Another earthing question......

[polo1]

Hi folks,
story is, TNS supply to one side of a semi detached property. A three phase head, one for the property I'm working in, one for the adjacent semi and one unused.

The supply for the adjacent semi runs in VIR singles in conduit from the main head. Whils working away on my job, the neighbour pops in and asks me if I can have a look at his installation as he has been suffering from the single RCD covering the board tripping over the last month or so.

So, late yesterday afternoon had a look. First thing is Ze is 514 ohms. The board is tucked away under a stair and really difficult to access. I wasn't able to see how the main earthing has been achieved, but I'm guessing it's via an old lead water main.

Now, the question. Is it acceptable/permissable to run a 16mm earth from the suppliers head (where the property with the high Ze takes its supply from) next door? If not, it's going to have to be PME if available or TT. There is something that strikes me as not quite right taking the earth from the main head in the neighbouring property but that might just be me - it would certainly be the easiest option.

Still have not had time to investigate the cause of the tripping.

Opinions/advice welcome!

Regards.

 

[GaryM]

My first port of call with regards to the high Ze would be the DNO.
They should attend pretty urgently.

 

[Engineer54]

As suggested this is a DNO problem, not so much yours!!

There are several old DNO served installations such as you describe. The earthing is obviously got to come from the supply cable sheathing on a TN-S system, whether that is via the iron clad service cut-out or not....

 

[polo1]

Thanks guys but will this not be a case of “ we did not supply this property with an earth so have no requirement to maintain”?

 

[oldtimer]

This is quite common the service head is 3 phase and is terminated into one of the semis the other is the same but in one of the terraced houses in the house with the service head there is a MET for both houses and because the mains is run in a metal conduit they (the DNOs) would earth clamp the conduit and both ends and then take a earth lead of say 6mm off each to go to the DB and the MET so my question is there any evidence of this and if so you can re-enstate it or you could let the DNO do the same thing One other thing the earth at the service head does not belong to the house that it is in it is what I would call a shared earth and neutral for both or in the case of a 3-4 house terrace properties.

 

[Engineer54]

If there is sign of any original earth provision from the service cut out to yours and/or the neighbors installation, then the DNO are duty bound to maintain that earthing provision...

 

[polo1]

If there is sign of any original earth provision from the service cut out to yours and/or the neighbors installation, then the DNO are duty bound to maintain that earthing provision...

No, there was nothing obvious. When I'm back at the job tomorrow, I'll try clamping the conduit to the MET and see what that brings the Ze down to. Failing that I'll give the DNO a call ( I can tell now how that conversation's going to go :sad3.

Thanks for your input all.

 

[oldtimer]

No, there was nothing obvious. When I'm back at the job tomorrow, I'll try clamping the conduit to the MET and see what that brings the Ze down to. Failing that I'll give the DNO a call ( I can tell now how that conversation's going to go :sad3.

Thanks for your input all.

This goes back to the days when the guy next door mains was about 11-12 metres away so they would run a conduit between the houses for mechanical protection plus if they did not earth clamp the conduit that was because the guy next door got his earth from the gas and water So whats changed ? well plastic pipes thats what and hence why we are seeing more of this as Transco are moving meters outside then dropping a card through your door to tell you to get your earth checked where a simple before and after test would suffice ie a card that says your Ze was 0.18 before the meter was moved to 29 ohms after with a please get this checked out by a qualified electrician on it but thats too easy

PS clamping both ends and fitting earth leads will bring the Ze down within limits but before you do that mention to the customer that this is chargeable plus he may want to get the DNO to upgrade the mains in due course. I got a job last month where BG yes them again told the customer she had no or a poor earth sure enough the DNO came out but would not connect the 6mm earth from the CU saying it was not their job and better get an electrician to look at it so I turned up and fitted a 600mm length of 16mm earth

 

[GaryM]

Failing that I'll give the DNO a call ( I can tell now how that conversation's going to go :sad3.

I'd be surprised if it didn't result in a site visit when you explain the Ze is 514ohms and way above what you would expect for a TN-S earthing system.
In fact, I once measured a Ze of 6ohms on a TN-S. The DNO were informed and they turned up the same afternoon. The end result was an external dig and a nice low Ze the day after.

 

[oldtimer]

I'd be surprised if it didn't result in a site visit when you explain the Ze is 514ohms and way above what you would expect for a TN-S earthing system.
In fact, I once measured a Ze of 6ohms on a TN-S. The DNO were informed and they turned up the same afternoon. The end result was an external dig and a nice low Ze the day after.

GaryM if they are told that a RCD is fitted they will come out in their own time ie 6-8 weeks as I have had this twice where they asked what is the Ze and I replied 1.9 and 26 ohms then they asked is there RCD protection and I confirmed this then they said we will plan it in our schedule plus both customers were charged for this

 

[GaryM]

The customer was charged by the DNO to maintain the earth that they initially provided, assuming it was also TN-S in your instance?
There was no charge to the customer in the instance I mentioned above.

 

[Engineer54]

The customer was charged by the DNO to maintain the earth that they initially provided, assuming it was also TN-S in your instance?
There was no charge to the customer in the instance I mentioned above.

Maintenance of DNO equipment and provisions are NOT and never have been chargeable to the customer. If they were charged by this DNO, then it's a letter to the DNO and ombudsman from your customers solicitor demanding a complete refund and any costs incurred....

 

[telectrix]

apart from the earthing question, it's time DNO came out to replace those VIR tails.

 

[polo1]

And for those interested....got the bum's rush from the DNO (oldtimer - almost word for word to your post!) - "we'll programme it in, but if we've never provided an earth.........etc". VIR tails - "very common in domestic properties, we have no plans to upgrade at present, but if you have any concerns, let us know".

So from the head I just bonded to either end of the conduit, then to the CU and got a very respectable Ze. Then got an urgent call and I still haven't checked out the nuisance tripping.:35:

 

[oldtimer]

And for those interested....got the bum's rush from the DNO (oldtimer - almost word for word to your post!) - "we'll programme it in, but if we've never provided an earth.........etc". VIR tails - "very common in domestic properties, we have no plans to upgrade at present, but if you have any concerns, let us know".

So from the head I just bonded to either end of the conduit, then to the CU and got a very respectable Ze. Then got an urgent call and I still haven't checked out the nuisance tripping.:35:

Well done i am glad it has come to a conclusion plus I bet you can sleep better knowing they have a good earth remember its not all about jobs worth it can be jobs satisfaction nice to hear that it has been sorted and as for the DNO well no excuse the pun shocks their then