Why did the Ra go from 180Ω to 14Ω?

[HappyHippyDad]

About a year ago I went to a property to do some work. It is a TT and the Ra was something like 300Ω or 400Ω, I cant remember exactly. I put in 2 new rods 5/8" and it came down to around 180Ω, still poor but <200Ω.

I went back today for some more work and the customer said an electrician had been out next door and got Western Power (WP) out with regards 'something to do with the earthing'.

Before I started my work I checked the Ra as I wanted to make sure it was stable at <200Ω. It measured as 14Ω.

My customer took me to the electricity pole (I guess this is the transformer) on her property (large field) and showed me where she thinks the work was carried out. On either side of the pole is a cable which runs into the ground, one of these is where she thinks the work was carried out. What would WP have done to improve the Ra? I thought the Ra was related just to the customers earthing system, i.e rod etc.

 

[SparkyChick]

They may have repaired the local earth spike for the transformer, or put a new rod in.

 

[SparkyChick]

But could be different ground conditions... we've had a lot of rain so the water table may be higher making the ground more conductive closer to the surface, thus improving the rods performance.

 

[AJshep]

Id agree with Sparkychick.
Answering your question, RA is the sum of the resistances of the earth electrode (to Earth) and its using the mass of earth as its conductor, so a nice new earth at a transformer would reduce your reading.

Sounds like your pretty close to the transformer so if you were getting a reading of 400ohms other people down the road probably had readings up in the 1000 I expect someone complained so DNO had to sort it out.

 

[Wilko]

Hi - that is their local rod for PME and your earth loop now goes to that by the looks .

 

[Bellendian]

Your rods might have a nice short route back to new dno rods, soil might be wetter than it was last time, or possibly nextdoor converted to pme, but old tt rod nextdoor still connected or bonded water mains close together giving a better path back. Loads of possibilities.

 

[Nigel]

There is no reason why anything the DNO does will affect your Ra.

In fact it is impossible.

 

[Ian1981]

Did you do a Ra electrode test or was it with a loop tester?

 

[westward10]

It must have been an elfi test.

 

[Bellendian]

Yep ok, agree it wouldn't affect Ra if measured with electrode tester.
But it could change a Ze done with a loop tester, which I guess is what you're getting at?

 

[UKMeterman]

They may have repaired the local earth spike for the transformer, or put a new rod in.

I know of one property where I had reported that the local earth conductor had been liberated from the local transformer pole...

 

[PEG]

There is no reason why anything the DNO does will affect your Ra.

In fact it is impossible.

I will have to say,that is incorrect,fella.
Unlikely,would be a fair statement,but certainly not impossible.

 

[Nigel]

I will have to say,that is incorrect,fella.
Unlikely,would be a fair statement,but certainly not impossible.

Please school me.

 

[PEG]

Would not dream of doing so,friend.
Just imagine the tasks one would perform,either intentionally,or deliberately,to improve the Ra on a particular installation.
Now consider,if any work or task,agents from a DNO would or could carry out,and how this could affect things.

 

[Nigel]

Would not dream of doing so,friend.
Just imagine the tasks one would perform,either intentionally,or deliberately,to improve the Ra on a particular installation.
Now consider,if any work or task,agents from a DNO would or could carry out,and how this could affect things.

I honestly swear I cannot think of anything.

 

[PEG]

Hi bud,the first i encountered,was at a Victorian semi,with cellars,where the TT earth connection,disappeared in to the cellar wall,as is common in this type of property,to,i suspect,the lead water or steel gas service pipe.
The DNO attended the property next door,where an elderly couple,had an electrician attend,and flagged an earthing issue,which the DNO accepted,was their responsibility to maintain,as they had completed repairs there, sometime in the seventies.

Whatever they did,during their 4 hour visit,plummeted the Ra at the property we were sorting,from the 30's,to just over 1.

I suspect their improvement,was back-feeding a benefit,through a shared or defunct service pipe,via the bonding.This improved Ra,remained the same,without variation,on subsequent visits.

The second occasion,was a rural property,where the TT rods,of which there were three,were in a line along the boundary. The ground was back-filled,and raised,and i would guess that this was the reason for the three rods.

The DNO moved a pair of transformer poles,to a new position,right next to the rods,due to some expansion,of the farm next door.
There was some carry-on,as the pole auger vehicle,got stuck,and the efforts to make a hard-standing for it to work,resulted in some top soil,being banked over the boundary,and two rods.

The links,between the rods,were underground,anyway,but the steady,summer Ra of about 150,was 20-30 when tested,2 days after they cleared site,and although i have only ever tested during summer,it appears stable.

It is not that long ago,that the work done,by any DNO,was a tad "individual",to say the least,where certain methods of work were driven more by sole characters ideas of methodology,and not rules and directives.
I have been told dozens of stories,by some of the older lads from National Grid,when i worked their sites at Carrington and Penwortham,of how they enabled expected earthing values,for a surprise visit from an engineer,Friday afternoon.....

Believe nothing you hear...only half of what you see...and never,say never

 

[Nigel]

Hi bud,the first i encountered,was at a Victorian semi,with cellars,where the TT earth connection,disappeared in to the cellar wall,as is common in this type of property,to,i suspect,the lead water or steel gas service pipe.
The DNO attended the property next door,where an elderly couple,had an electrician attend,and flagged an earthing issue,which the DNO accepted,was their responsibility to maintain,as they had completed repairs there, sometime in the seventies.

Whatever they did,during their 4 hour visit,plummeted the Ra at the property we were sorting,from the 30's,to just over 1.

I suspect their improvement,was back-feeding a benefit,through a shared or defunct service pipe,via the bonding.This improved Ra,remained the same,without variation,on subsequent visits.

The second occasion,was a rural property,where the TT rods,of which there were three,were in a line along the boundary. The ground was back-filled,and raised,and i would guess that this was the reason for the three rods.

The DNO moved a pair of transformer poles,to a new position,right next to the rods,due to some expansion,of the farm next door.
There was some carry-on,as the pole auger vehicle,got stuck,and the efforts to make a hard-standing for it to work,resulted in some top soil,being banked over the boundary,and two rods.

The links,between the rods,were underground,anyway,but the steady,summer Ra of about 150,was 20-30 when tested,2 days after they cleared site,and although i have only ever tested during summer,it appears stable.

It is not that long ago,that the work done,by any DNO,was a tad "individual",to say the least,where certain methods of work were driven more by sole characters ideas of methodology,and not rules and directives.
I have been told dozens of stories,by some of the older lads from National Grid,when i worked their sites at Carrington and Penwortham,of how they enabled expected earthing values,for a surprise visit from an engineer,Friday afternoon.....

Believe nothing you hear...only half of what you see...and never,say never

This sounds like Ze not Ra??

 

[davesparks]

There is no reason why anything the DNO does will affect your Ra.

In fact it is impossible.

I don't think it's impossible, just very unlikely.
It sounds like the DNO's earth rods are reasonably close to the installations earth rod.
My knowledge is a bit rusty here but I think if the DNO have increased the length of their earth rods the spheres of influence may now overlap where they did not before and thus affect Ra.
I think the effect on the measured Ra will depend on which test method is used, I doubt it would affect a 3 wire fall of potential test but a clamp tester or direct measurement may be affected.

Obviously if it is not Ra which is being measured but Ze by and efli test then whatever the DNO have done could have a big effect.

 

[Marvo]

....It is a TT and the Ra was something like 300Ω or 400Ω, I cant remember exactly. I put in 2 new rods 5/8" and it came down to around 180Ω, still poor but <200Ω......

Assuming you mean Ra and not Ze, you put new rods in and over time the disturbed ground around the rods will naturally settle and compact against the rod. This is especially so if you had long rods with couplings along their length.

I've installed deep earth rods and earth nests that have tested 6.5 ohms when installed and two years later were down to 2 ohms when retested.

 

[HappyHippyDad]

I think I should give some apologies here. I thought that the correct term for the measurement you get when doing the external earth fault loop impedance for a TT was Ra rather than Ze.

I have used a MFT to do a external EFLI test, so I should have said Ze.