Can't get earth rod under 200ohms

[Ash]

I just tested a shed TT, and the Ze came to 523Ω. I went and got a second rod, banged that into the ground, connected them together and then got 306Ω. I really don't want to hammer in a 3rd rod if I can help it, especially as about 10 miles away I did the exact same installation and got 36Ω. Is it the quality of the soil that's causing this or the distance from the substation?

does anyone have any advice or should i go buy a 3rd rod?

 

[Barx]

The fact that we just had a dry week or 2 wont help this......

You tried p**sing on it yet? Desparate times and all that. lol

 

[Adrian]

so what would be safer a high tt or tncs?

 

[Andy™]

deeper rod

and whilst adding something to it (barx), it will rise again once you leave. only a cowboy would do something like that

 

[SPECIAL LOCATION]

Need some more facts to make a decent descision.....

1/

Is this a new shed install or an existing installation you are working on?

2/

Is this a domestic garden shed?

3/

How far is the run from the main supply building?

4/

What is the earth arrangement at the main supply?

4/

Is it all wooden construction?

5/

Are there any extraneous parts in the shed?

:|

 

[Barx]

[quote name='Andy

 

[a1]

I took that as a "tongue in cheek" comment

from Barx, Andy.

 

[Revved Up Sparky]

I had the same problem at a house a few weeks ago where the Ra was 800 ohms. The house was on a hill and the terrain very rocky. I ended up hammering in eight rods (1200mm long) at five different locations--- and only then after all that effort, sweat and blistered hands, the reading was still 199 ohms !. The customer was so worried about the bill (I was all day hammering in rods) that I agreed to let him dig the trenches between the rods and lay the tubing and earths. I just hope when I eventually get back that he has left me a decent loop at each rod location for the earth connections !.

 

[Ash]

Need some more facts to make a decent descision.....1/

Is this a new shed install or an existing installation you are working on?

New shed built on concrete, it's a glorified shed to use as a workshop sorta thing

2/

Is this a domestic garden shed?

yeah

3/

How far is the run from the main supply building?

was 36m roughly from the house

4/

What is the earth arrangement at the main supply?

TN-C-S

4/

Is it all wooden construction?

it's mostly wooden

5/

Are there any extraneous parts in the shed?

there's was no metal structure that I bonded too, just the concrete floor

:|

answers in red

 

[kerching]

I had the same problem at a house a few weeks ago........ I ended up hammering in eight rods (a metre long) at five different locations--- and only then after all that effort, sweat and blistered hands, !.

Probably too late now [handy for next time though]...I got a broken SDS bit about 16mm , ground the end down to fit in the rod 'driving screw'. stick it in the SDS on roto-stop and blether away. Also did same on a broken K taper steel bit for my BIG Makita breaker...works a treat. simples

:coat

 

[SPECIAL LOCATION]

1/

Is this a new shed install or an existing installation you are working on?

New shed built on concrete, it's a glorified shed to use as a workshop sorta thing

2/

Is this a domestic garden shed?

yeah

3/

How far is the run from the main supply building?

was 36m roughly from the house

4/

What is the earth arrangement at the main supply?

TN-C-S

4/

Is it all wooden construction?

it's mostly wooden

5/

Are there any extraneous parts in the shed?

there's was no metal structure that I bonded too, just the concrete floor

Right..

IMHO this is one where you need to do a bit of risk assessment over costs of remedial improvements -vs- Realistic risk of danger..

First off..

If your distance had of been nearer the house

I probably wouldn't have bothered with TT'ing a wooden shed with no extraneous parts.

But as this is not the case on you job I would be thinking..

1/

How frequently is this shed going to be in use?

e.g General garden jobs -vs- a garden office with persons using it more than 6hrs per day?

2/

How hard or easy was it to sink the rods in the ground?

what soil types e.g. sandy, clay'ish, rocky etc..?

3/

Are the existing rods you have put in expandable?

i.e. can you go deeper with a coupler and threaded rod?

14mm x 1200mm Earth Rod ( 5/8" x 4ft )

4/

Do your RCD trip test's operate in good times?

Dependant upon the above answers..

I may just try to extend existing rod positions down deeper..

(e.g. just rod costs no more earth cable needed)

ALSO

If the shed is rarely used I may be inclined to leave it as is..

BUT arrange to go back a few months down the line to double check the

'Ra' to see if it has deteriorated or improved with time?

(remember 200ohms is NOT the max permissible..

just guidance where it MAY be unstable!)

IF it is going to be in use a lot of hours per day I may do a bit more..

but my gut feeling is the risks on a WOODEN SHED with no extraneous parts are not that much more than an extension lead up the garden plugged in at the house.

:C

 

[Revved Up Sparky]

Probably too late now [handy for next time though]...I got a broken SDS bit about 16mm , ground the end down to fit in the rod 'driving screw'. stick it in the SDS on roto-stop and blether away. Also did same on a broken K taper steel bit for my BIG Makita breaker...works a treat. simples :coat

Thanks for that Kerching . I did try something similar but the force of the hammer action kept breaking the couplers and I was losing rods underground. I take it you did'nt experience that problem ?

 

[Ash]

Right..IMHO this is one where you need to do a bit of risk assessment over costs of remedial improvements -vs- Realistic risk of danger..

First off..

If your distance had of been nearer the house

I probably wouldn't have bothered with TT'ing a wooden shed with no extraneous parts.

But as this is not the case on you job I would be thinking..

1/

How frequently is this shed going to be in use?

e.g General garden jobs -vs- a garden office with persons using it more than 6hrs per day?

unknown to me, maybe lots in one go then not at all for weeks

2/

How hard or easy was it to sink the rods in the ground?

what soil types e.g. sandy, clay'ish, rocky etc..?

quite rocky, but not a problem

3/

Are the existing rods you have put in expandable?

i.e. can you go deeper with a coupler and threaded rod?

14mm x 1200mm Earth Rod ( 5/8" x 4ft )

Yeah I always use the 5/8" ones anyway, problem is after a good hammer with the sledgehammer, it was hard to put the coupler on (any tips for this?)

4/

Do your RCD trip test's operate in good times?

problem at first was I couldn't do the 5x test because it was >333ohms. Now the RCD trips fine and ramp test was 27mA

Dependant upon the above answers..

I may just try to extend existing rod positions down deeper..

(e.g. just rod costs no more earth cable needed)

ALSO

If the shed is rarely used I may be inclined to leave it as is..

BUT arrange to go back a few months down the line to double check the

'Ra' to see if it has deteriorated or improved with time?

(remember 200ohms is NOT the max permissible..

just guidance where it MAY be unstable!)

IF it is going to be in use a lot of hours per day I may do a bit more..

but my gut feeling is the risks on a WOODEN SHED with no extraneous parts are not that much more than an extension lead up the garden plugged in at the house.

:C

I would have probably left it at 300ohms, but I was stubborn that the second rod didn't take it below 200,

however today I expanded it and drove it further down, now the Ze is 46.2ohms

phew

Lesson learnt for the future, go deeper rather than more rods at same depth

thanks for the posts all

 

[NozSpark]

If you have problems with the thread on the top f the rod after drivig it in then you're not doing it right

I always use a coupler and appropriatley sized bolt (driving stud) when knocking them in,,, I did however run into problems once in rocky ground and couldn't for the life of me remove the coupler & stud

so there's another trick for you,, fit the clamp before the coupler and stud, especially in rocky ground

 

[Apache]

Lesson learnt for the future, go deeper rather than more rods at same depth

Seems sensible as deeper = closer to water table and better conductivity. You'd hit bedrock around here though!

 

[steptoe]

always double up on depth before putting a second rod in,

as Apache says, deeper holds moisture more(closer to water table),

also remember that if you do need to drive a 2nd rod in to go twice the distance of depth of the deepest rod.

hope that makes sense....

 

[Manator]

I almost always drill first before trying to hammer a rod into the soil, and deeper is better than seperate rods. We have had a few good topics about the effectiveness of rods, and how to get the best from them.

 

[Revved Up Sparky]

I went to a house today to fix a high Ra (374 ohms) on a TT installation. I hammered in two long earth rods (coupled together) before testing Ra. It was 390 ohms ! grrr. I therefore coupled a third rod on and hammered that down too and hey presto I got an Ra of 73 ohms .... so as the others have said, going as deep as possible is definitely the answer. I find it fascinating that going just three foot deeper can dramatically reduce the reading. That is obviously where the moist earth is.

 

[m4tty]

I done one last week but had to break up a concrete path as nowhere else to put it. I decided to bang a thin rod through the expansion joint in the concrete path to test if I was going to get a decent reading. Banged it in a foot and took reading of 437 ohms then banged it in another 6 inches and it went down to 198 ohms. So broke some concrete then put in the 5/8 rod and got my best reading yet. 7.40 ohms. I was amazed bit then client told me the supply tx was 2 garages away lol.

 

[steptoe]

I done one last week but had to break up a concrete path as nowhere else to put it. I decided to bang a thin rod through the expansion joint in the concrete path to test if I was going to get a decent reading. Banged it in a foot and took reading of 437 ohms then banged it in another 6 inches and it went down to 198 ohms. So broke some concrete then put in the 5/8 rod and got my best reading yet. 7.40 ohms. I was amazed bit then client told me the supply tx was 2 garages away lol.

I think you shaved the sheath of the TNCS cable, :slap