TNCS system with supplementary TT rod

[SW1970]

Hi folks, went to fix a nuisance tripping problem today (success). Earthing was arranged as TN-C-S with a TT rod. See pic.



The top GY cable is the main earth conductor which goes to MET in the consumer unit (although haven't taken any measurements yet). The lower GY cable is connection to earth rod about 2 metres away. Rural property. Board currently has RCD as main switch. Likely to be doing a PIR followed by CU upgrade at the location to prepare for other works at the property. Any advice on this earthing arrangement and in particular how should testing of Ze be approached? Is this the sort of arrangement the DNO may have implemented or is this the work of an electrician attempting to get the numbers down?

Many thanks, SW

 

[topquark]

Are you saying (looking at the PICs) that the rod is connected to the head end earth? Sounds like the DNO getting the Ze down. Are there not many other buildings around?

 

[SW1970]

Thanks - yes - rod is connected directly to head end earth in the cutout. But I need to make enquiries to DNO to confirm that they have done this and it's not an improvised solution! The nearest building is about 150 metres away.

 

[topquark]

I would (if you can find someone that knows ). It would be worth finding out how old the rod is as well, it may well be deteriorating if it's been there for a long time.

 

[SW1970]

Thanks TQ - any thoughts on approaching testing Ze in this scenario?

 

[Engineer54]

Localised supplementary earth rod(s), on a PME supply system should be terminated at the MET , Not at the DNOs cut-out N-E connection.
It is highly unlikely this is a DNO installed ground rod, not unless they have been payed by the householder to provide one . Much more likely this was once a TT system converted to a PME supply. Either way, that rod needs to be connected to the MET, not the DNO's N-E connection point!!

 

[topquark]

You've two tests you can do, you can disconnect the rod (at the rod end !!!) and see if it's within spec, without the rod (that may answer the query about why it was done, but not who did it). Test again with the rod reconnected (either way leave the rod connected when you're finished, even if it's within spec (0.35 Ohms) without it ... technically I'd consider it DNO equipment (as it's attached inside the head end)).

Whether you record what you find will depend on the answer from the DNO as to whether they did it or not. If they didn't do it and it's not in spec then they'll need to come and sort it anyway. If they did do the work, then record the value with it connected.

The concern here is that disconnecting it could affect others in the area.

 

[Deleted member 9648]

It's also possible that someone has 'self connected' a PME and then found that the link from the N to PME terminal in the head is not in place,as a result they installed a rod and made it a TT,an RCD main switch would support this.
As above,testing of Ze on the PME terminal on the head would confirm/discount this.

 

[SW1970]

Thanks folks, just waiting on the order for the PIR and will report back findings. Cheers, SW

 

[Chr!s]

Is there a swimming pool or any metal clad structures?

 

[ackbarthestar]

From the Ops post. Are the DNOs seals in place?
That should answer your questions regarding whether or not earth electrode is part of the pme system.

 

[Noton1205]

There should be a sticker or plastic notice stating that there is pme at the cut out. If this is not there call the dno and ask if the property has pme. If it does do a ze between incoming live and neutral making sure you disconnect both of the GY cables.

 

[Noton1205]

From the Ops post. Are the DNOs seals in place?
That should answer your questions regarding whether or not earth electrode is part of the pme system.

On this kind of cutout you dont need to break the seals to get access to the NE block, that little black suare on the neutral cover pops out.

 

[platty1005]

You say it is a rural area ? is the installation supplied by overhead cables ? How do you know it is a TNCS system ?
Assuming overhead TNCS what would happen if the neutral broke - NO earth so the TT rod is probably there to provide a secondary earth system if overhead neutral breaks.
Testing Isolate supply at DB remove earthing conductor from head to MET at the MET and test Ze with both TNCS and TT systems connected
Then disconnect TT cable and test again To prove TNCS alone is less than 0.35 if TNCS alone is above 0.35 report to DNO it is their responsibilty to fix it

 

[IQ Electrical]

You say it is a rural area ? is the installation supplied by overhead cables ? How do you know it is a TNCS system ?
Assuming overhead TNCS what would happen if the neutral broke - NO earth so the TT rod is probably there to provide a secondary earth system if overhead neutral breaks. Testing Isolate supply at DB remove earthing conductor from head to MET at the MET and test Ze with both TNCS and TT systems connected
Then disconnect TT cable and test again To prove TNCS alone is less than 0.35 if TNCS alone is above 0.35 report to DNO it is their responsibilty to fix it

I hope not, you end up becoming the PEN for the whole street in the (very rare) event that an overhead neutral breaks.

It always sounds good in theory when people talk of 'tying down' the MET but in reality the probable Ra of the electrode is prohibitive and other damgers can actually be introduced.