external earthing

[love2know]

Hi,

Try and keep this short as possible.

Now referring to tncs my understanding is suppliers don't like the earthing to be exported externally in case of the loss of earth. So its to be treated externally as a TT system, all well and good.

My question is would it not be better to use the earth from the pme and then also make a TT system so you have infact got 2 earths? the more earths the better surely? can anyone foresee any problem doing it i this way?

Thanks.

 

[Sintra]

If you search the forum there are loads of threads on this subject. In most cases you are merely extending the earthed equipotential zone not exporting.

 

[Darkwood]

Yes plenty of threads on this one.. as Sintra has noted .

Key factors -

Is the shed/garage etc got incoming metal services also has the has the shed/garage etc got structural steel work if so then this may be a case for converting to TT and isolating the incoming earth at the shed/garage. (alternatively you can ensure earthing meets the requirements of table 54.8)

If no to the above then simply extending the existing equipotential earth is fine but would avoid fitting an earth rod if you have extended the Equip' zone.

 

[Engineer54]

Yes plenty of threads on this one.. as Sintra has noted .

Key factors -

Is the shed/garage etc got incoming metal services also has the has the shed/garage etc got structural steel work if so then this may be a case for converting to TT and isolating the incoming earth at the shed/garage. (alternatively you can ensure earthing meets the requirements of table 54.8)

If no to the above then simply extending the existing equipotential earth is fine but would avoid fitting an earth rod if you have extended the Equip' zone.

Why's that then??

Quite agree with you if were talking about banging in a thin twig!! lol!!

 

[Darkwood]

If as i said you don't require to TT the extended zone because it has not structural or incoming metal services then the supply earth to the shed/garage etc doesn't need to meet table 54.8 and because of this it is undersized in most cases to handle a pme broken N/E fault, so sticking a rod down will give a pme fault route to a outbuilding supply earth not sized to cope... you can only rod down if your earth is compliant with 54.8 table .... normally 10mm domestic ive just told a bloke the 4mm supply to his metal container requires a 50mm earth running as well or the shed isolating from the equipotential zone and rodding... the supply N requires 50mm services bonding ....

So imagine this wasn't a metal container and no incoming metal services then the 4mm earth was an extension of the the equip zone .. normally not a issue but if i rodded to ground then local network fault possibly 400amp could route though a 4mm cable earth... ive seen the after effects before ... the fire brigade even had issue with reports of shocks ... you drench an area with a pme fault with water it becomes a death trap because of an error of rodding a extended zone with an insufficient earth.

PME networks are designed that local supply N sizes match each other in each zone so domestic TNCS usually is only required to need a 10mm bond if the nature of the out building requires equip' bonding... in certain cases where neworks can lead to a possibility of a higher current loads in network faults finding a fault path through a premise then the DNO may request a larger earth than table 54.8 ..... ive also had this too where we priced for 50mm earth additional to the swa cable but were told by the DNO due to the local network we require a 70mm earth to handle any possible circulating N/E currents.

Correct me if im wrong!

 

[Engineer54]

Most European countries actually demand that a local rod is installed at every premises where a PME supply has been provided. It basically acts as another PME point but local to the installation. Obviously the typical crap 200 ohm on a twig isn't going to cut the mustard, but a well placed and installed deep rod will. The DNO work on a roded N-E point as being a max of 10 ohms, (but will often be much less). You can mix/combine a TT earth with any TN earth on an installation but you shouldn't mix TN systems past the service cut-out point...

Were only talking Domestic here, larger commercial and industrial earthing arrangements can complicate things out of context. I'm not following your train of thought, as a good local roded earth (the lower the Ra the better) will actually dissipate effects from a local upstream broken neutral, not enhance the effects. There are many examples of PME/TNC-S domestic installations that still only have 6mm main bonding, often where the supply has been retrofitted with PME from TT or a failed TN-S supply and from what i can see, is perfectly acceptable, from the many threads on the subject.

I'm not saying that i would install a rod where it's not needed, but if the rod was for example already in place and carried an acceptable Ra value with a suitably sized conductor, i'd be inclined to leave it in service, be it PME/TNC-S or TN-S...

 

[Darkwood]

Think we are both just using different approaches here to arrive at same answers, and yes im more on the industrial side and it does get more complex but overall agree as i already said earlier if the conductor is suitably sized (domestic) then no issue merging both set-ups. The networks are usually well designed when domestic areas are PME'd limiting excessive circulating PME currents in a fault using domestic equip bonding..