TT earth electrode

[Gardner]

What is the max resistance at the time of installation permitted on customer earth rods in a TT supply?

 

[uksparks]

200 Ohms is what is considered to be stable, ideally you should be aiming for a reading considerably lower than that, i small nest of rods i recently fitted, I achieved 2.5 Ohms.

 

[Gardner]

Got it.

8 feet minimum depth is considered to get this value stable, correct?

 

[davesparks]

What is the max resistance at the time of installation permitted on customer earth rods in a TT supply?

The regulations require that a stable and reliable value of Ra be achieved.
What is stable will depend entirely on ground conditions and soil resistivity.
Deep rods in multiple locations are the key to achieving stability.

No doubt someone will come on here to quote the idiotic 200ohm nonsense which has been misconstrued from a note in the regulations. The NICEIC have their own version of this which is 100ohm.

The problem it seems is that there are a lot of people who can't cope with the idea that a good and stable value will be very different in different parts of the country.
A value of 300ohms achieved with 4x 8' rods will be much more stable and therefore better than 50 ohms achieved with a single 4' rod

 

[telectrix]

depends on the ground. some areas give good results with 4ft. and others struggle with 12 ft.

 

[davesparks]

200 Ohms is what is considered to be stable, ideally you should be aiming for a reading considerably lower than that, i small nest of rods i recently fitted, I achieved 2.5 Ohms.

I thought you were NICEIC? Their version of that utter nonsense is 100ohm not 200.

 

[Gardner]

I guess I should drive them deep even if I get a low ohm reading with a 6 foot rod in the event the top layer of earth dries up.

 

[telectrix]

I guess I should drive them deep even if I get a low ohm reading with a 6 foot rod in the event the top layer of earth dries up.

obviously you're not in the UK. it's almost permanent monsoon here. nothing can dry up. :devilish:

 

[davesparks]

I guess I should drive them deep even if I get a low ohm reading with a 6 foot rod in the event the top layer of earth dries up.

You won't normally get a 6' rod, they come as 4' with a screw thread at each end to join them together

 

[Dave OCD]

Why exactly is 200 ohms max idiotic Dave ? With a decent quality 30 ma RCD you'll get perfectly reliable and acceptable [fast] disconnection times under fault conditions at much higher Ze than that. Here in Cornwall the ground is often very hard and rocky and you could easily spend a whole day trying to get multiple rods in and that would be with some luck, just not competitive I'm afraid.

 

[Gardner]

obviously you're not in the UK. it's almost permanent monsoon here. nothing can dry up. :devilish:

Is TT recommended outside of such climates? Im thinking in cases outside of monsoon over 8 feet might be needed to keep the reading stable.

 

[Leesparkykent]

I thought you were NICEIC? Their version of that utter nonsense is 100ohm not 200.

Yeah but they word it as ideally below 100 ohms.

 

[telectrix]

give it another 10 years and google will have every property in the world earthed by satellite

 

[davesparks]

Why exactly is 200 ohms max idiotic Dave ? With a decent quality 30 ma RCD you'll get perfectly reliable and acceptable [fast] disconnection times under fault conditions at much higher Ze than that. Here in Cornwall the ground is often very hard and rocky and you could easily spend a whole day trying to get multiple rods in and that would be with some luck, just not competitive I'm afraid.

Its idiotic because the idea that you can set a blanket figure for all parts of the country with the myriad different soil conditions and ground types is ridiculous.
There are parts of the country where multiple deep rods cannot get the Ra lower than 200ohm but it will be stable, and someone would waste a lot of time and money trying to achieve the magic 200ohm when it is entirely pointless. And then there are other places where you can get <50 ohms with a single 4' rod and this will not be stable at all.

I've happily signed off an installation with an Ra around 250ohms which we had achieved with a 16' rod at each of the four corners of the house. From the initial testing before we started first fix I realised it was going to be a swine so had ducts put in to get earth wires to the locations.

 

[Dave OCD]

Ah I mistakenly thought that you meant 200 ohms was too HIGH Dave, but seems we're on the same wavelength.

 

[uksparks]

I thought you were NICEIC? Their version of that utter nonsense is 100ohm not 200.

Very true, personally being one with little experience of TT systems I aimed on that last one to get a good reading and didn't want to settle for any less.

what I would say is that I measured the rods as I drove them in and it's very true that the deeper you go the better the result so I can only recommend going as deep as you can.

i think 3 rids deep it's quite s good depth and not too difficult to get to with a lump hammer if you don't have a better method of driving them in.

i think there's a limit to how far you go with a nest as you might keep banging rods in nice and deep and joining them together and it not making much difference so to you accept defeat at say 80 ohms or do you keep trying?

the only reason I kept going was because it seemed easy Ish to put them in and each stack gave good results.

 

[Gardner]

Very true, personally being one with little experience of TT systems I aimed on that last one to get a good reading and didn't want to settle for any less.

what I would say is that I measured the rods as I drove them in and it's very true that the deeper you go the better the result so I can only recommend going as deep as you can.

i think 3 rids deep it's quite s good depth and not too difficult to get to with a lump hammer if you don't have a better method of driving them in.

i think there's a limit to how far you go with a nest as you might keep banging rods in nice and deep and joining them together and it not making much difference so to you accept defeat at say 80 ohms or do you keep trying?

the only reason I kept going was because it seemed easy Ish to put them in and each stack gave good results.

what do most of your DNOs require?

 

[uksparks]

How do you mean?

 

[Deleted member 9648]

The regulations require that a stable and reliable value of Ra be achieved.
What is stable will depend entirely on ground conditions and soil resistivity.
Deep rods in multiple locations are the key to achieving stability.

No doubt someone will come on here to quote the idiotic 200ohm nonsense which has been misconstrued from a note in the regulations. The NICEIC have their own version of this which is 100ohm.

The problem it seems is that there are a lot of people who can't cope with the idea that a good and stable value will be very different in different parts of the country.
A value of 300ohms achieved with 4x 8' rods will be much more stable and therefore better than 50 ohms achieved with a single 4' rod

A misleading statement.
The regulations merely state that account is taken of the effect of soil drying and freezing, and of a possible increase in resistance due to corrosion. There is also a vague reference to values exceeding 200 ohms being unstable. That is somewhat at odds with the myriad self appointed TT regulatory authorities on this forum.

 

[davesparks]

Ah I mistakenly thought that you meant 200 ohms was too HIGH Dave, but seems we're on the same wavelength.

Fair enough, especially at the bottom of the scale where you have the domestic installations, as you move up to bigger installations with large sites and multiple buildings then there comes a point where you really have no excuse not to get a sub-1ohm Ra.

 

[Dave OCD]

Yes true,but such installations unless way out in the wilds would normally have the luxury of a DNO earth available.

 

[davesparks]

A misleading statement.
The regulations merely state that account is taken of the effect of soil drying and freezing, and of a possible increase in resistance due to corrosion. There is also a vague reference to values exceeding 200 ohms being unstable. That is somewhat at odds with the myriad self appointed TT regulatory authorities on this forum.

Also misleading, what it actually says on the subject is a note, not a vague reference, which states that the Ra should be as low as practicable, and that a value >200ohm may not be stable. It does not state that >200 is unstable as you suggest, as i explained earlier this would be a ridiculous statement as it is perfectly possible to achieve a value >200ohm which is stable just as you can get a value <200 which is unstable.

 

[davesparks]

Yes true,but such installations unless way out in the wilds would normally have the luxury of a DNO earth available.

Unless of course they are one of the many installations where the use of PME is not permitted and so a TT system must be created

 

[Gardner]

How do you mean?

Dont DNOs tell you what system to use? (TT, TN-S, PME)

 

[Deleted member 9648]

Also misleading, what it actually says on the subject is a note, not a vague reference, which states that the Ra should be as low as practicable, and that a value >200ohm may not be stable. It does not state that >200 is unstable as you suggest, as i explained earlier this would be a ridiculous statement as it is perfectly possible to achieve a value >200ohm which is stable just as you can get a value <200 which is unstable.

Exactly my point, it's a note, almost guidance....open to interpretation and hence vague. It's NOT a regulation. There is no regulation requiring values to be stable, merely a note stating values exceeding 200 ohms may be unstable, well so what? There's no requirement in 7671 for them to be stable. We are back to self appointed regulation writers again.

 

[davesparks]

Exactly my point, it's a note, almost guidance....open to interpretation and hence vague. It's NOT a regulation.

And elsewhere in the regulations we are directed to bs7430 for further guidance on the design of earth electrodes.

 

[Deleted member 9648]

And elsewhere in the regulations we are directed to bs7430 for further guidance on the design of earth electrodes.

More 'guidance' then....?

 

[davesparks]

More 'guidance' then....?

Yes, the whole damned book is guidance

 

[Lazlo]

You are more than likely to be relying on RCDs for additional and fault protection, 1667ohms is the maximum Zs permitted for TT, you would need to be certain that your Ra plus R2 would never exceed this value. I usually ensure I get something sub 100ohms Ra. I do find the coupled 8ft rods to be a pain to drive and I always doubt the conductivity of the screw thread on the coupler and rods where they join.

 

[Deleted member 9648]

You are more than likely to be relying on RCDs for additional and fault protection, 1667ohms is the maximum Zs permitted for TT, you would need to be certain that your Ra plus R2 would never exceed this value. I usually ensure I get something sub 100ohms Ra. I do find the coupled 8ft rods to be a pain to drive and I always doubt the conductivity of the screw thread on the coupler and rods where they join.

1667 is the maximum where a 30ma RCD is used, not all TT installs use 30ma RCD's, values of 500,167 and 100 apply to 100,300 and 500 ma RCD's respectively

 

[uksparks]

1667 is the maximum where a 30ma RCD is used, not all TT installs use 30ma RCD's, values of 500,167 and 100 apply to 100,300 and 500 ma RCD's respectively

Yes, ultimately it depends on what your final circuits are as to what RCD is used if any I suppose.

 

[Lazlo]

Yes 50\0.03 = 1667ohm max Zs can only apply to 30mA?

 

[Guest77]

200 Ohms is what is considered to be stable, ideally you should be aiming for a reading considerably lower than that, i small nest of rods i recently fitted, I achieved 2.5 Ohms.

well we have all heard that story haven't we?

 

[Leesparkykent]

Yes 50\0.03 = 1667ohm max Zs can only apply to 30mA?

Yes that's why 50V/0.03 for 30mA.

 

[Lazlo]

50v being touch voltage,I wasn't asking the question, I was wondering why I was being told Zs varies depending on Ma rating of RCD, 1667 only applies to 30mA...zzzzzzz simple maths......

 

[Deleted member 9648]

You are more than likely to be relying on RCDs for additional and fault protection, 1667ohms is the maximum Zs permitted for TT, you would need to be certain that your Ra plus R2 would never exceed this value. I usually ensure I get something sub 100ohms Ra. I do find the coupled 8ft rods to be a pain to drive and I always doubt the conductivity of the screw thread on the coupler and rods where they join.

50v being touch voltage,I wasn't asking the question, I was wondering why I was being told Zs varies depending on Ma rating of RCD, 1667 only applies to 30mA...zzzzzzz simple maths......

You were being told because your post quoted here may be taken by some to apply to all TT installs with an RCD of any type.