Supply to shed

[Davethsparky]

I thought  21 was a  stated  figure from the DNO , at their transformer,  and then, as you say, over 200 is said to become unstable .

Nope, there is a minor note in the regulations which suggests that values above 200 may not be stable, nothing to do with 'what is said to become unstable'. It's not any form of recommended or maximum value just a very annoying note which a lot of people seem to take as a target value

 

[apprentice87]

Deke is right in that the DNO have to achieve a value of 21 ohms or less for their earth spike at the transformer end..

john..

 

[Andy™]

yes, but that doesnt mean Ze at an installation must be under 21

 

[Davethsparky]

yes, but that doesnt mean Ze at an installation must be under 21

No, it would hopefully be a damned sight less than that!

 

[Sidewinder]

However, if you are installing a means of earthing/earthing system, then you have to look further than BS7671, to BS7430, which is a CoP.

Being a CoP it has more quasi legal status than BS7671.

 

[Andy™]

No, it would hopefully be a damned sight less than that!

why? for TT it can be anything upto 1667 with a 30mA RCD, although around here you rarely see over 100 with a single rod, usually 30-80

 

[Davethsparky]

why? for TT it can be anything upto 1667 with a 30mA RCD, although around here you rarely see over 100 with a single rod, usually 30-80

Stability and reliability would be a good start, and hopefully removing reliance on RCDs. 

A well installed earth electrode system with a low overall Ra is going to have far better long term reliability and fault current handling ability than a single rod of high value. 

Plus of course there is the ridiculously old fashioned concept of doing a good job and taking pride in your work, but I don't suppose that really counts for anything does it

 

[Andy™]

so you really think its possible to get Ze on a TT to a lot less than 21 ohms on every installation? and do you really think you can always get Zs on final circuits low enough to not need RCD's? as much as i would like that to happen, it doesnt

 

[Evans Electric]

Yes , I know about the 200 as the dreaded "Unstable" limit   , for some reason I put the 21 down , hoping to make Horace sound as if he has an understanding of it  for the assessor.    Don't even know where I got 21 from now TBh.

Asking the max Ze s seems to be a favourite question at assessments .

 

[Davethsparky]

so you really think its possible to get Ze on a TT to a lot less than 21 ohms on every installation? and do you really think you can always get Zs on final circuits low enough to not need RCD's? as much as i would like that to happen, it doesnt

No, and that's not what I said. 

It is not possibly to get it below 200 ohms sometimes, but what is always possible is to install a decent earth made up of multiple electrodes which achieves a good degree of stability and reliability.

what is not acceptable in my opinion is to install a single 4' rod and rely on that.

Yes , I know about the 200 as the dreaded "Unstable" limit   , for some reason I put the 21 down , hoping to make Horace sound as if he has an understanding of it  for the assessor.    Don't even know where I got 21 from now TBh.

Asking the max Ze s seems to be a favourite question at assessments .

21 is the value which you can expect a DNO to quote if you enquiry about the Ze of a TT supply. Much like the 0.35 and 0.80 values which get bandied around. 

Pit is important to note however that these are not maximum values, they are guidelines based on the standard 100A supply. Other size supplies will have other guideline max Ze

 

[Andy™]

what is not acceptable in my opinion is to install a single 4' rod and rely on that.

that depends where you are. ground conditions around here are very good for rods, a single rod even partly in will almost always be under 200, fully in its usually 30-80, and the readings rarely change by much once installed. so yes, perfectly acceptable and stable. but then there are other areas (as a few posters here have said), where even with multiple rods they still cant get an acceptable reading

 

[Davethsparky]

that depends where you are. ground conditions around here are very good for rods, a single rod even partly in will almost always be under 200, fully in its usually 30-80, and the readings rarely change by much once installed. so yes, perfectly acceptable and stable. but then there are other areas (as a few posters here have said), where even with multiple rods they still cant get an acceptable reading

How have you proved the stability of those readings? Being only 4' deep it will be far more susceptible to seasonal changes than a deeper electrode. 

Around here 2x 8' rods spaced around 12' apart normally achieve an Ra below 20ohm 

that at all depends on how you define acceptable, a value of 300ohm achieved through multiple deep rods or horizontal tapes will be more stable than a value of 100ohm achieved with a single short rod

 

[apprentice87]

yes, but that doesnt mean Ze at an installation must be under 21

Never said it did....

john....

 

[Horace]

Will study some more about max Ze values, ive not had alot of dealings with TT systems. I need to brush up on alot of the regs tbh, they do strangely make interesting reading now im getting towards the big 40 !

 

[leesparkykent]

Installing one 4ft twig isn't going to give you stability.

 

[Evans Electric]

Will study some more about max Ze values, ive not had alot of dealings with TT systems. I need to brush up on alot of the regs tbh, they do strangely make interesting reading now im getting towards the big 40 !

Really !!!        Horace you need to get out more !!

 

[steptoe]

Installing one 4ft twig isn't going to give you stability.

Since when?

As I've said in,a different thread somewhere,

I have sub 1ohm readings on a "4' twig" as you call it, ok, so that's Ze, are you seriously sating that's going to suddenly jump up to over 500 ohms now after a few years of being installed,?'?!?! :shakehead

Wide sweeping misinformed statements are worse than no comments at all.!!!

 

[leesparkykent]

Since when?

As I've said in,a different thread somewhere,

I have sub 1ohm readings on a "4' twig" as you call it, ok, so that's Ze, are you seriously sating that's going to suddenly jump up to over 500 ohms now after a few years of being installed,?'?!?! :shakehead

Wide sweeping misinformed statements are worse than no comments at all.!!!

Do you monitor this to check stability out of interest as the first 4ft of ground is the most susceptible to seasonal changes? Do you think you would have a sub 1Ohm reading when the ground it frozen being only 4ft in? I honestly think its great you have a sub 1Ohm reading on a 4ft twig...something I've never seen. Tbh though I wouldn't be banging in one 4ft twig in the ground and walking away from jobs with no regard for stability......

 

[kerching]

Satisfy my curiosity please......

is 'twig' another of these hip names , similar to 'twin and skin'

also these 4' twigs....are they the ones that cannot be extended?

just voicing my bewilderment over the nomenclature

 

[bluetobits]

I used to have a 6' twig on my car for the CB many moons ago.

I hope that's not the sort of thing they're burying in their gardens