Earth electrode tester or not?

[1Justin]

I did some work today (added a new circuit) to a TT where earth electrode/Ra (bonding disconnected) was 198 Ohms. Bit close to limit where advice is to "investigate".

Interestingly the figure I got is precisely the same as that measured in 2010 by previous electrician, so it does appear to be fairly stable. Maybe grounds to leave it all alone since it's well below 500 (Property has 100mA RCD also on it's PV system) . Despite this I advised householder that probably good idea to re-check frequently, so I put 2 years on my EIC.

Were I to install a new electrode to improve the figure (another reason to upgrade might be that the electrode is a thin mild steel rod, not great looking, and with rust). - Then I believe it's OK (according to GN3) to simply measure Ra for the new one? (We have 30mA RCD's).

Soil type is mostly a sandy area. Alongside the electrode is an old (actually rather nicer) galvanized rod (now disconnected). I get exactly the same figure if I check this one also, which implies it might just be the soil (or they are both equally bad).

Is it the case that if I were to re-install, I could measure only Ra? - Or would I need to get my hands on an Earth Electrode Resistance Tester and do it properly?

 

[GLENNSPARK]

what is it your wanting to do here Justin?
if its just Ra/Ze you want to measure...then a loop tester will cut it...
if its specific ground resistance you want to measure....then you need a ground resistance tester....
its a 4 wire test as well is that....

 

[1Justin]

Am I right in thinking that a four wire ground resistance tester would only be necessary on a empty (no supply) field site prior to DNO setting a supply?

As I have a supply, and a house, and just a dodgy looking electrode, then do I just bang in a new one (should I want to ), and measure Ra with standard high current Ze/loop tester?

 

[GLENNSPARK]

yep...use the high current setting on the loop tester....
alternatively you could get an earth resistance tester....but then you need the test leads and spikes to go with it.....can be pricy...
use your loop tester Justin....go for a min. of 2 X 5/8" rods....site em well....say about a meter or so from any structures....and someware theres going to be no digging activity....
also protect the main earth cable with a leg of conny....

 

[1Justin]

Hi Glenn,

Thanks for that. Useful tips.

BTW I got one of those mini current clamps you recommended a few weeks ago. Nice little instrument, works well

 

[GLENNSPARK]

Hi Glenn,

Thanks for that. Useful tips.

BTW I got one of those mini current clamps you recommended a few weeks ago. Nice little instrument, works well

you mean that leakage clamp i told you to get?
you happy with it then justin?

- - - Updated - - -

evening eng...

 

[Engineer54]

Interestingly the figure I got is precisely the same as that measured in 2010 by previous electrician, so it does appear to be fairly stable. Maybe grounds to leave it all alone since it's well below 500

From where did you get the 500 ohms as being the maximum from?? Even the numpty recommendation in the BGB is 200 ohms, and your value is only 2 ohms below that figure!!

Bare steel and galvanised steel electrodes are not ideal by a long choc for many different soil conditions, sandy soils would be one of them. Copper clad steel rods are pretty much immune (within reason) to all soil conditions, as far as accelerated corrosion is concerned, and have a far better conductivity value than steel, galvanised or otherwise.

 

[PEG]

That sandy soil is crying out for some proper rodding...Put one in recently in sandy soil with red clay strata 2m down,got three 5/8ths in with old Hilti TE72 in 40 minutes.Conditions dry now but 8 ohms,will probably get to retest in Winter.Go on,get piling....

 

[Deleted member 9648]

I did some work today (added a new circuit) to a TT where earth electrode/Ra (bonding disconnected) was 198 Ohms. Bit close to limit where advice is to "investigate".

Interestingly the figure I got is precisely the same as that measured in 2010 by previous electrician, so it does appear to be fairly stable. Maybe grounds to leave it all alone since it's well below 500 (Property has 100mA RCD also on it's PV system) . Despite this I advised householder that probably good idea to re-check frequently, so I put 2 years on my EIC.

Were I to install a new electrode to improve the figure (another reason to upgrade might be that the electrode is a thin mild steel rod, not great looking, and with rust). - Then I believe it's OK (according to GN3) to simply measure Ra for the new one? (We have 30mA RCD's).

Soil type is mostly a sandy area. Alongside the electrode is an old (actually rather nicer) galvanized rod (now disconnected). I get exactly the same figure if I check this one also, which implies it might just be the soil (or they are both equally bad).

Is it the case that if I were to re-install, I could measure only Ra? - Or would I need to get my hands on an Earth Electrode Resistance Tester and do it properly?

Cant possibly be stable though.....well, according to ....?

 

[Engineer54]

Cant possibly be stable though.....well, according to ....?

HaHa, Still trying to be clever!!!

 

[D Skelton]

Cant possibly be stable though.....well, according to ....?

Haha! Brilliant!

 

[Engineer54]

Haha! Brilliant!

Of course, ...he's your Dad!!

 

[D Skelton]

Of course, ...he's your Dad!!

That's cruel WP can't be that old!

 

[Deleted member 9648]

That's cruel WP can't be that old!

Older than the hills mate...:sailor:

 

[Guest55]

ignoring the old fossils for a moment........

as for the OP's original enquiry regards getting a dedicated ground resistance meter , its not really worthwhile for your average jobbing spark.
The IET and all their official literiture now all but endorse the earth loop method of checking Ra as an acceptable alternative.

 

[1Justin]

Hi Engineer54

From where did you get the 500 ohms as being the maximum from?? Even the numpty recommendation in the BGB is 200 ohms, and your value is only 2 ohms below that figure!!

By my understanding, 200 Ohm is recommended for investigation (ie is it stable..). The well known 1666 ohms max for 30mA RCD (= 50V/30mA) and for 100 ma RCD (which customer has on PV system) 500 ohms (=50V/100mA). See GN3 Pg 48.

QED?

Rgds

 

[GLENNSPARK]

Hi Engineer54



By my understanding, 200 Ohm is recommended for investigation (ie is it stable..). The well known 1666 ohms max for 30mA RCD (= 50V/30mA) and for 100 ma RCD (which customer has on PV system) 500 ohms (=50V/100mA). See GN3 Pg 48.

QED?

Rgds

200 ohms is the max value whilst the Ra will remain stable....
its a case of the Ra increasing by up to 8 times (worst case)....so 8 X 200 is 1600....so still under the value of 50V/0.03 = 1667
but in reality its a con....
the NIC say 100 ohms....
again..a con....

 

[Engineer54]

Hi Engineer54



By my understanding, 200 Ohm is recommended for investigation (ie is it stable..). The well known 1666 ohms max for 30mA RCD (= 50V/30mA) and for 100 ma RCD (which customer has on PV system) 500 ohms (=50V/100mA). See GN3 Pg 48.

QED?

Rgds

The very last place i would take advice from on any TT system is BS7671 or it's OSG's, Wirepuller is your man here!!

If both the house and the PV system is TT, id be asking why has the PV system been given a 100mA RCD and the house 30mA...?? If the house is on a TN system, again, why they have thought it necessary to TT the PV system?

 

[Deleted member 9648]

Hi Engineer54



By my understanding, 200 Ohm is recommended for investigation (ie is it stable..). The well known 1666 ohms max for 30mA RCD (= 50V/30mA) and for 100 ma RCD (which customer has on PV system) 500 ohms (=50V/100mA). See GN3 Pg 48.

QED?

Rgds

You are correct Justin,it comes down to whether or not you accept the 7671 values/recommendations.
I get slated on here for accepting those....by the very same people who also slate the reliance on RCD's as primary earth fault protection and yet fall over themselves to praise an Ra of 15 ohms!'
Either accept higher Ra values and reliance on RCD's, or obtain a stable TN value of Ra and back up RCD's with effective OCPD earth fault protection. There's no in-between', despite the 'holier than thou' rubbish spouted by certain members on this subject.

 

[Engineer54]

You are correct Justin,it comes down to whether or not you accept the 7671 values/recommendations.
I get slated on here for accepting those....by the very same people who also slate the reliance on RCD's as primary earth fault protection and yet fall over themselves to praise an Ra of 15 ohms!' Normally you quote 10 ohms?? ...anyway it's only you falling over yourself seeing, or rather wanting to see something that's never been implied or stated. Either accept higher Ra values and reliance on RCD's, or obtain a stable TN value of Ra and back up RCD's with effective OCPD earth fault protection. There's no in-between', despite the 'holier than thou' rubbish spouted by certain members on this subject.

Say's the man that only talks about spikes for sheds and garages!! lol!!

I don't get paid good money to run multi million dollar projects by spouting rubbish!! I'll leave all the rubbish spouting, to the clever but lazy buggers that openly wouldn't even try to install a decent TT system, and fall back on the crap that written in the BGB...