earthing systems

[sparky0000]

at a job today and the earthing system looks like a tnc-s first look


but when the main fuse was took out and the small cover was removed the line neutral and earth was all seperate

which makes me think is it a tns?

its a shelterd accomanation with about 100 plus flats
the line neutral and earth to the cutout is coming thrugh a concreat floor in plastic conduit

 

[Taylortwocities]

Photos, we need photos!!

 

[sparky0000]

i will get some pics tomoz

it just looks like a tn-c-s but the neutral and earth are not combined in anyway a all with what i can see

 

[ban-all-sheds]

which makes me think is it a tns?

its a shelterd accomanation with about 100 plus flats
the line neutral and earth to the cutout is coming thrugh a concreat floor in plastic conduit

it just looks like a tn-c-s but the neutral and earth are not combined in anyway a all with what i can see

Why do all of those observations make you roll your eyes?

 

[sparky0000]

i just like the symble

 

[Taylortwocities]

And I like this one

 

[ericmark]

How can any electrician not in the employ of the DNO ever know it is TN-S without being issued with paperwork from DNO that states it's TN-S?

If at any point between transformer and head the earth and neutral are combined it becomes a TN-C-S supply. May be the DNO will state it's a TN-S supply which allows a ELI of 0.8 ohms but when I have tried to get paperwork to state it's TN-S so I can use it to feed caravans or boats except where a dedicated transformer is used I have failed.

 

[LoveRocket]

How can any electrician not in the employ of the DNO ever know it is TN-S without being issued with paperwork from DNO that states it's TN-S?

If at any point between transformer and head the earth and neutral are combined it becomes a TN-C-S supply. May be the DNO will state it's a TN-S supply which allows a ELI of 0.8 ohms but when I have tried to get paperwork to state it's TN-S so I can use it to feed caravans or boats except where a dedicated transformer is used I have failed.

If I may highjack this rather poor (in terms of original question, grammar and spelling) post; why is there such a difference between how TNC-S and TNS supplies must be treated (for example with regards to exporting earths or for caravan/dock supplies)?

Is it because of the risks of a broken neutral past the last rod on a TNC-S supply?
Or is it because with TNC-S the last connection to real earth may be some distance away from the installation? (If this is the case, does this not assume that a TNS supply will be earthed along its length by its outer metallic sheath? Obviously this isn't the case with modern fully insulated cables - so the connection to real earth is all the way back at the transformer!)

 

[ericmark]

Main point boat and caravan are often metal construction and if the neutral earth was lost with such a good conductor no second chance.

House often brick or other non conducting material and even if was a conducting material it would be bonded so likely it would hold the supply neutral earth down anyway.

The fact that petrol stations, caravans and boats can't use TN-C-S shows it is dangerous and really the whole idea of TN-C-S is now flawed with the use of RCD protection. Before RCD it did have some good points with 0.35 ohm ELI instead of 0.8 ohm ELI but now only advantage is cheaper for DNO.

 

[bongos]

why is there such a difference between how TNC-S and TNS supplies must be treated (for example with regards to exporting earths or for caravan/dock supplies)?

I think it's because in the case of an open circuit neutral you would also lose your earth and everything before the neutral break would be at line potential including anything attached to the MET (I think).

 

[ricicle]

The circulating currents in normal use or earth current in the event of a broken Neutral could result in current passing from earthed metalwork to ground (possibly arcing) eg underground petrol tanks !

That is the reason why PME is forbidden on petrol stations.

 

[LoveRocket]

That make sense, pretty obvious really! Thanks guys.