Earth arrangements confusion

[alan132]

Working on a old property in a slightly remote area installing a new circuit.

When I looked at the earthing it looked like a tns but I investigated and found a old 2.5mm running towards the rod.

My problem is I am getting a ze reading of 0.25 ohms.

Ino we had a really heavy down poor the day before but to be that low?

I am wait for the dno to confirm the earthing but they ain't rushing.

Any thoughts
I know a good reading for tt is 200 ohms or lower. This is the first one I have worked on

 

[westward10]

Have you got all main protective bonding disconnected to remove parallel paths. How did it look TN-S for it to then become TT.

 

[alan132]

Thats is just the main cable nothing else

When I manage to peak around the wooden board covering the mains it looked like a swa with a earth strap.
It's under the stairs and I could hardly fit my head in there.

 

[Richard Burns]

If you have an earth connection to the sheath of the supply cable and the Ze reading with all other earthing connection disconnected is 0.25Ω then the supply is TNS.
If it has a rod connected to the MET then there is an earth rod connected to the MET it would not change the earthing of the supply.

 

[alan132]

As I said I assumed it was a swa.
But looking out side there was only 2 cables going in.
So I would say that was a tube of some sort

This place has had works done but no one has left any certificates

I will have to contact dno again to comfirm the earth.

 

[mhar]

You say it is remote, is it on it's own and does it have it's own transformer? If so is the transformer earthed at the pole?

 

[Wilko]

Hi Alan - I've been building up my TT portfolio lately using the larger 5/8" 4ft rods, two at a time. Around here in wet clay this has been good for 10 Ohms or less. I would need to parallel up many many of these to get your numbers, so it's def a DNO means of earthing in my humble opinion. Have you checked Ze against short circuit impedance? Avoid the non trip setting, if you get 0.25 for both then perhaps DNO has PME'd an old TT at some point.

 

[alan132]

It's a area they just keep adding to
There is power pole then when you get to a built up area they stop.

Can't find a substation near by.

I don't know if this may affect it but there is a dyke around the place and it on a flood plane
Ground soft and soak water in

 

[mhar]

Properties can be fed from pole mounted transformers not necessarily near a substation.
There is a method of earthing very occasionally used on single properties in remote locations fed from their own transformer (pnb).
Have you looked at the pole(s) supplying? If a pme overhead supply there should be a pme label on every third or fourth pole plus you will see an earthing conductor running down the pole.

 

[buzzlightyear]

I don't know if this may affect it but there is a dyke around the place and it on a flood plane
Ground soft and soak water in

you are not dowsing for water ,you need to take pic of the set up and show us if poss and speak to dno.

 

[Midwest]

Hi Alan - I've been building up my TT portfolio lately using the larger 5/8" 4ft rods, two at a time.

Sorry OP for hijacking your thread. Wiko what surveying do you do, if any, when choosing a place to install your earth rods, so as not to hit any other services?

Just curious as I've not done one yet, and if the 18th has its way, I'd better get some practise in.

 

[Toneyz]

If you have an earth connection to the sheath of the supply cable and the Ze reading with all other earthing connection disconnected is 0.25Ω then the supply is TNS.
If it has a rod connected to the MET then there is an earth rod connected to the MET it would not change the earthing of the supply.

18th compliance already !.

 

[ruston]

Properties can be fed from pole mounted transformers not necessarily near a substation.
There is a method of earthing very occasionally used on single properties in remote locations fed from their own transformer (pnb).
Have you looked at the pole(s) supplying? If a pme overhead supply there should be a pme label on every third or fourth pole plus you will see an earthing conductor running down the pole.

I have a three wire overhead supply from a pole transformer, bare and only the line (live lol) insulated from the last pole to the house. Three houses on this one supply.

 

[Phil Thompson]

Hi Alan - I've been building up my TT portfolio lately using the larger 5/8" 4ft rods, two at a time. Around here in wet clay this has been good for 10 Ohms or less. I would need to parallel up many many of these to get your numbers, so it's def a DNO means of earthing in my humble opinion. Have you checked Ze against short circuit impedance? Avoid the non trip setting, if you get 0.25 for both then perhaps DNO has PME'd an old TT at some point.

I found those rods give a much better reading myself, going to use them from now on.

 

[Midwest]

I found those rods give a much better reading myself, going to use them from now on.

Same question for you Phil;

Sorry OP for hijacking your thread. Wiko what surveying do you do, if any, when choosing a place to install your earth rods, so as not to hit any other services?

Just curious as I've not done one yet, and if the 18th has its way, I'd better get some practise in.

 

[Wilko]

I think you are right to be concerned as there will always be risks banging in a rod. As we spend a bit of time looking at the existing bonding, I have a go at estimating where I think the services would run from that, look of water connection at property boundary etc. I speak to owner and get their take on where services may be. If I've any doubt remaining, I dig a bit by hand which is the only thing that saved me in the attached pic. The cost of ground penetrating radar hire is £300 or so per day, and probably be about the same if you get a specialist company to do a report on a domestic plot. Folks won't pay for that to have a new circuit in their house, but it's the only way to be sure about what's below.

 

[Phil Thompson]

Double post

 

[Phil Thompson]

I try and opt to locate spike to the rear of a property to avoid any services (here they're always brought from the road/street to the front of a property) and assess drainage and guttering on a property so as to avoid any drainage pipes.

If frontal location was only option and I wasn't 99% certain I was safe to put a spike down I'd likely have to get further assessment done as whilst a cable finder will locate telephone, virgin and mains etc, water and gas pipes are ran in pvc pipe. So would not show up on any scanner that I've seen.
Although in general the services are brought the most direct route from street to gas metering equipment/internal stop cock. But not always. Although it's very rare to find them travelling an indirect route here.
Fortunately it's very rare for me to find TT properties here with no spike already installed, so it's not actually something I have to do very often. That was first this year and only did 2 last year and 1 of those was a new build property in the countryside where all services were visible.

But common sense and a basic knowledge of how services are routed to properties helps decide.

 

[Midwest]

Thanks for the replies chaps, that was a close en Wiko. My son's having an extension done. When they were digging the footings, they found some unexpected drainage pipes. Thames Water came out and camera'd them, and found another waste pipe. Both were 3' feet down, and if I'd look for a place to put a rod, I'd hit both!

If this 18th proposal goes ahead, it might prove expensive.

 

[Phil Thompson]

Definitely, Those cable finders are £500 and upwards, but doesn't help with gas and water pipe location.