tt earthing on farm

[Marvo]

21 ohms is useless if you're relying on it to make fault disconnection times with no other parallel paths. We just installed a local earth at a remote pumping station, took 64 x 2.4m rods in 4 separate earth pits and 70m of 25mm earth wire to achieve a sub-ohm Ra. As Davesparks says a single rod won't help.

 

[Engineer54]

Zs = 21 ohms at a socket on db.( my mistake before calling it ze) 3 phase tt supply. No earth rod. Supply goes from rcd main switch to metal clase switched fuse. Swa then across yard to db in a shed. Did notice where db was that the steel work of building was bonded, so i think this would be giving a low reading. So if i put rod at mains and get a low reading do i need to put one at every board even if i get a low reading. Thanks

Basically you have an incoming DNO supply without any earthing, so if you have no earth electrode connection of any description at the source then it isn't even a TT system.... Seems the whole electrical system is relying on bonding connections, probably the water supply for the farmhouse and steelwork for this steel framed horse barn. I can see this situation turning into quite an expensive operation to rectify, and one that the farmer ain't gonna be too happy about, seeing how they hate spending money for something/anything they can't actually see.

Where livestock is concerned, things need to be done correctly, our 4 legged friends don't normally get to have second chances!!

 

[Simon-0116]

Thanks for the genuine replys. Just went to look at job so never tested at incomer. Probably should have worded q better, thanks

 

[Engineer54]

21 ohms is useless if you're relying on it to make fault disconnection times with no other parallel paths. We just installed a local earth at a remote pumping station, took 64 x 2.4m rods in 4 separate earth pits and 70m of 25mm earth wire to achieve a sub-ohm Ra. As Davesparks says a single rod won't help.

I think you've frightened the crap out of him!! lol!!

Perhaps you should have mentioned that you are driving rods into compacted sand, and needed to go that deep (38+m) X 4...

 

[Bobster]

I think you've frightened the crap out of him!! lol!!

Perhaps you should have mentioned that you are driving rods into compacted sand, and needed to go that deep (38+m) X 4...

He'd get a cracking Ra over here with that though!!

 

[Marvo]

I think you've frightened the crap out of him!! lol!!

Perhaps you should have mentioned that you are driving rods into compacted sand, and needed to go that deep (38+m) X 4...

We had 4 rods per pit, each one was 4 rods coupled together and we angled the rods at about 20degrees from vertical so the depth of each rod was only around 8 meters. I wish it was sand but alas it was half way up a mountain so very rocky. Even digging the pit was a job for jackhammers and pickaxes. We deliberately used the thinner 5/8" rods because they deviate more readily when they hit rock, the other option is a 1 1/2" rod which will go through rock but there was no way to get a pneumatic driver up there.


Final Ra was 0.092ohms (worst result of 3 tests) after soaking the pits to backfill them. I've got to go back and confirmation test in 3 months and again in a year. Should breeze the 3 month test because it will be mid-winter and very wet, the 1 year retest should be the interesting one.

 

[Engineer54]

View attachment 28457
We had 4 rods per pit, each one was 4 rods coupled together and we angled the rods at about 20degrees from vertical so the depth of each rod was only around 8 meters. I wish it was sand but alas it was half way up a mountain so very rocky. Even digging the pit was a job for jackhammers and pickaxes. We deliberately used the thinner 5/8" rods because they deviate more readily when they hit rock, the other option is a 1 1/2" rod which will go through rock but there was no way to get a pneumatic driver up there.


Final Ra was 0.092ohms (worst result of 3 tests) after soaking the pits to backfill them. I've got to go back and confirmation test in 3 months and again in a year. Should breeze the 3 month test because it will be mid-winter and very wet, the 1 year retest should be the interesting one.

Hang on, you said you used 64 rods in 4 pit locations, that to my calculation is 16 rods a per pit??

''took 64 x 2.4m rods in 4 separate earth pits''

 

[Knobhead]

4 x 16 = ?

Although Marvo’s initial description is misleading.

 

[Marvo]

Probably didn't word it very well, we did use 64 x 2.4m rods in total. There were 4 separate pits as pictured above.

 

[GB.Kayak]

Probably didn't word it very well, we did use 64 x 2.4m rods in total. There were 4 separate pits as pictured above.

M, I got the gist ... 64 in total not 256!

 

[Knobhead]

I’ve got this silly image in my mind of Marvo belting rods in and hitting rock that bend the rods back upwards, they appear out of the ground behind him.

Sorry kid.

 

[Marvo]

It wouldn't be the first time, we knocked a rod in once at the corner of a factory unit and it surfaced about 12m away in the middle of the car park. You never really know how deep or what direction they go unless it's sand or unless you use large diameter rods like 1 1/2" or 2", with the 5/8ths rods it's just a numbers game where some will go straightish and some will deviate toward horizontal.

 

[Knobhead]

Something I wished I’d tried at the foundry.

Push a 6” nail in and see what reading it would give. The ground was full of iron oxide, carbon, sulphur, lime slag and to make it fun, cyanide.

PS, the land is up for sale as a prime building site, don’t take up gardening.

 

[Engineer54]

Yep, i was obviously led astray.. lol!