :S

 

[ColJack]

you don't get a choice in the matter, you get what you're given so what's the point of worying which is better..

I've never heard that TN-C-S is any worse for nuisance tripping than TN-S..

 

[Spark123]

Given the choice TN-S wins every time for me.
The major difference is that you have a separate earth core all the way back to the substation in a TN-S whereas in a TN-CS the earth and neutral are the same core (CNE).
This means that if this was to break then all your metalwork can become live at mains potential, hence it has to be tied down using electrodes along it's length to minimise the risk, (Protective multiple earthing).
Owing to the risk of an open circuit neutral TN-CS is banned from caravan sites and marinas in the UK. I think it is banned places such as petrol stations too and extra precautions need to be applied for some special locations.
Inside your house as long as your main bonding is up to scratch the risk is reduced but your appliances might be a bit upset about the battle of the phases downstream of an open circuit neutral, but theres not a lot you can do about that.

 

[Joeyoc2006]

basically my son has asked me, cause he has a question on it in his school work from college, and i cant think of an advantage of having TNC-S over TNS

 

[Spark123]

It's cheaper for the DNO is the only real advantage I can think of.

Give him a clip around t' lug and tell him to pay attention in class

 

[ban-all-sheds]

But DON'T tell him that the impedence is higher with TN-C-S.

 

[ericmark]

You are correct that the permitted maximum impedance for TN-C-S is 0.35Ω and for TN-S it is 0.8Ω outside the consumers installation.

You are also correct that neutral – earth faults are more likely to trip an RCD in the TN-S than TN-C-S as the voltage between earth and neutral is likely to be higher.

As already stated some locations we are not allowed to use TN-C-S but having said that it is not always easy to identify between the two. As a result many people play safe and use a TT system for supply to Caravans. Boats are another story as we have to consider cathode protection and erosion of sacrificial anodes.

As Spark123 has said TN-C-S is cheaper for the DNO and because it uses multiple earthing the earth - line loop impedance and the neutral – line loop impedance is lower due to parallel paths. Before the near blanket use of RCD’s this may have been an advantage. A D32 MCB on a TN-C-S system would trip on the magnetic part of the trip if the internal cable was short but with TN-S a B32 MCB was largest that could be used.

However with the blanket use of RCD’s these advantages have now gone.

 

[ban-all-sheds]

You are correct that the permitted maximum impedance for TN-C-S is 0.35Ω and for TN-S it is 0.8Ω outside the consumers installation.

You are also correct that neutral – earth faults are more likely to trip an RCD in the TN-S than TN-C-S as the voltage between earth and neutral is likely to be higher.

i know that the impedance on the TNC-S is higher and you get nusance tripping

 

[ericmark]

You are correct that the permitted maximum impedance for TN-C-S is 0.35Ω and for TN-S it is 0.8Ω outside the consumers installation.

You are also correct that neutral – earth faults are more likely to trip an RCD in the TN-S than TN-C-S as the voltage between earth and neutral is likely to be higher.

i know that the impedance on the TNC-S is higher and you get nusance tripping

OK point taken "What is higher" I like many others refer to changing to a higher gear in my car as speed increases although it is a lower gear ratio and I had assumed he was talking in same vain. And if measured in Siemens he would be correct and he did not say what units he was using!!!

 

[Spark123]

It isn't often we work in admittances or conductances, we tend to use resistances or impedances.

 

[ban-all-sheds]

OK point taken "What is higher" I like many others refer to changing to a higher gear in my car as speed increases although it is a lower gear ratio and I had assumed he was talking in same vain. And if measured in Siemens he would be correct and he did not say what units he was using!!!

 

[NotHimAgain]

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.

 

[Grizzly]

And if measured in Siemens he would be correct and he did not say what units he was using!!!

If he was talking in siemens, then he would have said admittance, not impedance, wouldn't he?