Change to Earthing system

[studentspark]

Property had poor Ze reading on a TNS system.

This (eventually) was changed by the DNO to a TNCS system.

It got me thinking about the checks that the DNO do to the property when changing the earthing type.
No comment or advice was given to the home owner about the possible consequences, and no checks were made.

I am thinking about exporting the PME.

This is a big house with a large garage and a office 'shed' which could easily have earthy metal.
I would have thought that the DNO would be checking that the change to the earthing would not affect the installation, but it appears not.
I've been quite surprised by my recent dealings with the DNO.

 

[RF Lighting]

What do you think they should have checked for, or given you advice about?

 

[Iggifer]

I would have thought beyond checking the Ze was fine that would be where their responsibility ends.

Obviously this could mean the Zs values on circuits are too high but this isn't really the DNO's problem. A word of advice would be nice though

 

[studentspark]

- Given the home owner advice -

Given that the earthing system has an effect on the safety of the installation, and changing that system could effect that safety of a installation constructed for the prior method, I think guidance to the home owner would be advisable.

I guess you think not?

 

[Iggifer]

No I do agree with you, I know it doesn't seem like it.

But a meter technician I met the other day didn't know what an RCD/RCBO was and I imagine the majority of DNO engineers are of a similar level of knowledge.

There is nothing worse than incorrect advice. An information leaflet for customers who have a TNS combined to PME might be a good thing, but I don't think it will happen

 

[RF Lighting]

There's a whole world of difference in the knowledge required to be a meter changer and someone who can safely work on the network.

What would be in this leaflet? I'm not aware of any differing requirements in BS7671 between a TN-S supply and a TN-C-S supply.

 

[flameport]

The vast majority of home owners wouldn't know what earthing types were or what the consequences of each type would be, or care about the electrical installation beyond lights and appliances actually working.
If the DNO gave them any information it would probably be completely ignored or would just lead to more questions and queries. Either way it would cost the DNO money for absolutely no purpose.

If homeowners were concerned about electrical safety in the slightest they would have EICRs (PIRs previously) completed on a regular basis, say 5 or 10 years, all of the electrical work in their home would comply with BS7671 and they would have a big folder containing all of the certificates for everything since the place was built.

 

[westie101]

If a request is made regarding earthing from an electrician we would deal with them and expect them to agree with the house owner what is needed on the installation. As I see is the case here.

If we decide to do network modifications and the instigation for the earthing change is ours we have to write to the customer and explain the issues, often we would upgrade installations if needed in these cases.

 

[studentspark]

RF Lighting, you make me doubt my self, as you are a well respected member on here with a lot more experience that me, but is there not an issue with exporting an earth to an out building that is outside the equipotential zone. And a failure on a PME supply can raise metal work to a high voltage

 

[RF Lighting]

Exactly the same rules apply if you are exporting an earth from a TN-S system.

In reality the dangers could be a bit worse with TN-C-S but as far as the regs are concerned all TN systems are treated the same.

 

[ericmark]

I read your post and started to consider were changing from TN-S to TN-C-S would matter.

Caravan and boat supplies was first to come to mind and also Radio Hams who may have large earth mats. However in the main there would be little difference between the two systems.

It has been talked about a few times as to how one knows one has a TN-S system? The bond between neutral and earth does not have to be on the customers premises if next door was converted to TN-C-S then your house would also be TN-C-S even though the connection was not made in your house between neutral and earth.

This has been raised a few times with marinas who think they have a TN-S supply so have supplied earths to each hook-up point. But taking a meter to supply earth and water one has rather a high voltage well over the 2.4 volt the diode will allow for.

So what would you change on a installation if the earth was changed from TN-C-S to TN-S or TN-S to TN-C-S? In most houses I would not expect any change.

Now TT is another story but that's not the case here.

 

[studentspark]

OK thanks.

http://www.google.co.uk/url?sa=t&rc...yey_Cw&usg=AFQjCNHdTWfigRIv_mVA-SEjvMCbiYcuAw

wow thats a long link

So just to clarify then, supplies to an out building for both TNS and TNCS are the same.

If no earthy metal all is well

If earthy metal it has to be MPB to the met or TT it

But what if that earthy metal i.e a water pipe comes from the house and is bonded there? but runs under the ground for a bit and comes out in the garage

 

[bernardgreen]

But what if that earthy metal i.e a water pipe comes from the house and is bonded there? but runs under the ground for a bit and comes out in the garage

It has to have a one metre or longer section of plastic ( or other non conducting ) pipe inserted to prevent there being a metalic connection between the TT earth and the non TT "earth".

 

[chapeau]

Good on you studentspark, keep on asking questions, seems you know a lot more than many.

I think you are completely correct. The DNO should at least check the equipotential bonding is in place as required by BS7671, or advise customer to do the same. Should they do this to other houses in the street if they convert one to PME?

 

[riveralt]

Good on you studentspark - seems you know a lot more than many.

I think you are completely correct. The DNO should at least check the equipotential bonding is in place as required by BS7671

And even more than you perhaps although studentspark should be fully aware BS7671 does not apply to DNO as per BRB regulation 110.2 (1)