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.

 

[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.

 

[D Skelton]

What is peoples obsession with suppliers providing earths?

TT systems are by far the safest (if installed correctly) and most effective types of earthing, far superior to any TN-S or TN-C-S.

 

[Crosswire]

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

Ok. That makes sense. That is a definite benefit of a TT system.

Thanks!

 

[Engineer54]

To be totally honest, i'm amazed that a working electrician is actually asking the question in the first place!! Unless of course we are yet again, dealing with another 17 day/electrical trainee...

 

[Crosswire]

What is peoples obsession with suppliers providing earths?

TT systems are by far the safest (if installed correctly) and most effective types of earthing, far superior to any TN-S or TN-C-S.

Please explain in further detail, if you would be so kind

 

[Crosswire]

To be totally honest, i'm amazed that a working electrician is actually asking the question in the first place!! Unless of course we are yet again, dealing with another 17 day/electrical trainee...

Thankyou for expressing your amazement.

I did notice a total lack of an explanation of the relative benefits of different earthing systems, however.

If I ever post a thread entitled " Please express your amazement!" , I will be sure to PM you.

Cheers,
Crosswire.

 

[ipf]

What is peoples obsession with suppliers providing earths?

TT systems are by far the safest (if installed correctly) and most effective types of earthing, far superior to any TN-S or TN-C-S.

The (bracketed) tells the story.

 

[derek]

Engineer 54 has posted many interesting facts on TT systems and there are many debates on that particular earthing system if you use the search function, it is all food for thought IMHO.

 

[Crosswire]

Engineer 54 has posted many interesting facts on TT systems and there are many debates on that particular earthing system if you use the search function, it is all food for thought IMHO.

I will be very happy to do that.

Why didn't he simply post a link instead of making snide remarks?

 

[D Skelton]

Please explain in further detail, if you would be so kind

There is far less to go wrong, and if it does, it's easily rectified. Both TN-S and TN-C-S are prone to faliure whereas a well installed TT system will outlast any other means of earthing. TT systems benefit from having noise free earthing. TT systems are far more cost effective from a suppliers point of view. You also have the benefit of having a true earth reference, rather than zero potential introduced by the star point, this is one of the reasons why TN-C-S is not allowed on final connection to caravans and mobile and transportable units.

 

[Crosswire]

There is far less to go wrong, and if it does, it's easily rectified. Both TN-S and PME are prone to faliure whereas a well installed TT system will outlast any other means of earthing. TT systems benefit from having noise free earthing. TT systems are far more cost effective from a suppliers point of view. You also have the benefit of having a true earth reference, rather than zero potential introduced by the star point, this is one of the reasons why PME is not allowed on final connection to caravans and mobile and transportable units.

Thanks dude