TT system bad earth help

[J25GTi]

Hi,


My question is as follows -


I have recently gone to a site to replace some lighting, and before I started I noticed the fuseboard/Earth were in a bad condition so I carried out a Ze test before starting works.


With all parallel paths disconnected and just the earth electrode I got a reading of 122.4ohms.


Now this was on a day where we have just had 2.5 weeks worth of solid rain and the ground was saturated.


I believe that when the ground is dry this will raise to more than 200ohms.


But my boss is telling me to carry on with the work as if it is less than 200ohms "It doesnt matter"


Now I remember reading somewhere in bs7671 or a GN that if the earth electrod Ze is over 100ohms then a latent defect could exist and requires investigation? But I cant find it for the life of me!


Help!


Thanks

 

[jumpin jax]

200 ohms is guidance in regs, niceic used to have guidance of 100 ohms but not now i have heard

 

[nickblake]

as jumpin jacks says 200 ohms below that the supply could be come unstable ,if you are concerned try adding additional earth rods or tape but install it in an area which may be damp during dry time ie next to a gully but be careful so you dont damage the drains and ensure you use the proper earth rod clamp

 

[J25GTi]

So its changed in the latest edition? That's gonna bugger me up so to speak... I thought it was still over 100ohms needed investigation. As I believe once it dries out it will be a nightmare.

 

[telectrix]

it's probably a 3' twig. screwed rods driven deeper will improve it. with a TT system, though, all circuits should be RCD protected.

 

[sjm]

200 ohms is the reccommended max value that an earth rod should not exceed. The reason that 200 is used is because the value of resistance can alter by up to 8 times depending on soil conditions etc. In other words 8 times 200 ohms gives a value of 1600 ohms which is still below the theorectical max of 1667 ohms Zs value when protected by a 30 mA RCD.

Hope this helps.

 

[J25GTi]

Yes of course on all points. Thanks for the input guys. I will thank you all later when I'm on the laptop.

So is the regulation about if its 100ohm or over a latent defect exists not applicable/in use anymore?

 

[Paul505]

Have the obvious things been checked.....is the cable connection to the earth rod okay, correct earth rod clamp used, installed correctly and protected by an earth inspection pit,is it in a suitable location for the existing rod and also does the rod feel loose in the ground..... etc
Maybe your boss should consider your opinion and listen to your concerns, which you think could potentially become a problem and raise above the 200ohms, and then explain the problem to the customer. Is your boss signing off the work by the way?

 

[J25GTi]

No our nic qualifying supervisor is. My boss is only going on what they have told him. Yes everytrhing is okay with the earth rod. Part of the problem is severe corrosion on the cable etc. And requires replacing.

 

[telectrix]

the 100 ohm was just a niceic guideline. BS7671 recommends 200 ohms. still crap, though.

 

[Paul505]

it's probably a 3' twig. screwed rods driven deeper will improve it. with a TT system, though, all circuits should be RCD protected.

Years ago i had a problem with a high Ze on a TT plus rod not installed very well....drove in 3x heavy duty rods screwed together and got the reading down to 30 ohms.
Also another electrician told me that it helps to install a rod on the north facing side of a building.... if thats an option available to you.

 

[J25GTi]

Why on the north side? Yeah I've done a few TT installations just the qualifying supervisor (who has never ever been to the site) is trying to tell me I'm wrong!

 

[SPARTYKUS]

the 100 ohm was just a niceic guideline. BS7671 recommends 200 ohms. still crap, though.

So, just a made up thing then!!

 

[Paul505]

Why on the north side? Yeah I've done a few TT installations just the qualifying supervisor (who has never ever been to the site) is trying to tell me I'm wrong!

The ground/soil in that area will mostly be in the shade, which should have a better soil resistivity apparently.
Northern hemisphere countries only though...lol

 

[Engineer54]

200 ohms is the recommended max value that an earth rod should not exceed. The reason that 200 is used is because the value of resistance can alter by up to 8 times depending on soil conditions etc. In other words 8 times 200 ohms gives a value of 1600 ohms which is still below the theoretical max of 1667 ohms Zs value when protected by a 30 mA RCD.

Hope this helps.

And that makes a 200 ohm Ra a good earth then does it??? lol!!! ...Do me a favour!!!

From what's been posted by the OP, this TT system is in trouble, it's certainly not showing signs of being a stable system... Probably as usual, just a single 1.2m 3/8 rod that can't be extended has been used.

By the way, any reading over a 100 ohms, especially in rain saturated ground is suspect and should be investigated, and where necessary improved...