explaining earthing systems

[beanzntoast]

Hi,firstly just to introduce myself which i think would be rude if i didn't seen as i'm new to the forum & asking for advise,i'm jason & through studying & self taught i have learnt & found out quite alot about electrics with alot more to learn,i am on a course studying 2330 level 2,3 & nvq level 3, the question is on a TNC-S earthing system am i right in saying that the neutral & earth are connected together at the supply side only & they seperate at the consumers side to give two seperate conductors,if this is correct what advantages does it have over a TN-S( why is it used more than the TN-S on new installations is it supposed to be safer because it is connected to earth at various points from the supply transformer).If this is true could somebody explain how come at the points where it is connected to earth by the use of earthing cables running down the side of wooden electric poles on the supply side,that why at these points they dont carry voltage/current to earth when the neutral returns the voltage/current to the transformer(i'm obviously missing something somewhere).if i've got this wrong i apologise in advance but if so could somebody correct me

cheers jason

 

[Guest123]

Ok mate here goes.

We are all used to TT, TNS and TNC-S as supply types.

I'll start with TNC-S. As everyone knows, the customer is provided with a connection to the System Neutral to use as their main earth. The Neutral will be earthed at the star point of the supply transformer, again at the end of the main, at or beyond the connection of the furthest customer on that cable. In addition, earths are added to the neutral along the main at appropriate points, to reduce the System Neutral to earth potential under normal conditions, and to limit the potential on the System Neutral compared to earth in the event of a break occurring on the Neutral conductor back towards the supply transformer.

With TNS, the star point of the supply transformer is earthed. A separate conductor is provided back to the supply transformer to connect to the customer's earth transformer. Virtually all TNS supplies are provided on lead mains, the lead sheath / steel armouring effectively acts like a large earth rod, equivalent to the length of the cable.

With TT, the star point of the supply transformer is earthed. No earth connection is provided to the customer.

Both have disadvantages TN-S for instance - if the sheathing of the cable breaks down this could go un-noticed for years as the bonding conductors would be providing a paralell path to earth. You would know straight away if a TN-C-S conductor was lost.

Most DNO's stste that anyone on TN-S should be "upgraded to TN-C-S as TN-S is no longer supplied on new installations

Take a look at the link below aswell.


Hope this helps.


http://---------------/Publish/Wire..._matters_earthing_your_questions_answered.pdf

 

[kung]

Good explination and link Lenny.
Regards
Kung.

 

[Spudnik]

On a side note, most DNO's have to upgrade TT systems, i think before 2012 or similar.

A lot of the time they will do it for free too!