fluctuating readings on a tt rod

[Gavin John Hyde]

Been at a rural cottage today doing a pre install survey for a EV charger,
House is 400 and something years old and situated on a hill with lots of rocky ground, mainly limestone based rocks. Owner tells me the rod is in his own words ' two joined together to make a long one' as he helped bang it in.
The following are taken from certs they have from previous work
Jan 17 - 140 ohms
July 17 - 274 ohms
Dec 17 - 128 ohms
3rd Feb 18 - 112 ohms - Measured by myself
Today 355 ohms!!!
The ground is very dry and even digging down a foot quickly with a spade the ground is dry. little if any damp. Any moisture quickly goes down the hill the property is situated on. the ground is hard rocky and little actual soil.
Certainly fluctuates with the seasons.. can't help but think this prolonged dry spell has impacted the reading..
its got me thinking about whether to put another rod in as we are told anything above 200 ohms is 'unstable'
Down side is not sure how on earth I get a rod in when the ground has so little soil etc... might be spending some time playing with the SDS!
What would you guys do? earth plate or tape? as this reading is not ideal

 

[123]

As existing setup is 2 4 foot rods joined together that the owner helped bang in...let him have another go at doing the same thing where you need it.

 

[Deleted member 9648]

Well at the end of the day it complies with Bs7671 which does not give a max value other than the max Zs of 1667 ohms, and nowhere does it state the reading must be stable. Having said that I would parallel in another rod to get it down .

 

[Gavin John Hyde]

I was working on the basis that the 2nd rod would in an ideal world need to be 8 foot from the last rod. which would literally put it slap bang in the middle of his garden! the garden from the front to the bit closest to house has a difference on the side of approx 4 feet, as the side road is a hill but owing to the narrow front dimensions of said house cant put it in the deeper part as id be in the neighbours garden or in the middle of the road.

 

[Gavin John Hyde]

. Having said that I would parallel in more rods to get it down.

I thought about asking him to train the dogs to pee on the existing rod and install a sprinkler to keep it moist!

 

[Yozza19]

It is a real hard one I had the same issue at Lancaster university recently putting a temp supply in I ended up putting a 900x900 earth mat in a metre down on a bed of marconite managed to get reading down to 34 ohm

Spoke to lightning protection lads recently they said any wer north of Preston usually causes them problems

 

[Deleted member 9648]

I had one a few weeks ago, on a hill ,the existing was 300 odd ohms, doubling up got it to 180 odd. Still not a reading many would accept but in full compliance.
You hear a lot of figures plucked out of nowhere with no logic at all. I recall one poster on here saying he wont accept below 80 ohms....Why ffs! Unless you can back up a minimum figure with technical reasoning you may as well go with the Bs7671 fudge. It complies....onward and upward!

 

[Strima]

I would consider either a mat or tape in ground like that. TBH 355 ohms after the recent dry spell is really worst case scenario for that installation, although not great is still compliant with BS7671 for 30mA RCD protection.

 

[Wilko]

Hi - have you had a look at the rod connections? When there's evidence that things have changed perhaps wander lead can help confirm the cause.

 

[Yozza19]

Also what is method of measurement?

 

[Andy-1960]

Earth mat in Bentonite should improve the situation

 

[Gavin John Hyde]

Measured using method 2 as per Gn3 didnt have access to a dedicated rod testing kit, as I so rarely deal with them in my day to day house bashing work within the local area.
could have borrowed one if needed but given the location, the distances needed between rods would have meant it wasnt practical in anycase as I would have test leads over the road, up the hill and part way to the pub!

 

[Gavin John Hyde]

Hi - have you had a look at the rod connections? When there's evidence that things have changed perhaps wander lead can help confirm the cause.

Checked connection to the rod and all is very good and covered with suitable enclosure from elements.

 

[magnoliafan89]

In college i was told the rods had to be a minimum of 1 metre from the dwelling. What is the reasoning for this as that was never actually explained

 

[FatAlan]

I take it DNO can’t provide a PME connection?