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.