Benefits of a TT system?

[Crosswire]

If an installation is supplied by a two wire system (E.G. a TT system) , what are the benefits of using an earth electrode instead of simply joining earth to neutral at the supply head to create a TNCS system?

Surely using the supplier's neutral is a more effective return path for fault current than relying on the earth electrode and the solid mass of earth?

All replies welcome,

Cheers,
Crosswire.

 

[Octopus]

Are you suggesting you would add the link?

 

[trev]

Surely you're taking the mick, please tell us you are.
Lots of people, and you appear to be one of them, are frightened of TTs, presumably because you see the 200 ohms figure in various publications and compare that to the values for TNs.
So here's a bit of news for you, with a bit of thought, time and effort, in most cases TN values can be achieved.
Now, are you seriously suggesting you're going to go around knocking out imitation PMEs?

 

[Crosswire]

I'm asking what the the relative benefits are of the two different systems. The link could be added by myself or the supplier, it doesn't really matter.

What is important is how this will affect the installation.

 

[Simonslimline]

The benefit of a DNO provided earth is you don't have to sink rods in until you get a nice low value.

 

[Crosswire]

Surely you're taking the mick, please tell us you are.
Lots of people, and you appear to be one of them, are frightened of TTs, presumably because you see the 200 ohms figure in various publications and compare that to the values for TNs.
So here's a bit of news for you, with a bit of thought, time and effort, in most cases TN values can be achieved.
Now, are you seriously suggesting you're going to go around knocking out imitation PMEs?

This is partly why I stopped posting here.

Instead of simply providing a technical explanation of the relative benefits of the two different systems, the thread dissolves into personal attacks within two replies.

For your information I have never done this. It was a hypothetical instance, purely because I am interested in the relative benefits of different earthing systems..

I am not "frightened" of the different required values, merely interested. I clearly chose the wrong forum to initiate a discussion of earthing systems in.

Thanks all the same,

Crosswire

 

[trev]

First off, you are not allowed to install the link, there are specific requirements for TN-C-S and simply bunging in a DIY link between N & E doesn't cut the mustard. If you've done this in the past or are proposing to do it in the future then I'm questioning your competence.
Secondly, a properly thought out and installed TT system can easily achieve Ra values equivalent to a TN so what's the problem?
You work with what you've got mate.

 

[derek]

TN C-S also known as PME, how do you know unless you work for the DNO whether it is PME or not; you could loose the neutral therefore your path to earth, if the wrong type of supply cable is used as far as I understand it. I could be mistaken mind

 

[trev]

Well perhaps if you'd worded it a bit better you wouldn't have had this "attack" you speak of.

If you read your opener back then you'll see where it came from.

 

[Crosswire]

First off, you are not allowed to install the link, there are specific requirements for TN-C-S and simply bunging in a DIY link between N & E doesn't cut the mustard. If you've done this in the past or are proposing to do it in the future then I'm questioning your competence.
Secondly, a properly thought out and installed TT system can easily achieve Ra values equivalent to a TN so what's the problem?
You work with what you've got mate.

As I have already made clear, this is a hypothetical situation. Consider it a thought experiment, if you will.

My "problem" , as you say, is that I dont understand how the general mass of earth via an earth electrode, provides a superior fault current return path than the the low resistance path that already exists in the form of the suppliers neutral.

I'm assuming there must be other benefits to using a TT system, of which I am currently unaware.

This is the reason I initiated the discussion. It is a quest for information on electrical theory.

 

[derek]

As I have already made clear, this is a hypothetical situation. Consider it a thought experiment, if you will.

My "problem" , as you say, is that I dont understand how the general mass of earth via an earth electrode, provides a superior fault current return path than the the low resistance path that already exists in the form of the suppliers neutral.

I'm assuming there must be other benefits to using a TT system, of which I am currently unaware.

This is the reason I initiated the discussion. It is a quest for information on electrical theory.

What happens if the suppliers neutral breaks, and it isn't PME?
Edit: As far as I understand it with a TT installation with a 100mA time delayed protective device then under a fault to earth condition the protective device will operate. If you are relying on the suppliers neutral and it breaks and it is not PME then you are relying on extraneous conductive parts to provide your fault path and as they are not necessarily designed for that purpose they can't be relied on, I could be wrong though.

 

[Crosswire]

What happens if the suppliers neutral breaks, and it isn't PME?

I'm guessing that I will lose both my neutral and my earth. But surely this must be true for all TNCS systems?

 

[Paulgibson]

I think all systems dont have advantages to say, but they are chosen due to they're circumstance. TN-C-S has got to be safest with the DNOs having they're own electrodes in all the substations(say something go wrong with the supply side). TT I'd imagine that its used because TN-C-S can't reach to "out of no where" locations and meet d/c times, or the "earth" is more conductive in those locations and so provides better protection.

I'm not trying to be a teacher or anything so correct me if Im wrong.

Keen Learner

 

[derek]

I'm guessing that I will lose both my neutral and my earth. But surely this must be true for all TNCS systems?

My understanding of it is that PME is protective MULTIPLE earthing i.e. earthed at many points along the route to the suppliers stepdown transformer for the area. If the neutral breaks on a TNC-S/PME system there is more of a chance that there will still be an earth path for fault currents back to the transformer (I have no idea what the Ze would be in this scenario).

 

[Crosswire]

What happens if the suppliers neutral breaks, and it isn't PME?
Edit: As far as I understand it with a TT installation with a 100mA time delayed protective device then under a fault to earth condition the protective device will operate. If you are relying on the suppliers neutral and it breaks and it is not PME then you are relying on extraneous conductive parts to provide your fault path and as they are not necessarily designed for that purpose they can't be relied on, I could be wrong though.

I agree that the requirement for TT systems to include a residual current device at origin is definitely is a good thing, but again this requirement could easily be extended to all earthing systems.

I'm assuming it is only compulsory in TT systems due to the high resistance earth fault loop path, unless I am missing something?

 

[Crosswire]

My understanding of it is that PME is protective MULTIPLE earthing i.e. earthed at many points along the route to the suppliers stepdown transformer for the area. If the neutral breaks on a TNC-S/PME system there is more of a chance that there will still be an earth path for fault currents back to the transformer (I have no idea what the Ze would be in this scenario).

I agree. Sounds like a better system than one earth electrode hammered in at the installation origin with a (maximum) resistance of 200 Ohms.

 

[derek]

I agree that the requirement for TT systems to include a residual current device at origin is definitely is a good thing, but again this requirement could easily be extended to all earthing systems.

I'm assuming it is only compulsory in TT systems due to the high resistance earth fault loop path, unless I am missing something?

I think adequate equipotential bonding and RCDs provide reasonable protection from electric shocks as long as they are not between Line and Neutral.
Do you believe it is good practice to use the supplier's neutral as TNC-S if you don't know if it is PME?

 

[ipf]

I think all systems dont have advantages to say, but they are chosen due to they're circumstance. TN-C-S has got to be safest with the DNOs having they're own electrodes in all the substations(say something go wrong with the supply side). TT I'd imagine that its used because TN-C-S can't reach to "out of no where" locations and meet d/c times, or the "earth" is more conductive in those locations and so provides better protection.

I'm not trying to be a teacher or anything so correct me if Im wrong.

Keen Learner

Forgetting TN-S, for starters.

 

[Crosswire]

I think adequate equipotential bonding and RCDs provide reasonable protection from electric shocks as long as they are not between Line and Neutral.
Do you believe it is good practice to use the supplier's neutral as TNC-S if you don't know if it is PME?

Again, I would never do this in reality. But I would be fairly confident that the suppliers neutral returned to the supply transformer and formed an electrical circuit, based on the functioning of the installation in question.

For this reason I think that it would make a good return path for fault current to operate the OCPD within the required time.

 

[derek]

Again, I would never do this in reality. But I would be fairly confident that the suppliers neutral returned to the supply transformer and formed an electrical circuit, based on the functioning of the installation in question.

For this reason I think that it would make a good return path for fault current to operate the OCPD within the required time.

Neutrals break on TT systems in storms etc. that is why the supplier has not made them TNC-S.