Loop test

[1stfix2ndfix]

136.4 ohms at lights whilst doing the loop test before we change the board. Guessing they've been lucky ? Plus side is I get to put an earth stake in were see how much I remeber from the one I did 2 years ago

 

[HappyHippyDad]

Who the hell in the IET came up with a figure of ≤200Ω wants taking out and shooting at dawn!

How many in the trade have to same attitude as HHD “Oh well, it’s near enough, it’ll do.”

Thats more like it Tony, just like old times :smiley2:

Give me a 'realistic' idea of how to go about getting a sub 1 result and I'll listen Tony? Even with an SDS adaptor you'll still need a HUGE rod to get that sort of figure.

I really would like to hear from you about the following as you will know the answers and I am just talking from theory not from experience:

1. What Ra would you class as acceptable in a domestic situation AND why?

2. How would you achieve this figure taking into account the figures in post 15? (An SDS adaptor and 5m rod is not going to get the Ra below 1Ω)

Also...... Are there ANY electricians on this site who have got <1Ω for a domestic TT installation? Apart from you Trev

 

[telectrix]

it might if the ground conditions are right. option is to add more rods. problems arise when you hit bedrock at 4 ft. that's when you go horizontal with earth mat/plates.

 

[Bobster]

Thats more like it Tony, just like old times :smiley2:

Give me a 'realistic' idea of how to go about getting a sub 1 result and I'll listen Tony? Even with an SDS adaptor you'll still need a HUGE rod to get that sort of figure.

I really would like to hear from you about the following as you will know the answers and I am just talking from theory not from experience:

1. What Ra would you class as acceptable in a domestic situation AND why?

2. How would you achieve this figure taking into account the figures in post 15? (An SDS adaptor and 5m rod is not going to get the Ra below 1Ω)

Also...... Are there ANY electricians on this site who have got <1Ω for a domestic TT installation? Apart from you Trev

Well you're going to have to balance cost and quality. I'd accept <10Ω for a domestic install. Yes you're still relying on the RCD's but it's low enough to not fluctuate from that reading by much, providing a stable system.

Depends where you are in the country 5m may well get you <1Ω. There is always an option of adding another rod if needed.

 

[Silly Sausage]

Thats more like it Tony, just like old times :smiley2:

Give me a 'realistic' idea of how to go about getting a sub 1 result and I'll listen Tony? Even with an SDS adaptor you'll still need a HUGE rod to get that sort of figure.
........................

Welcome to the world of Engineering, finding the 'perfect' compromise between the ideal solution and cost!
As a designer of your own installs, you're going to have to work that out between yourself and your customers.

 

[HappyHippyDad]

Well you're going to have to balance cost and quality. I'd accept <10Ω for a domestic install. Yes you're still relying on the RCD's but it's low enough to not fluctuate from that reading by much, providing a stable system.

Depends where you are in the country 5m may well get you <1Ω. There is always an option of adding another rod if needed.

Thanks Rob, I just struggle with the 'accepted figure' for an Ra to achieve stability. BS7671 says ≤200Ω, you say <10Ω (which I have heard before).

It would be good to know just how much the resistance varies with change in moisture content then I could see for myself if a value of 200 could ever exceed 1667 during a heat wave. GN8 simply shows that the soil resistivity can increase 'dramatically' when moisure content decreases from 20% - 10% but it doesn't give actual figures.

Ps.. I like your signiature quote!

 

[Farmelectrics]

Not had a chance to read all threads but I asked this morning what type of earthing arrangement it was I'm gathering it's a tt will read threads though confused lol

 

[Farmelectrics]

Question why and I'm sure we've all come across it do people mix earthing arrangements up not saying this is the case here but why do people want to drive rods into the ground when the earthing system is a tns or a tnc-s and what dangers can occur seen this a few times over the years anyone else had similar

 

[Simonslimline]

I was under the impression that adding a rod to a TN system was fine. But mixing TN systems is a no no.

Sure there was a thread on here about it a while back.

 

[davesparks]

It's not just a case of going deeper to get a better Ra.

If permanently installing earth electrodes I will start with 2 x 8' rods spaced 10' apart as my baseline installation and work from that.
That usually returns a result in the region of 20 to 30 ohms.

But you get to know your local area and can make a good estimate before you start

For temporary installations (generators for outdoor events usually) I'll use single rods spaced around the site and connected to the distribution system at each major DB.

Or for a small system at a firework gig when working in a water logged field and the generator is sitting in 6" of water It'll get a twig poked in the ground.

 

[Knobhead]

Domestic wasn’t my field at all so only did one TT install, my own house. The limestone was only a matter of feet down so I put an earth mat in.

Like Rob I worked in quarrying so deep rods weren’t a problem.

 

[1stfix2ndfix]

This has all become a bit confusing

 

[Bobster]

Whys that?

 

[1stfix2ndfix]

Dont know what im doing lol

technical stuff aside

I went to a job today and started to test the lights. First of all my tester kept saying 0v so i ignored it and went to another light same thing next one same thing and so on. Getting a bit annoyed I went back and said the testers being funny and not giving a reading. Of i go again this time setting it at 2000ohms instead of 20 which I normally use. It works but ive just seen 136 odd on the screen. Alarm bells are ringing lol but Ive got an idea that a tt might be higher then the 7 something im use to it having to be within. From the thread Ive gained the information that it has to be equal to or less then 200 ohms.

anyway ive had to put in an earth rod (im not sure why from this thread as if its less then or equal to 200 ohms its fine) it was fed by 10mm earth cable and the only thought on its location was position (close) and the technical bits (measurement from the window) and guessing part (where the foundations finished) decided where it went (in between driveway block paving)

After doing all of that it went down to 83 ohms pretty much, more then half of what its ok to be?

Thats all ive got

 

[Bobster]

The 200ohm is recommended in the Bgb. A number they, as far as I can tell pulled out of thin air.

 

[1stfix2ndfix]

It must actually be greater then 200 because the eath can change its resistivity so they must of allowed for that.